the digestive systemmyresource.phoenix.edu/secure/resource/sci241r4/sci241...the digestive system: 3...

(Luis Real/Age Fotostock America, Inc.)

C H A P T E R 3 C O N C E P T S

� All plant and animal life is made up of cells, which form tissues that compose the organs and organ systems of a living organism.

� The food we eat is digested in the gastrointestinal tract and itsnutrients are absorbed into the body.

� Hormones released into the blood and enzymes released into the gastrointestinal tract facilitate the digestion of food and the absorption of nutrients.

� Digestion begins in the mouth. Food is then swallowed and movesdown the esophagus to the stomach for storage and further digestion.

� The small intestine is the primary site of digestion and absorption.� Water-soluble materials are absorbed into the blood. Most

fat-soluble materials are absorbed into the lymph.� Changes in the gastrointestinal tract can alter digestion

and absorption and affect nutritional status.� Nutrients delivered to the cells can be used to produce energy

in the form of ATP and to synthesize molecules for immediate use or for storage.

� Materials that are not absorbed pass into the large intestine and areexcreted in feces. The waste products generated inside the body by metabolism are eliminated via the lungs, skin, and kidneys.

Why does your mouth water at the sight or smell of food?

Why are you hungry very soon aftereating some meals while others stickwith you longer ?

Is it healthy to have bacteria living in your gastrointestinal tract?

Ju s t A Ta s t e

WebMD Feature Archive

Bariatric Surgery: A Radical Obesity FixBy Catherine Guthrie

Dec. 18, 2000—Two years ago, Rhonda Bailey was fatand miserable. The 38-year-old bore 245 pounds on her 5-foot-1 frame. Squeezing into a restaurant booth,airplane seat, or amusement park ride was unimaginable.Walking made her joints stiff and sore. She leaned heavilyon a cane to get from the handicapped parking space toher desk at work.

Today, Bailey is literally half the woman she used to be.Over the past 18 months, she has shed 50% of her bodyweight. Her waist shrunk from a size 26 to a size six.Now, at 125 pounds, the Southern California residentjogs daily, bicycles with her stepdaughter, and rejoices inher ability to do life’s little things—like tying her shoes—without asking her husband for help. Bailey’s secret isn’tthe latest fad diet or radical weight loss drug. She owesher slimmed-down body to gastric-bypass surgery. Theprocedure is just one of several weight loss operationsthat fall under the heading of bariatric surgery.

As obesity rates creep skyward, so do the number ofAmericans turning to surgery as a weight loss tool.

To read the rest of this article, go to www.my.webmd.com/content/article/14/1689_51239.htm.

55

3The Digestive System:From Meals to MoleculesOur Complex Bodies Are Built from Simpler UnitsCells Form Tissues, Tissues Form Organs, Organs Form

Organ SystemsOrgan Systems Work Together to Digest, Absorb,

and Utilize Nutrients

The Digestive System Digests Foodand Absorbs NutrientsThe Gastrointestinal Tract Is a Long, Hollow TubeDigestive Tract Secretions Aid DigestionThe Wall of the Gastrointestinal Tract Has Four LayersThe Gastrointestinal Tract Protects Us from Infection

Food Must Be Digested forNutrients to Be AbsorbedDigestive Activity Begins with the Sights, Sounds,

and Smells of FoodDigestion Begins in the MouthSwallowing Occurs in the PharynxSwallowed Food Travels Down the Esophagus

to the StomachThe Stomach Mixes and Stores Food and Begins

Protein DigestionThe Small Intestine Is the Site of Most Digestion

and AbsorptionThe Large Intestine Is the Last Chance for Absorption

Alterations in the GI Tract CanAffect Digestion and AbsorptionDigestive System Problems and Discomforts

Are CommonYou Can Obtain Nutrients Even If You Can’t EatDigestive Function Varies with Life Stage

The Blood Delivers AbsorbedNutrients to Body CellsThe Cardiovascular System Delivers Nutrients

and Removes WastesWater-Soluble Nutrients Go to the LiverFat-Soluble Substances Enter the Lymphatic SystemNutrients Are Transported into the Body Cells

Metabolic Reactions Synthesize and Break Down MoleculesCellular Respiration Breaks Down Nutrients

to Produce EnergyCarbohydrate, Protein, and Fat Are Used for Energy

or for Synthesis

Waste Products Must Be Eliminatedfrom the Body

INTRODUCTION

ariatric surgery is used to treat obesity. It reducesthe size of the stomach to the size of your thumbso it can’t hold as much. It also reroutes the in-testines so less of what is eaten can be absorbed.After this surgery, people can’t eat as much and

can’t absorb as much, so they lose weight.Was this surgery a cure for Rhonda’s obesity? She has

lost weight, but not without consequences. Her digestivetract has been permanently altered and so has her ability toget the nutrients she needs. She can no longer eat a largemeal or absorb all the nutrients from that meal. If she doesovereat she may experience pain, sweats, chills, and nausea.

B

56 Chapter 3 The Digestive System: From Meals to Molecules

Even when she is careful to eat small meals, diarrhea can be a problem. Bypassing partof her intestines also put her at risk for a number of vitamin deficiencies. Understand-ing how the gastrointestinal tract functions will help you grasp the consequences ofthis type of surgery as well as appreciate how important this organ system is for ob-taining nutrients.

Our Complex Bodies Are Built from Simpler Units

When it comes down to it, we are all made of atoms. Atoms can be linked by chemicalbonds to form molecules. The carbohydrates, lipids, proteins, and vitamins in our dietand in our bodies are organic molecules. Water and minerals are inorganic molecules.These molecules are organized to form cells, the smallest unit of plant and animal life(Figure 3.1). These cells are then grouped to provide different structures and functionsin the body.

Cells form tissues, tissues form organs,organs form organ systemsIn any living system, whether a sunflower, a cow, or a human being, molecules are or-ganized into cells. Cells that are similar in structure and function form tissues. Thehuman body contains four types of tissue: muscle, nerve, epithelial, and connective.These tissues are organized in varying combinations into organs. Organs such as theheart, the stomach, and the kidneys are discrete structures that perform specializedfunctions in the body (Figure 3.2).

F I G U R E 3 . 1

All living things are made up of cells. Thisphoto shows the cells that make up the roottip of an onion plant. Various types of cellsmake up all the tissues of the human body.(© Lester V. Bergman/Corbis Images)

Atoms

Molecule

Cells Tissues

OrganOrganismOrgan

systems

NC

OH

F I G U R E 3 . 2

The organization of life begins with atoms that form molecules, which are then organized into cells to form tissues,organs, organ systems, and organisms. (Brian Bailey/Stone/Getty Images)

Atoms The smallest units of an elementthat still retain the properties of that element.�*

Chemical bonds Forces that holdatoms together.�*

Molecules Units of two or more atomsof the same or different elements bondedtogether.

�*

Organic molecules Molecules thatcontain two or more carbon atoms.�*

Inorganic molecules Substances thatdo not contain carbon-to-hydrogen bonds.�*

Cells The basic structural andfunctional units of plant and animal life.�*

Organ A discrete structure composedof more than one tissue that performs aspecialized function.

�*

Most organs do not function alone but are part of organ systems. The organ sys-tems in humans include the nervous system, respiratory system (lungs), urinary sys-tem (kidneys and bladder), reproductive system, cardiovascular system (heart andblood vessels), lymphatic/immune system, muscular system, skeletal system, en-docrine system (hormones), integumentary system (skin and body linings), and diges-tive system (Table 3.1). An organ may be part of more than one organ system. Forexample, the pancreas is part of the endocrine system as well as the digestive system.

Organ systems work together to digest,absorb, and utilize nutrientsThe digestive system is the organ system primarily responsible for the movement ofnutrients into the body; however, several other organ systems are also needed to regu-late these processes and use the nutrients. The endocrine system secretes hormonesthat help regulate how much we eat and how quickly food and nutrients travelthrough the digestive system. The nervous system sends nerve signals that help controlthe passage of food through the digestive tract. The cardiovascular system transportsnutrients to individual cells in the body. The urinary, respiratory, and integumentarysystems eliminate wastes generated within the body.

Our Complex Bodies Are Built from Simpler Units 57

T A B L E 3 . 1What Do Organ Systems Do?

Organ System What It Includes What It Does

Nervous Brain, spinal cord, Responds to stimuli from the external and internal and associated nerves environments; conducts impulses to activate muscles

and glands; integrates activities of other systems

Respiratory Lungs, trachea, and air Supplies the blood with oxygen and removes carbon dioxidepassageways

Urinary Kidneys and their Eliminates wastes and regulates the balance of water, associated structures electrolytes, and acid in the blood

Reproductive Testes, ovaries, and their Produces offspringassociated structures

Cardiovascular Heart and blood vessels Transports blood, which carries oxygen, nutrients, and wastes

Lymphatic/Immune Lymph and lymph structures, Defends against foreign invaders, picks up fluid leaked white blood cells from blood vessels, transports fat-soluble nutrients

Muscular Skeletal muscles Provides movement and structure

Skeletal Bones and joints Protects and supports the body, provides a framework for the muscles to use for movement

Endocrine Pituitary, adrenal, thyroid, Secretes hormones that regulate processes such as growth, pancreas, and other reproduction, and nutrient useductless glands

Integumentary Skin, hair, nails, and sweat Covers and protects the body; helps control body temperatureglands

Digestive Mouth, esophagus, stomach, Ingests and digests food, absorbs nutrients into the blood, intestines, pancreas, liver, eliminates unabsorbed food residue and other waste productsand gallbladder

Adapted from E. N. Marieb. Human Anatomy and Physiology, 5th ed. Redwood City, California: Benjamin/Cummings Publishing Co.,2000.

Hormones Chemical messengers thatare produced in one location, released intothe blood, and elicit responses at otherlocations in the body.

�*


The Digestive System Digests Food and Absorbs Nutrients

The digestive system provides two major functions: digestion and absorption. Mostfood must be digested in order for the nutrients it contains to be absorbed into thebody. For example, a slice of bread does not travel through the digestive system intact.First it is broken apart, releasing its carbohydrate, protein, and fat. Most of the carbo-hydrate is digested to sugars, the protein to amino acids, and the fat to fatty acids.Sugars, amino acids, and fatty acids can be absorbed. The fiber from the bread cannotbe digested and therefore is not absorbed into the body. Fiber and other unabsorbedsubstances pass through the digestive tract and are excreted in the feces.

The gastrointestinal tract is a long, hollow tubeThe main part of the digestive system is the gastrointestinal tract. It is also referred toas the GI tract, gut, digestive tract, intestinal tract, or alimentary canal. It can bethought of as a hollow tube, about 30 feet in length, that runs from the mouth to theanus. The organs of the gastrointestinal tract include the mouth, pharynx, esophagus,stomach, small intestine, large intestine, and anus (Figure 3.3). The inside of the tubethat these organs form is called the lumen. Food within the lumen of the gastrointesti-nal tract is not technically inside the body because it has not been absorbed. There-fore, if you swallow something that cannot be digested, such as an unchewed raisin orcorn kernel, it will pass through your digestive tract and exit in the feces without everbeing broken down or entering your blood or cells. Only after substances are trans-ferred into the cells that line the intestine by the process of absorption are they actu-ally “inside” the body. The amount of time it takes for food to pass through the length

Mouth

Salivary glands

Pharynx

Esophagus

Liver

Gallbladder

Smallintestine

Stomach

Pancreas

Largeintestine

Anus

Colon

Rectum

F I G U R E 3 . 3

The digestive system consists of the organs of the gastrointestinal tract: themouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus,as well as a number of accessory organs: the salivary glands, liver, gallbladder,and pancreas.

Digestion The process of breakingfood into components small enough to beabsorbed into the body.

�*

Absorption The process of takingsubstances into the interior of the body.�*

Fatty acids Chains of carbon atomsthat are often present as components oflarger molecules known as triglycerides,which we commonly call fats.

�*

Feces Body waste, includingunabsorbed food residue, bacteria, mucus,and dead cells, which is excreted from thegastrointestinal tract by passing throughthe anus.

�*

Gastrointestinal tract A hollow tube consisting of the mouth, pharynx,esophagus, stomach, small intestine, largeintestine, and anus, in which digestion and absorption of nutrients occur.

�*

Lumen The inside cavity of a tube,such as the gastrointestinal tract.�*

of the GI tract from mouth to anus is referred to as transit time. The shorter the tran-sit time, the more rapid the passage through the digestive tract. In a healthy adult,transit time is about 24 to 72 hours. It is affected by the composition of the diet,physical activity, emotions, medications, and illnesses.

Digestive tract secretions aid digestionThe digestive process is aided by substances that are secreted into the digestive tractfrom cells lining the digestive tract and from a number of accessory organs. One ofthese substances is mucus. It moistens, lubricates, and protects the digestive tract.Enzymes are another component of digestive system secretions. Enzymes are proteinmolecules that speed up chemical reactions without themselves being consumed orchanged by the reactions (Figure 3.4). In the digestive tract, enzymes break down foodinto absorbable units. Different enzymes are needed for the breakdown of differentfood components. For example, enzymes called amylases digest carbohydrate but haveno effect on fat, and enzymes called lipases digest fat but have no effect on carbohydrate.

The digestive system also releases hormones. Hormones are chemical messengersthat are secreted into the blood by one organ to regulate body functions elsewhere. Inthe gastrointestinal tract, hormones regulate the rate at which food moves through thesystem by sending signals that help prepare different parts of the GI tract for the ar-rival of food. For example, a hormone called cholecystokinin signals the pancreas andgallbladder to secrete digestive substances into the GI tract.

The wall of the gastrointestinal tract has four layersThe wall of the gastrointestinal tract contains four layers (Figure 3.5). The layer liningthe lumen, which is in contact with the gut contents, is the mucosa. Because mucosalcells are in direct contact with churning food and harsh digestive secretions, they liveonly about 2 to 5 days. The dead cells are sloughed off into the lumen, where somecomponents are digested and absorbed and the rest are excreted in the feces. Mucosalcells reproduce rapidly to replace those that die. To allow for this rapid replacement,the mucosa has high nutrient requirements and is one of the first parts of the body tobe affected by nutrient deficiencies. Surrounding the mucosa is a layer of connectivetissue containing nerves and blood vessels. This layer provides support, delivers nutri-ents to the mucosa, and provides the nerve signals that control secretions and musclecontractions. Layers of smooth muscle—the type over which we do not have voluntary

The Digestive System Digests Food and Absorbs Nutrients 59

Largemolecule

Smallermolecules

Enzyme

Enz

yme E

nz yme F I G U R E 3 . 4

Enzymes speed up chemical reactions withoutthemselves being altered by the reaction. Inthis example, an enzyme breaks a largemolecule into two smaller ones.

Transit time The time between theingestion of food and the elimination of thesolid waste from that food.

�*

Mucus A viscous fluid secreted byglands in the gastrointestinal tract andother parts of the body. It acts to lubricate,moisten, and protect cells from harshenvironments.

�*

Enzymes Protein molecules thataccelerate the rate of specific chemicalreactions without being changedthemselves.

�*

Mucosa The layer of tissue liningthe gastrointestinal tract and other bodycavities.

�*


control—surround the connective tissue. The contraction of smooth muscles in theGI tract mixes food, breaks it into smaller particles, and propels it through the diges-tive tract. The final, external layer is also made up of connective tissue and providessupport and protection.

The gastrointestinal tract protects us from infectionThe gastrointestinal tract is an important part of the immune system, which protectsthe body from foreign invaders. It limits the absorption of toxins and disease-causingorganisms. If an invading substance, or antigen, does get past the intestinal cells, theimmune system has a number of weapons that can destroy it. Some of these are cellsand some are chemicals released into the blood. The different types of white bloodcells make up the cells of the immune system. Many of these reside in the gastroin-testinal tract, so a foreign substance or harmful organism that enters the body throughthe mucosa can be destroyed quickly. The first types of cells to come to the body’s de-fense are called phagocytes. They will target any invader and act by engulfing and de-stroying foreign substances. These cells also release chemicals that signal otherimmune system cells called lymphocytes to come join the fight. Lymphocytes are veryspecific about what they will attack. Some lymphocytes bind directly to a specific in-vader or antigen and destroy it. This type of lymphocyte helps eliminate cancer cells,foreign tissue, and cells that have been infected by viruses and bacteria. Other lym-phocytes produce and secrete protein molecules called antibodies. Antibodies bind toinvading antigen and help to destroy it. Each antibody is designed to fight off onlyone specific type of antigen. Once the body has made antibodies to a specific antigen,it remembers and is ready to fight that antigen any time it enters the body.

Our immune system protects us from many invaders without our even being awareof it. Unfortunately, the response of the immune system to a foreign substance is alsoresponsible for allergic reactions. An allergic reaction occurs when the immune systemproduces antibodies to a substance that is present in our diet or environment. For ex-ample, a food allergy occurs when proteins present in food are seen as foreign andtrigger an immune response. The immune response causes symptoms that range fromhives to life-threatening breathing difficulties.

Connective tissueVilli

Nerves

Smooth muscle

Blood vessels

Mucosa

Connective tissue

F I G U R E 3 . 5

This cross-section through the wall of thesmall intestine shows the four tissue layers:mucosa, connective tissue, smooth musclelayers, and outer connective tissue layer.

When you receive a vaccination orimmunization you are receiving asmall dose of harmless antigen,such as an inactivated or killedvirus. It does not make you sick butis seen as foreign, and your bodyproduces antibodies to thatantigen. Then, if you are exposed tothe same antigen again, yourimmune system remembers how tomake the specific antibody, and youare able to quickly fight off theinfection. Antibodies are discussedmore in Chapter 6.

Antigen A foreign substance (almostalways a protein) that, when introducedinto the body, stimulates an immuneresponse.

�*

Antibodies Proteins produced bycells of the immune system that destroy orinactivate foreign substances in the body.

�*

Food Must Be Digested for Nutrients to Be Absorbed

Food Must Be Digested for Nutrients to Be Absorbed 61

Your Choice: When Food Is the Foe

Food provides nourishment along with interesting tastesand smells that stir our senses. However, for someone with a food allergy, unfamiliar foods and flavors must beconsidered with caution. Food allergies occur when theimmunological components of the digestive tract becomeconfused and see a particular food as an invader rather than a source of nutrition. Once you have developed a food allergy, exposure to the offending food can causeuncomfortable and even deadly reactions. How does food become the foe?

The first time a food is consumed, it does not trigger anallergic reaction, but in a susceptible person, this begins theprocess. As the food travels down the digestive tract it ismechanically broken apart, exposed to acid, and chemicallydigested, but tiny fragments of undigested protein remain.These fragments are absorbed into the body. In susceptibleindividuals this triggers the production of a particular typeof antibody. This antibody binds to cells called mast cells.When the offending food is eaten again, the food proteinsbind to the antibody and trigger the mast cells to releasechemicals, such as histamine, that cause redness and swellingand other symptoms of an allergic reaction. The tiny proteinfragments that cause allergies are not destroyed by cooking,broken down by stomach acid, or digested by enzymes. Theyare able to cross the gastrointestinal lining and travelthrough the bloodstream.1

The symptoms of a food allergy change depending onwhere in the digestive tract and the body these protein

fragments trigger the release of histamine. For example,when the food enters the mouth the allergic person mayexperience an itching or tingling sensation on the tongue or lips. As it travels down to the stomach and intestines the food may cause vomiting and cramps. After it isabsorbed and travels through the bloodstream, theseprotein fragments may cause a drop in blood pressure,hives, and when it reaches the lungs, asthma.

Food allergies affect about 1.5% of adults and up to 6% of children under 3 years of age. They are responsiblefor 150 deaths each year.1 Foods that most commonly causeallergies in adults include seafood, peanuts, tree nuts, fish,and eggs. In children allergies to eggs, milk, peanuts, soy,and wheat are most common. The best way to avoid allergicsymptoms is to choose foods that are free of ingredientsthat trigger an immune response. This sounds easy, but it can be difficult to identify all the ingredients in foods.Reading food labels is essential, but many of the foods weeat don’t come out of a package. For most of us, trying anew entrée at a restaurant or going to a potluck dinner isan adventure in eating, but for those with food allergies, it is a potential danger. Careful reconnaissance is requiredbefore a new food can be considered safe.

Reference1. Formanek, R. Food allergies: when food becomes the enemy. FDA Consumer,

July/Aug, 2001. Available online at www.fda.gov/fdac/features/2001/401food.html/Accessed Nov. 17, 2003.

To be used by the body, food must be consumed and digested, and the nutrients mustbe absorbed and transported to the cells of the body. Because most foods are mixturesof carbohydrate, fat, and protein, the digestive tract is designed to allow the digestionof all of these components without competition among them. The following sectionsof this chapter will trace a meal through all these processes, from the body’s anticipa-tion of food to the elimination of waste products.

Digestive activity begins with the sights,sounds, and smells of foodImagine slices of oven-roasted turkey served on fresh baked whole grain bread accom-panied by a crisp apple and a tall glass of cold milk. Is your mouth watering? You don’teven need to put something in your mouth for activity in the digestive tract to begin.As the meal is being prepared, sensory input such as the sight of a turkey being liftedout of the oven, the clatter of the table being set, and the smell of freshly baked breadmay make your mouth water and your stomach begin to secrete digestive substances.

You can’t digest food withoutputting it in your mouth, butyou can get the juices flowing.Just looking at or smellingappetizing food can sendsignals to your salivary glandsto make your mouth waterand to your stomach to secretegastric juice.


This response occurs when the nervous system signals the digestive system to readyitself for a meal.

Digestion begins in the mouthTake a bite (Figure 3.6). The mouth is the point at which food enters the digestivetract. The presence of food in the mouth stimulates the flow of saliva from the sali-vary glands (see Figure 3.3). Saliva moistens food and carries dissolved food moleculesto the taste buds. Signals from the taste buds along with the smell of food allow us toexperience the full flavor of food. Saliva helps digest food because it contains the en-zyme salivary amylase, which begins breaking starches into sugars. Saliva also helpsprotect against tooth decay because it washes away food particles and contains sub-stances that inhibit the growth of bacteria that may cause tooth decay.

Chewing food is important for digestion. Chewing breaks food into small piecesand thereby increases the surface area that is in contact with digestive juices. Adult hu-mans have 32 teeth, specialized for biting, tearing, grinding, and crushing foods. Thetongue helps mix food with saliva and aids chewing by constantly repositioning foodbetween the teeth. Chewing also breaks apart fiber that traps nutrients. If the fiber isnot broken, some nutrients cannot be absorbed. For example, the peel of an apple is asource of vitamins and minerals; these nutrients cannot be absorbed without firstbeing released from the fiber in the peel.

Swallowing occurs in the pharynxThe meal that entered the mouth as a turkey sandwich, apple, and milk has now beenformed into a bolus, a ball of chewed food mixed with saliva. From the mouth, thebolus moves into the pharynx, the part of the gastrointestinal tract responsible for swal-lowing. The pharynx is shared by the digestive tract and the respiratory tract: foodpasses through the pharynx on its way to the stomach, and air passes on its way to andfrom the lungs. During swallowing, the air passages are blocked by a valve-like flap oftissue called the epiglottis, so food goes to the stomach, not the lungs. Sometimes eat-ing too quickly or talking while eating can interfere with the closing of the epiglottisand food can pass into an upper air passageway. It is usually dislodged with a cough,but if it becomes stuck it can block the flow of air and cause choking. A quick responseis required to save the life of a person whose airway is completely blocked. The Heim-lich maneuver, which forces air out of the lungs with a sudden application of pressureto the upper abdomen, can blow an object out of the blocked air passage (Figure 3.7).

Swallowed food travels down the esophagusto the stomachThe esophagus passes through the diaphragm, a muscular wall separating the ab-domen from the chest cavity where the lungs are located, to connect the pharynx andstomach. In the esophagus the bolus of food is moved along by rhythmic contractionsof the smooth muscles, a process called peristalsis. The contractions of peristalsis arestrong enough that even food swallowed while you are standing on your head willreach your stomach. This contractile movement, which is controlled automatically bythe nervous system, occurs throughout the gastrointestinal tract, pushing the foodbolus along from the pharynx through the large intestine.

To move from the esophagus into the stomach, food must pass through a sphincter,a muscle that encircles the tube of the digestive tract and acts as a valve. When thesphincter muscle contracts, the valve is closed (Figure 3.8). The gastroesophagealsphincter, located between the esophagus and the stomach, prevents foods from mov-ing back out of the stomach. Occasionally, materials do pass out of the stomachthrough this valve. The pain of heartburn is caused when some of the acidic stomachcontents leak up and out of the stomach into the esophagus, causing a burning sensa-tion. Vomiting is the result of reverse peristalsis that causes the sphincter to relax andallow the food to pass upward out of the stomach toward the mouth.

F I G U R E 3 . 6

By the time you take a bite, the sights, smells,sounds, or thoughts of food have alreadytriggered activity in your digestive tract.(Ann Ackerman/Taxi/Getty Images)

Saliva A watery fluid produced andsecreted into the mouth by the salivaryglands. It contains lubricants, enzymes,and other substances.

�*

Pharynx A funnel-shaped opening thatconnects the nasal passages and mouth tothe respiratory passages and esophagus. Itis a common passageway for food and airand is responsible for swallowing.

�*

Esophagus A portion of thegastrointestinal tract that extends from the pharynx to the stomach.

�*

Peristalsis Coordinated muscularcontractions that move food through thegastrointestinal tract.

�*

Sphincter A muscular valve thathelps control the flow of materials in the gastrointestinal tract.

�*

The stomach mixes and stores food and begins protein digestionThe stomach is an expanded portion of the gastrointestinal tract that serves as a tem-porary storage place for food. While held in the stomach, the bolus is mixed withhighly acidic stomach secretions to form a semiliquid food mass called chyme. Themixing of food in the stomach is aided by an extra layer of smooth muscle in thestomach wall. While most of the gastrointestinal tract is surrounded by two layers ofmuscle, the stomach contains a third layer, allowing for powerful contractions thatthoroughly churn and mix the stomach contents. Some digestion takes place in thestomach, but, with the exception of some water, alcohol, and a few drugs such as as-pirin and acetaminophen (Tylenol), very little absorption occurs here.


Bolus

Epiglottis

EsophagusPassageway

to lungs

(a)

(b)

Diaphragm

F I G U R E 3 . 7

(a) When a bolus of food is swallowed, it pushes the epiglottis down over the opening to the air passageways. (b) If fooddoes become lodged in the air passageways, it can be dislodged by the Heimlich maneuver, illustrated here.

Do you have a sweet tooth? Most ofus do. Our taste for sweet as well assalty foods is inborn. Thesepreferences may have helped ourancestors get enough energy andsodium, which was scarce in theprimitive diet. Today, sugar and saltare added to many foods, and thesedesirable tastes may lead us tooverconsume them. Our aversion forsour and bitter may have helped usavoid bitter toxins.

Bolus

Sphincterclosed

Sphincteropened

Esophagus

Stomach

F I G U R E 3 . 8

The rhythmic contractions of peristalsispropel food down the esophagus, through theopen sphincter, and into the stomach.


Gastric juice contains acid and enzymes Stomach secretionscalled gastric juice are produced by gastric glands in the lining of the stomach. One ofthe components of gastric juice is hydrochloric acid. This acid kills most of the bacte-ria present in food. It also stops the activity of the carbohydrate-digesting enzyme sali-vary amylase and helps begin the digestion of protein by unfolding proteins andactivating the enzyme pepsin. The digestion of the starch from the bread stops in yourstomach, and digestion of the protein from the turkey, milk, and bread begins. Theprotein of the stomach wall is protected from the acid and pepsin by a thick layer ofmucus. If the mucus layer is penetrated, pepsin and acid can damage the underlyingtissues and cause peptic ulcers, erosions of the stomach wall or some other region ofthe gastrointestinal tract. One of the leading causes of ulcers is infection of the stom-ach lining by acid-resistant bacteria called Helicobacter pylori. These bacteria damagethe gastrointestinal tract wall and destroy the protective mucosal layer.1

Nerves and hormones regulate stomach motility and se-cretions How much your stomach churns and how much gastric juice is releasedis regulated by signals from both nerves and hormones. These signals originate fromthree different sites—the brain, stomach, and small intestine. The thought, smell,sight, or taste of food causes the brain to send nerve signals that stimulate gastric se-cretion, preparing the stomach to receive food (Figure 3.9). Food entering the stom-ach then also stimulates secretions and motility by stimulating local nerve signals,sending signals to the brain, and by stimulating gastrin secretion. Gastrin is a hor-mone that triggers the release of gastric juice and increases stomach motility.

Food entering the small intestine triggers hormonal and nervous signals that candecrease stomach motility and secretions. This slows the release of food into the intes-tine, ensuring that the amount of chyme entering the small intestine does not exceedthe ability of the intestine to process it.

The size and composition of the meal affect stomachemptying Chyme normally empties from the stomach in 2 to 6 hours. The rateof stomach emptying is determined by the size and composition of the meal and iscontrolled by signals from the small intestine. To move from the stomach to the smallintestine, chyme must pass through the pyloric sphincter. This sphincter helps regu-late the rate at which food enters the intestine. The small intestine stretches as it fills

Decrease GastricSecretion

Increase Gastric

Food

Secretion

Food

Gastrin

Hormones

FoodF I G U R E 3 . 9

The sight, smell, and taste of food cause the brain to signal an increase in gastric secretions.When food enters the stomach, gastric secretions are stimulated by the stretching of localnerves, signals sent to the brain, and the release of gastrin. As food enters the small intestine,gastric secretions are inhibited by nervous and hormonal signals. In this diagram the zigzagarrows represent nerve signals, and the dashed arrows represent hormonal signals.

Over half of the world’s populationis infected with the bacteriumHelicobacter pylori. It causes ulcersin the stomach and esophagus, andlong-term infection is associatedwith the development of gastriccancer. But not everyone who isinfected will develop any symptoms.The good news for those who do isthat 2 weeks of antibiotic therapycan kill the bacteria, allowing theulcer to heal.

Pepsin A protein-digesting enzymeproduced by the stomach. It is secreted inthe gastric juice in an inactive form andactivated by acid in the stomach.

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Peptic ulcer An open sore in the liningof the stomach, esophagus, or small intestine.�*

Gastrin A hormone secreted by thestomach mucosa that stimulates thesecretion of gastric juices.

�*

with food; this distension inhibits stomach emptying. A large meal takes longer tocompletely leave the stomach than a small meal, and a solid meal leaves the stomachmore slowly than a liquid meal. The nutritional composition of a meal also affectshow long it stays in the stomach. The meal of a sandwich, apple, and milk that wehave been following through the GI tract contains protein, carbohydrate, and fat andis partly solid and partly liquid. It will leave the stomach in an average amount oftime, about 4 hours. A higher-fat meal would stay in the stomach the longest becausefat slows stomach emptying. A meal that is primarily protein will leave more quicklythan a high-fat meal, and a meal of mostly carbohydrate will leave the fastest. Becausethe nutrient composition of a meal affects how quickly it leaves your stomach, it af-fects how soon after eating you feel hungry again. Thus, what you choose for breakfastcan affect when you become hungry for lunch. Toast and coffee will leave your stom-ach far more quickly than a larger meal with more protein and some fat, such as abowl of cereal with low-fat milk and toast with peanut butter. Factors besides foodcomposition also can affect gastric emptying. For example, sadness and fear tend toslow emptying, while aggression tends to increase gastric motility and speed emptying.

The small intestine is the site of mostdigestion and absorptionThe small intestine is a narrow tube about 20 feet in length. It is divided into threesegments. The first 12 inches are the duodenum, the next 8 feet are the jejunum, andthe last 11 feet are the ileum. The small intestine is the main site of digestion of foodand absorption of nutrients.

Digestive secretions from accessory organs aid digestionIn the small intestine, secretions from the pancreas and gallbladder as well as from theintestine itself aid digestion. The pancreas secretes pancreatic juice, which contains bi-carbonate and digestive enzymes. The bicarbonate neutralizes the acid in the chyme,making the environment in the small intestine neutral or slightly basic rather thanacidic as it is in the stomach. This neutrality allows enzymes from the pancreas andsmall intestine to function. Pancreatic amylase continues the job of breaking starchesinto sugars that was started in the mouth by salivary amylase. Pancreatic protein-digesting enzymes, including trypsin and chymotrypsin, continue to break proteininto shorter and shorter chains of amino acids. Intestinal digestive enzymes, found at-tached to or inside the cells lining the small intestine, aid the digestion of sugars intosingle sugar units and the digestion of short amino acid chains into single aminoacids.

The gallbladder stores and secretes bile, a substance produced in the liver that isnecessary for fat digestion and absorption. Bile secreted into the small intestine mixeswith fat and breaks it into smaller globules, allowing pancreatic enzymes, called lipases,to more efficiently access the fat and digest it. The bile and digested fats then formsmall droplets that facilitate the absorption of fat into the mucosal cells.

Absorption occurs by passive and active mechanisms Thesmall intestine is the primary site of absorption for water, vitamins, minerals, and theproducts of carbohydrate, fat, and protein digestion. To be absorbed, these nutrientsmust pass from the lumen of the GI tract into the mucosal cells lining the tract andthen into the blood or lymph. Several different mechanisms are involved (Figure3.10). Many nutrients can cross into the mucosa by diffusion, which is the movementof substances from an area of higher concentration to an area of lower concentration.In this case nutrients move from the lumen, where their concentration is high, intothe cells of the intestine, where their concentration is lower. In simple diffusion, sub-stances pass freely across cell membranes and no energy is required. Vitamin E andfatty acids are absorbed by simple diffusion. Water is also absorbed by diffusion. Thediffusion of water is called osmosis. In osmosis there is a net movement of water in adirection that will balance the concentration of dissolved substances on either side of amembrane. For example, if there is a high concentration of sugar in the lumen of the


The reason some meals stickwith you longer relates to theamount of carbohydrate, fat,and protein they contain. Ahigh-fat, high-protein mealsuch as fried chicken withbuttered corn bread willempty from your stomachslowly and keep you full longerthan a high carbohydrate mealof rice and steamed vegetables.

Pancreas An organ that secretesdigestive enzymes and bicarbonate into thesmall intestine during digestion.

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Gallbladder An organ of the digestivesystem that stores bile, which is producedby the liver.

�*

Bile A substance made in the liverand stored in the gallbladder. It is releasedinto the small intestine to aid in fatdigestion and absorption.

�*

Lipases Fat-digesting enzymes.�*

Simple diffusion The movement of substances from an area of higherconcentration to an area of lowerconcentration. No energy is required.

�*

Osmosis The diffusion of water acrossa membrane to equalize the concentrationof dissolved substances on both sides.

�*


intestine, water will actually move from the mucosal cells into the lumen. As the sugaris absorbed, and the concentration in the lumen decreases, water will move back intothe mucosal cells by osmosis.

Some nutrients cannot pass freely across cell membranes; they must be helpedacross by carrier molecules in a process called facilitated diffusion. They still movefrom an area of higher concentration to one of lower concentration, and no energy isneeded for their transport. The sugar fructose found in the apple is absorbed by facili-tated diffusion.

Substances unable to be absorbed by simple or facilitated diffusion must enter thebody by active transport, a process that requires both a carrier molecule and energy.This use of energy allows substances to be transported from an area of lower concentra-tion to an area of higher concentration. The sugar glucose from the breakdown ofstarch in the bread and amino acids from the protein in the milk, turkey, and bread areabsorbed by active transport. Active transport allows these nutrients to be absorbedeven when they are present in higher concentrations inside the mucosal cells than inthe lumen. More specific information about the absorption of the products of carbo-hydrate, fat, and protein digestion will be discussed in Chapters 4, 5, and 6, respectively.

The small intestine has a huge surface area for absorb-ing nutrients The structure of the small intestine is specialized to allow maxi-mal absorption of the nutrients. In addition to its length, the small intestine has threeother features that increase the area of its absorptive surface (Figure 3.11). First, theintestinal walls are arranged in circular or spiral folds, which increase the surface areain contact with nutrients. Second, its entire inner surface is covered with fingerlikeprojections called villi (singular, villus). And finally, each of these villi is covered withtiny microvilli, often referred to as the brush border. Together these features provide asurface area that is about the size of a tennis court (300 m2 or 3229 ft2). Each villuscontains a blood vessel and a lymph vessel or lacteal, which are located only one celllayer away from the nutrients in the intestinal lumen. Nutrients must cross the mu-cosal cell layer to reach the bloodstream or lymphatic system for delivery to the tissuesof the body.

Cellmembrane

Energy

Active transport

(a) (b) (c)

Simplediffusion

Facilitateddiffusion

F I G U R E 3 . 1 0

Nutrients are absorbed from the lumen across the cell membrane into the cell by several mechanisms. (a) Simplediffusion, which requires no energy, is shown here by the purple balls that move across the membrane from an areaof higher concentration to an area of lower concentration. (b) Facilitated diffusion, which requires no energy, isshown here by the yellow cubes that move across the membrane from an area of higher concentration to an areaof lower concentration with the help of a carrier. (c) Active transport, which requires energy and a carrier, is shownhere by the red pyramids that move across the membrane from an area of lower concentration to an area ofhigher concentration.

Facilitated diffusion The movementof substances across a cell membrane froman area of higher concentration to an areaof lower concentration with the aid of acarrier molecule. No energy is required.

�*

Active transport The transport ofsubstances across a cell membrane withthe aid of a carrier molecule and theexpenditure of energy.

�*

Villi (villus) Fingerlike protrusions of the lining of the small intestine thatincrease the absorptive surface area.

�*

Microvilli Minute brushlikeprojections on the mucosal cell membranethat participate in the digestion andabsorption of foodstuffs.

�*

Lymph vessel or lacteal A tubularcomponent of the lymphatic system thatcarries fluid away from body tissues. Lymphvessels in the intestine are known aslacteals and can transport large particlessuch as the products of fat digestion.

�*

The protein in our cells could bedigested by the same enzymes thatdigest the protein in our food, butthey aren’t. One reason is thatdigestive enzymes like pepsin andtrypsin are produced in inactiveforms so they can’t harm the glandsthat produce them. They areactivated only after they are in thelumen of the GI tract, which isprotected by a layer of mucus.

Circular orspiral folds

Microvilli

Villi

Microvilli

LactealCapillary

Artery

Vein

Lymphvessel

SMALL INTESTINE

The large intestine is the last chance for absorptionMaterials that have not been absorbed in the small intestine enter the large intestinethrough a sphincter. This sphincter prevents material from the large intestine fromreentering the small intestine. The large intestine is about 5 feet long and includes thecolon and rectum. Although most absorption occurs in the small intestine, water andsome vitamins and minerals are also absorbed in the colon. Peristalsis here is slowerthan in the small intestine. Water, nutrients, and fecal matter may spend 24 hours inthe large intestine, in contrast to the 3 to 5 hours it takes material to move throughthe small intestine. This slow movement favors the growth of bacteria. These bacteria,called the intestinal microflora, are permanent beneficial residents of this part of thegastrointestinal tract. They break down unabsorbed portions of food, such as the fiber,producing nutrients that the bacteria themselves can use or, in some cases, that can beabsorbed into the body. For example, the microflora synthesize small amounts of someB vitamins and vitamin K, some of which can be absorbed. As the microflora breakdown food, they produce gas, which causes flatulence. In normal adult humans, be-tween 200 and 2000 ml of gas is produced per day.

Materials not absorbed in the colon are excreted as waste products in the feces. Thefeces are a mixture of undigested unabsorbed matter, dead cells, secretions from theGI tract, water, and bacteria. The amount of bacteria varies but can make up morethan half the weight of the feces. The amount of water in the feces is affected by fiberand fluid intake. Fiber retains water, so when adequate fiber and fluid are consumed,feces have a high water content and are easily passed. When inadequate fiber or fluidis consumed, feces are hard and dry, and constipation can result.

The end of the colon is connected to the rectum, where feces are stored prior todefecation. The rectum is connected to the anus, the external opening of the digestivetract. The rectum and anus work with the colon to prepare the feces for elimination.Defecation is regulated by a sphincter that is under voluntary control. It allows the


F I G U R E 3 . 1 1

The small intestine contains folds, villi, andmicrovilli, which increase the absorptivesurface area. (S. Ito and D. W. Fawcett/VisualsUnlimited)

Colon The largest portion of the largeintestine.�*

Rectum The portion of the largeintestine that connects the colon and anus.�*

Intestinal microflora Microorganismsthat inhabit the large intestine.�*

Anus The outlet of the rectum throughwhich feces are expelled.�*

It is normal and healthy tohave bacteria living in yourcolon. Although you’re notaware of their presence, youmiss them when they are gone.For example, taking antibioticsto kill harmful bacteria oftenalso kills the beneficial bacteriain our gut, causing diarrhea.


Your Choice: Should You FeedYour Flora?

Bacteria growing in your gut? Sounds bad, but the largeintestine of a healthy adult is home to 300 to 500 differentspecies of bacteria. In fact, the number of bacterial cellsliving in your gut is about 10 times greater than the totalnumber of cells in your entire body.1 The type and numberof bacteria in your gastrointestinal tract are affected byboth your diet and your health. In turn, the type andamount of bacteria in your GI tract can affect your health.

Most of the time these microflora provide beneficialeffects. They break down non-digestible dietary substances;improve the digestion and absorption of essential nutrients;synthesize vitamins, some of which can be absorbed; andmetabolize harmful substances, such as ammonia, thusreducing their concentration in the blood. They areimportant for intestinal immune function, proper growthand development of cells in the colon, and optimalintestinal motility and transit time.2 A strong population of healthful bacteria can also inhibit the growth of harmfulbacteria. If the wrong bacteria take over, they can causediarrhea, infections, and perhaps an increased risk of cancer.Should we be supplementing our diet with beneficialbacteria or eating certain foods to promote the growth of these good bugs?

The recognition that intestinal bacteria are important to human health is not new. In the early 1900s, NobelPrize–winning Russian scientist Eli Metchnikoff observedthat Bulgarian peasants who consumed large quantities of yogurt and other fermented milk products lived longhealthy lives. He proposed that the bacteria in these foods positively affected the microflora in the colon and“prevented intestinal putrification” and helped “maintainthe forces of the body.”3 Modern research continues tosupport Metchnikoff’s hypothesis that the bacteria infermented dairy products, which include Bifidobacteriumand Lactobacillus, provide health benefits.

Today, the consumption of products, such as yogurt, thatnaturally contain these bacteria and supplements containinglive bacteria are referred to as probiotic therapy. Probioticsare living organisms which when consumed in adequateamounts confer a health benefit on the host. Whenconsumed, some of these organisms survive passagethrough the upper GI tract and live temporarily in the colonbefore they are excreted in the feces. Some of the healthbenefits of probiotics are well documented. For example,they improve the digestion of the sugar lactose in lactose-

intolerant people (see Chapter 4). They have been shown to prevent diarrhea associated with antibiotic use and toreduce the duration of diarrhea due to intestinal infectionsand other causes.4,5 Probiotic bacteria have also been foundto have beneficial effects on immune function in theintestine.6 They have been shown to reduce the productionof toxic substances in the colon and inhibit the activity ofenzymes that produce carcinogens.1 Although probioticshave not been clearly demonstrated to reduce the risk ofcolon cancer in humans, studies in animals have shown thatthey can prevent the formation and growth of cancerouscells in the colon.7 There is also some evidence thatprobiotics may have other beneficial effects, including relief from constipation, reduction of allergic symptoms,and reduction of blood cholesterol levels.

One problem with probiotics is that when they are nolonger consumed, the added bacteria are rapidly washedout of the colon. However, a healthy population of bacteriacan be promoted by consuming substances that encouragethe growth of particular types of bacteria. These substances,called prebiotics, pass undigested into the colon andstimulate the growth and/or activity of certain types ofbacteria. Prebiotics are currently sold as dietary supplementsand are added to commercially prepared tube feedingformulas to promote gastrointestinal health.

Our understanding of probiotics and prebiotics isexpanding. We know that the risks of using these productsare negligible, but their specific health benefits are stillbeing investigated. Soon we may be able to take probiotics,instead of antibiotics, to eliminate hazardous bacteria in ourintestines. And we may be paying attention to what we arefeeding our flora—as well as ourselves.

References1. Bengmark, S. Ecological control of the gastrointestinal tract: the role of

probiotic flora. Gut 42:2–7, 1998.2. Guarner, F., and Malagelada, J.R. Gut flora in health and disease. Lancet

361:512–519, 2003.3. Metchnikoff, E. The Prolongation of Life. New York: GP Putnam’s Sons, 1908.4. D’Souza, L., Rajkumar, C., Cooke, J., and Bulpitt, C.J. Probiotics in prevention of

antibiotic associated diarrhea: meta–analysis. BMJ 324:1361–1366, 2002.5. Martineau, P.R., de Vrese, M., Cellier, C.J., and Schrezenmeir, J. Protection from

gastrointestinal diseases with the use of probiotics. Am. J. Clin. Nutr.73(suppl):430S–436S, 2001.

6. Isolauri, E., Sütas, Y., Kankaanpää, P., et al. Probiotics: effects on immunity. Am.J. Clin. Nutr. 73(suppl):444S–450S, 2001.

7. Brady, L. J., Gallaher, D.D., and Busta, F.F. The role of probiotic cultures in theprevention of colon cancer. J. Nutr. 130:410S–414S, 2000.

feces to be eliminated at convenient and appropriate times. The digestion and absorp-tion of carbohydrate, fat, and protein are summarized in Figure 3.12.

Alterations in the GI Tract Can Affect Digestion and Absorption 69

Aminoacids

Brushborder

enzymes

Acids andgas

Largeintestine Bacterial

enzymes

Fiber

FecesBacteria, fiber not broken down,

dead cells, other undigested food products

Fiber, single sugars

Bile

Pancreaticlipase

Portalveinto

liver

Lymphvessel

toblood

Most fattyacids

AF TProtein andpolypeptides

Trypsinand otherenzymes

Pancreaticamylase

Fiber, double sugars

Sugars, smaller starches, fiber

PROTEIN

Polypeptides

Smallintestine

Acid

Pepsin

CARBOHYDRA(sugars, starch, fiber)

Salivaryamylase

Sugars, smaller starches, fiber

Mouth

Stomach

TE

F I G U R E 3 . 1 2

This overview of the digestion and absorption ofa meal illustrates the digestion of carbohydratesinto smaller starches and eventually singlesugars, the breakdown of proteins into shorteramino acid chains called polypeptides andeventually into amino acids, and the digestionof fats into fatty acids, most of which must beabsorbed into the lymph.

Alterations in the GI Tract Can Affect Digestion and Absorption

Each of the organs and processes of the digestive system is necessary for the proper di-gestion of food and absorption of nutrients. Alterations in the GI tract due to illnessor injury can affect your ability to obtain nutrients from food. Life stage, such as preg-nancy, infancy, and old age, also affects the physiology of the digestive tract. Some di-gestive problems and their causes, consequences, and solutions are given in Table 3.2.

Digestive system problems and discomforts are commonAlmost everyone experiences digestive discomforts from time to time, limiting thetypes of food that can be consumed. For example, dental problems can make it diffi-cult to chew. When food is poorly chewed, contact between digestive enzymes and thenutrients in food is reduced, thereby limiting absorption.

The brown color of feces comesmainly from a pigment calledbilirubin. It arises from thebreakdown of red blood cells andenters the GI tract in bile. Bilirubinis yellow, but bacterial actionconverts it to a brown compound.


T A B L E 3 . 2Digestive Problems Have Nutritional Consequences

Problem What Causes It What Are the Consequences What Can Be Done to Help

Dry mouth Disease, medications Decreased food intake due to Change medications, use changes in taste, difficulty artificial salivachewing and swallowing, increased tooth decay, and gum disease

Dental pain Tooth decay, gum disease Reduced food intake due to Change consistencyand loss of impaired ability to chew; of foods consumedteeth reduced nutrient absorption

due to incomplete digestion

Heartburn and Stomach acid leaking into Pain and discomfort after Reduce meal size, avoid high-fat GERD esophagus, caused by eating; ulcers; increased foods, consume liquids between

overeating, anxiety, stress, cancer risk rather than with meals, remainpregnancy, and upright after eating, takedisease processes antacids and other medications

Ulcers Infection of stomach by Pain, bleeding, and possible Antibiotics to treat infection; H. pylori, acid-resistant abdominal infection antacids to reduce acid; bacteria that penetrate the change medicationsmucous layer and damage the epithelial lining; chronic use of drugs such as aspirin and ibuprofen that erode the mucosa

Vomiting Bacterial and viral Dehydration and electrolyte Medications to treat infection; infections, medications, imbalance; if chronic, can fluid and electrolyte replacementother illnesses, eating damage the mouth, gums,disorders, pregnancy, esophagus, and teethfood allergies

Diarrhea Bacterial and viral Dehydration and electrolyte Medications to treat infection; infections, medications, imbalance fluid and electrolyte replacementfood intolerance

Constipation Low fiber intake, low fluid Discomfort, intestinal blockage, High-fiber, high-fluid diet; intake, high fiber in formation of outpouchings in the exercise; medicationscombination with low intestinal wall (diverticula)fluid intake, weak intestinal muscles

Pancreatic Cystic fibrosis or Malabsorption of fat, fat-soluble Oral supplements of digestive disease other chronic vitamins, and vitamin B12 due to enzymes

pancreatic diseases reduced availability of pancreaticenzymes and bicarbonate

Gallstones Deposits of cholesterol, Pain and poor fat digestion and Low-fat diet, surgical removal bile pigments, and calcium absorption of the gallbladderin the gallbladder or bile duct

Heartburn occurs when the acidic stomach contents leak back into the esophagus,causing a burning sensation in the chest or throat. If the caustic fluid is tasted in theback of the mouth, it is sometimes called acid indigestion. The more technical termfor the leakage of stomach contents back into the esophagus is gastroesophageal reflux.Occasional heartburn from reflux is common, but if it occurs more than twice a weekit may indicate a condition called gastroesophageal reflux disease (GERD). If left un-treated it can eventually lead to more serious health problems such as bleeding, ulcers,

Gastroesophageal reflux disease(GERD) A condition in which the acids fromthe stomach move backward from thestomach into the esophagus.

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Off the Label: Antacids: Gettingthe Drug Facts

Medication is an often overlooked source of nutrients. Insome cases, these nutrients are a welcome addition to thediet, but in others they may have a negative impact onnutritional status. Heartburn remedies, which are amongthe most popular over-the-counter drugs, are often a sourceof nutritive minerals.

Heartburn is a common minor digestive complaint that iscaused by stomach acid leaking into the esophagus. The acidin stomach secretions is necessary for digestion; it enhancesthe absorption of certain nutrients and minimizes bacterialgrowth. But excess stomach acid or stomach acid that comesin contact with other parts of the gastrointestinal tract cancause pain and discomfort.

When choosing a product to help your heartburn (or anyother ailment), be sure to check the label. Drug Facts labelson nonprescription drugs are designed to help consumerstake medications correctly as well as understand thebenefits and risks and any nutritional impact the drugsmight have. The label must present information in astandardized, easy-to-follow format that is as readable andas consistent as the Nutrition Facts label found on food

products. Under the title “Drug Facts,” the product’s activeingredients must be listed first, along with the purpose foreach, followed by uses, warnings, and directions, and theninactive ingredients.1

The label shown here indicates that Reducid containsfamotidine, a drug that works by reducing the amount of acid secreted by the stomach. It also contains calciumcarbonate and magnesium hydroxide, compounds thatneutralize the acidity of the stomach contents as well ascontribute nutrients to the diet. The label tells you that asingle tablet contains 800 mg of calcium carbonate and 165mg of magnesium hydroxide. This is equivalent to about 320mg of calcium (32% of the Daily Value) and 69 mg ofmagnesium (17% of the Daily Value). Both of thesenutrients are typically consumed in less than therecommended amounts in the American diet. The calciumcarbonate in this product is a good source of well-absorbedcalcium and the magnesium hydroxide will add magnesiumto the diet. Higher doses of magnesium can cause diarrhea.In fact, when taken in high doses, magnesium antacids suchas milk of magnesia can be used as laxatives.

ReduReducidcidAntacid/Acid Reducer

FlavoredChewable

tabletsUse

Famotidine 10 mg

relieves heartburn associated with acid indigestion and sour stomach

Calcium carbonate 800 mgMagnesium hydroxide 165 mg

Acid reducerAntacidAntacid

DUAL ACTIONDUAL ACTIONwith

Reduces and Neutralizes Acid

Relieves Heartburn Due To Acid Indigestion

RRRR

Drug Facts

WarningsAllergy alert: Do not use if you are allergic to famotidine or other acid reducers

Do not use

Ask a doctor or pharmacist before use if you are presently taking a prescription drug. Antacidsmay interact with certain prescription drugs.

Stop use and ask a doctor if • stomach pain continues • you need to take this product for more than 14 days

If pregnant or breast-feeding, ask a health professional before use. Keep out of reach of children. In case of overdose, get medical help or contact a Poison ControlCenter right away.

• if you have trouble swallowing • with other famotidine products or acid reducers

• adults and children 12 years and over:

• read the directions and warnings before use• keep the carton and package insert. They contain important information.• store at 25–30°C (77-86°F) • protect from moisture

call toll-free 1–800–555–4008

• do not swallow tablet whole: chew completely• to relieve symptoms, chew 1 tablet before swallowing• do not use more than 2 chewable tablets in 24 hours

• children under 12 years: ask a doctor

(continued)

Directions

Other information

cellulose acetate, corn starch, dextrates, flavors, hydroxypropyl cellulose, hydroxypropylmethylcellulose, lactose, magnesium stearate, pregelatinized starch, red iron oxide, sodiumlauryl sulfate, sugar

Inactive ingredients

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and cancer. Whether you have occasional heartburn or GERD, it can cause discom-fort after eating a large meal and limit the types and amounts of food you can con-sume. Symptoms can be reduced by eating small meals and avoiding spicy foods, fattyand fried foods, citrus fruits, chocolate, caffeinated beverages, tomato-based foods,garlic and onions, and mint-flavored foods. Remaining upright after eating, wearingloose clothing, avoiding smoking and alcohol, and losing weight may also help relievesymptoms. For many people medications are needed to manage symptoms.

Ulcers can occur in both the esophagus and the stomach. They can result fromGERD but, as discussed above, are more often caused by a bacterial infection in thestomach. Mild ulcers may limit food choices to those that do not cause discomfort,but more severe ulcers can cause bleeding that is life-threatening.

Changes in accessory organs of the GI tract can also affect digestion. If the pan-creas is not functioning normally, the availability of enzymes needed to digest carbo-hydrate, fat, and protein will be reduced, limiting the ability to absorb these essentialnutrients. A common condition that affects the gallbladder is gallstones. These clumpsof solid material that form in the gallbladder can cause pain when the gallbladder con-tracts in response to fat in the intestine. Gallstones can interfere with bile secretionand reduce fat absorption. They are usually treated by removing the gallbladder. Inthis case, bile simply drips into the intestine as it is produced rather than being storedand squeezed out in larger amounts when fat enters the intestine.

Common discomforts that are related to problems in the intestines include diar-rhea and constipation. Diarrhea occurs when the intestines do not absorb enoughwater, resulting in frequent watery stools. This occurs when the lining of the small in-testine is inflamed so nutrients and water are not absorbed. The inflammation may becaused by infection with microorganisms or by other irritants. Constipation refers tohard dry stools that are difficult to pass. Constipation can be caused by a diet thatcontains insufficient fluid or fiber, lack of exercise, or a weakening of the muscles ofthe large intestine.

You can obtain nutrients even if you can’t eatSometimes, because of serious problems with the GI tract, people are unable to con-sume food or digest and absorb the nutrients needed to meet their requirements. Forthese cases, there are several alternative feeding methods that can be used to providenutrients. People who are unable to chew or swallow can be fed a liquid diet through atube inserted into the stomach or intestine. Tube feeding can provide a balanced dietcontaining all the essential nutrients. It can be used in patients who are unconsciousor have suffered an injury to the upper gastrointestinal tract. For individuals whosegastrointestinal tract is not functional, nutrients can be provided directly into thebloodstream. This is referred to as total parenteral nutrition (TPN). Carefully plannedTPN can provide all the nutrients essential to life. When not all nutrients are pro-vided in a TPN solution, nutrient deficiencies develop quickly. Inadvertently feeding

Other antacids contain minerals that are not beneficialadditions to the diet. For example, Alka-Seltzer containssodium bicarbonate. Each tablet of Alka-Seltzer contains567 mg of sodium, about 24% of the Daily Value. Sodium-containing antacids contribute a significant amount ofsodium and would not be recommended for someone on asodium-restricted diet. Aluminum is also found in a numberof antacids. Aluminum binds to phosphate in the gut andlimits phosphorus absorption. It may also cause constipation.In addition to minerals, a look at the inactive ingredientsshows that many products marketed to treat heartburncontain sugar (added to make them more palatable) and

starch (added as filler). The amounts are small, but individualswho have diabetes or are consuming low-calorie diets shouldconsider these in diet planning.

If you are looking for an over-the-counter medication totreat occasional heartburn, understanding how to read thelabel helps you to use it correctly and to know if the productwill affect your nutritional health as well as reduce yourheartburn.

Reference1. Nordenberg, T. New drug label spells it out simply. FDA Consumer, July/August

1999. Available online at www.fda.gov/FDAC/features/1999/499_otc.html.Accessed August 12, 2004.

For more information on diet and digestivedisease, go to theNational Digestive

Diseases InformationClearinghouse at

www.digestive.niddk.nih.gov/

Total parenteral nutrition (TPN)A method of providing complete nutritionwithout use of the gastrointestinal tract by infusing a nutrient-rich solution directlyinto the bloodstream.

�*

patients incomplete TPN solutions has helped nutritionists determine the functionsand essentiality of several trace minerals.

Digestive function varies with life stageThere are some differences in the way the digestive system functions during pregnancyand infancy and with advancing age. These differences affect the ability to ingest anddigest food and absorb nutrients. However, if the diet is properly managed, nutri-tional status can be maintained at all stages of life.


P I E C E I T T O G E T H E R

Changes in Your GI Tract Affect Your Nutritional HealthThis chapter has followed the path of a turkey sandwich, anapple, and a glass of milk through the processes of digestion andabsorption. During the journey from mouth to anus, manyfactors affect how well these processes work. For each situationdescribed below, think about how digestion, absorption, and the GI tract itself might be altered.

You are taking medication that reduces the amount ofsaliva you produce.

HOW MIGHT THIS AFFECT YOUR FOOD INTAKE?

When you don’t have enough saliva, it is difficult to swallowand you can’t taste food well. These factors decrease theappeal of food and may reduce intake. Also, because salivahelps protect your teeth, the likelihood of cavities isincreased.

WHAT NUTRIENTS MIGHT NOT BE ABSORBED FROM THEAPPLE IF YOU JUST HAD SOME DENTAL WORK DONE?

If you don’t chew food well, the enzymes needed fordigestion do not contact all the components of the food. Ifthe fiber in the apple skin is not chewed, the vitamins andminerals it contains may not be available for absorption.

After consuming the turkey sandwich, apple, and glass of milk, you have a large slice of high-fat cheesecake fordessert.

HOW WOULD THIS AFFECT THE TRANSIT TIMEOF YOUR MEAL?

Transit time would be longer because the cheesecake is high in fat and makes the meal larger, both of which slowstomach emptying.

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You have a disease of the pancreas that causes a deficiencyof pancreatic enzymes.

HOW WOULD THIS AFFECT THE DIGESTIONAND ABSORPTION OF YOUR MEAL?

Pancreatic enzymes are needed to digest carbohydrate, fat,and protein. If these enzymes are lacking, digestion will beincomplete, and nutrient absorption will be compromised.Carbohydrate-digesting enzymes in the mouth and intestinalbrush border, as well as protein-digesting enzymes in thestomach and mucosal cells, can partially compensate for a reduction in pancreatic enzymes.

You are ill and have become malnourished. The malnutritioncauses the villi in your intestine to become flattened.

HOW WOULD THIS AFFECT YOUR NUTRIENT ABSORPTION?

Your answer:

You eat the turkey sandwich and apple but choose not todrink the milk or any other liquid.

HOW MIGHT THIS AFFECT YOUR FECES?

A diet low in fluid and high in fiber could result in hard, dryfeces and cause constipation.

You have gallstones, which cause pain when the gallbladdercontracts.

WHAT TYPE OF FOODS SHOULD YOU AVOID?

Your answer:

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Nausea, heartburn, and constipation are common dur-ing pregnancy Physiological changes that occur during pregnancy often causegastrointestinal problems (see Chapter 12). During the first 3 months, many womenexperience nausea, referred to as morning sickness. This term is a misnomer, because itcan occur at any time of the day. Morning sickness is believed to be due to pregnancy-related hormonal changes. In most cases, its symptoms can be limited by eating fre-quent small meals and avoiding foods and smells that cause nausea. Eating dry,starchy foods like crackers or cereal may also help. In severe cases in which uncontrol-lable vomiting occurs, TPN may be needed to provide adequate nutrition.

Later in pregnancy, the enlarged uterus puts pressure on the stomach and in-testines, which can make it difficult to consume large meals. In addition, the placentaproduces the hormone progesterone, which causes the smooth muscles of the digestivetract to relax. The muscle-relaxing effects of progesterone may relax the gastroe-sophageal sphincter enough to allow stomach contents to move back into the esopha-gus, causing heartburn. In the large intestine, relaxed muscles and the pressure of theuterus cause less efficient peristaltic movements and may result in constipation. In-creasing water intake, eating a diet high in fiber, and maintaining a regular exerciseroutine can help prevent constipation.

Infants’ digestive tracts are not fully developed At birth, ababy’s digestive tract is functional, but obviously a baby cannot consume an adult diet(see Chapter 12). Infants can only consume liquids. They are born with a suckling re-flex that allows them to consume liquids from a nipple placed toward the back of themouth. They also have a protrusion reflex that causes anything placed in the front ofthe mouth to be pushed out by the tongue. As head control increases this reflex disap-pears, making spoon feeding possible.

Digestion and absorption also differ between infants and adults. The enzymes inthe infant digestive tract are better suited to digesting the type of carbohydrate and fatfound in milk. Infants have a greater ability to absorb whole proteins. This allows

So, What Should I Eat?

Eat foods you enjoy

Avoid foods that cause digestive problems for you

Reduce your risk of choking• Take small bites• Chew each bite thoroughly• Don’t take a bite when talking or laughing

Reduce the chances of heartburn• Eat enough to satisfy your hunger but not so much you are stuffed• Wait 10 minutes between your first and second course to see how full you feel• Stay upright after you eat—don’t flop on the couch in front of the tube

Prevent constipation by having enough fiber and fluid• Choose a whole grain cereal like oatmeal or raisin bran for breakfast• Double your servings of vegetables at dinner• Take two pieces of fruit for lunch• Switch to whole grain bread• Have one or two beverages with or between each meal

For more information onnormal digestion, go tothe National DigestiveDiseases Information

Clearinghouse, atwww.digestive.niddk.nih.gov/ddiseases/a-z.asp, and click on digestion.

them to absorb immune factors from breast milk that provide temporary immunity tocertain diseases. However, it also makes them susceptible to food allergies.

The bacteria in the large intestine of infants are also different from those in adultsbecause of their all-milk diet. This is why the feces of breast-fed babies are almostodorless. Another feature of the infant digestive tract is the lack of voluntary controlof elimination. Between the ages of 2 and 3 years, this ability develops and toilet train-ing is possible.

Changes occur with aging that can affect GI functionWith aging, changes in the digestive tract and other systems may affect the palatabilityof food and the ability to obtain proper nutrition (see Chapter 13). The senses ofsmell and taste are often diminished or even lost with age, reducing the appeal offood. A reduction in the amount of saliva may make swallowing difficult, decrease thetaste of food, and promote tooth decay. Loss of teeth and improperly fitting denturesmay limit food choices to soft and liquid foods or cause solid foods to be poorlychewed. A condition called atrophic gastritis, which occurs commonly in older peo-ple, causes a reduction in the secretion of stomach acid. This may decrease the absorp-tion of several vitamins and minerals and may allow bacterial growth to increase.Constipation is a common complaint among the elderly; it may be caused by de-creased elasticity of the colon, weakened abdominal and pelvic muscles, and a decreasein sensory perception.

The Blood Delivers Absorbed Nutrients to Body Cells 75

The Blood Delivers Absorbed Nutrients to Body Cells

Once food has been digested and absorbed, the nutrients have to be delivered to thecells. This delivery is handled by the cardiovascular system, which consists of the heartand blood vessels. Some nutrients enter the bloodstream at the intestine. Amino acidsfrom protein, simple sugars from carbohydrate, and the water-soluble products of fatdigestion are absorbed directly into the bloodstream. However, the products of fat di-gestion that are not water-soluble are taken into the lymphatic system before enteringthe blood. The villi of the intestine contain both capillaries, which are part of the car-diovascular system, and lacteals, which are small vessels of the lymphatic system.

The cardiovascular system delivers nutrients and removes wastesThe cardiovascular system is a closed network of tubules through which blood ispumped. Blood carries nutrients and oxygen to the cells of all the organs and tissues ofthe body and removes waste products from these same cells. Blood also carries othersubstances, such as hormones, from one part of the body to another (Figure 3.13).

The heart and blood vessels make up the cardiovascularsystem The heart is the workhorse of the cardiovascular system. It is a muscularpump with two circulatory loops—one that delivers blood to the lungs and one thatdelivers blood to the rest of the body. The blood vessels that transport blood and dis-solved substances toward the heart are called veins, and those that transport blood anddissolved substances away from the heart are called arteries. As arteries carry bloodaway from the heart, they branch many times to form smaller and smaller blood ves-sels. The smallest arteries are called arterioles. Arterioles then branch to form capillar-ies. Capillaries are thin-walled vessels that are just large enough to allow one red bloodcell to pass at a time. From the capillaries, oxygen and nutrients carried by the bloodpass into the cells, and waste products pass from the cells into the capillaries. In thecapillaries of the lungs, blood releases carbon dioxide to be exhaled and picks up oxy-gen to be delivered to the cells. In the capillaries of the GI tract, blood delivers oxygenand picks up water-soluble nutrients absorbed from the diet. Blood from capillariesthen flows into the smallest veins, the venules, which converge to form larger and

An average person has about10,000 taste buds, located mostlyon the tongue. As people age, thenumber of taste buds they havedeclines and so does their sense of taste.

Atrophic gastritis An inflammation ofthe stomach lining that causes a reductionin stomach acid and allows bacterialovergrowth.

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Lymphatic system The system ofvessels, organs, and tissues that drainsexcess fluid from the spaces between cells,transports fat-soluble substances from thedigestive tract, and contributes to immunefunction.

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Capillaries Small, thin-walled bloodvessels where the exchange of gases andnutrients between blood and cells occurs.

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Veins Vessels that carry blood towardthe heart.�*

Arteries Vessels that carry blood awayfrom the heart.�*


larger veins for return to the heart. Therefore, blood starting in the heart is pumpedthrough the arteries to the capillaries of the lungs, where it picks up oxygen. It thenreturns to the heart via the veins and is pumped out again into the arteries that lead tothe rest of the body. In the capillaries of the body, blood delivers oxygen and nutrientsand removes wastes before returning to the heart via the veins.

Blood flow can be directed or diverted, depending onneed The amount of blood and hence the amounts of nutrients and oxygen that aredelivered to a specific organ or tissue depend on the need. When you are resting,about a quarter of your blood goes to your digestive system, a fifth to your skeletalmuscles, and the rest to the heart, kidneys, brain, skin, and other organs.2 But thischanges when you eat or exercise. When you have eaten a large meal, a greater propor-tion goes to your digestive system to allow digestion and absorption and to transportnutrients. When you are exercising strenuously, about 85% of your blood will be di-rected to your skeletal muscles to deliver nutrients and oxygen and remove carbondioxide and waste products. This is why you get cramps if you exercise right after youeat a big meal. Your body cannot direct blood to the intestines and the muscles at thesame time. The muscles win, and food remains in your intestines, often causingcramps.

Water-soluble nutrients go to the liverIn the small intestine, water-soluble molecules, including amino acids, sugars, water-soluble vitamins, and the water-soluble products of fat digestion, cross the mucosalcells of the villi and enter capillaries. These capillaries merge to form venules at thebase of the villi. The venules then merge to form larger and larger veins, which even-tually form the hepatic portal vein. The hepatic portal vein transports blood directlyto the liver, where absorbed nutrients are processed before they enter the general circu-lation (Figure 3.14).

LUNGS

CapillariesArteries

HEAR

BODYCapillaries

Veins

T

F I G U R E 3 . 1 3

Blood is pumped from the heart through the arteries to the capillaries of the lungs, where it picks up oxygen. It then returns to the heart via the veins and is pumped out again into the arteries that lead to the restof the body. In the capillaries of the body, blood delivers oxygen andnutrients and picks up wastes before returning to the heart via the veins. In this figure, red indicates blood that is rich in oxygen, and bluerepresents blood that is oxygen-poor and carrying more carbon dioxide.

Did your mother ever tell you not togo swimming right after eatinglunch? The reason is that food inyour stomach is likely to causecramping when you exercise. Tomake sure your stomach is emptywhen you exercise, don’t eat a high-fat meal, which takes a long time toleave the stomach. But don’t gohungry either. The best choice ishigh-carbohydrate foods like pastaand pancakes, eaten about 2 hoursbefore you exercise.

Hepatic portal vein The vein thattransports blood from the gastrointestinaltract to the liver.

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The liver acts as a gatekeeper between substances absorbed from the intestine andthe rest of the body. Some nutrients are stored in the liver, some are changed into dif-ferent forms, and others are allowed to pass through unchanged. The liver decideswhether individual nutrients will be stored or delivered directly to the cells dependingon the needs of the body. The liver is also important for the synthesis and breakdownof amino acids, proteins, and fats. It modifies the products of protein breakdown toform molecules that can be safely transported to the kidney for excretion. The liveralso contains enzyme systems that protect the body from toxins that are absorbed bythe gastrointestinal tract.

Fat-soluble substances enter the lymphaticsystemThe lymphatic system consists of a network of tubules (lymph vessels), structures,and organs that contain infection-fighting cells of the immune system. The lymphaticsystem is an important part of the immune system. Fluid that has accumulated in tis-sues drains into the lymphatic system, where it is filtered past a collection of infection-fighting cells before it is returned to the bloodstream. If the fluid contains antigen itwill trigger an immune response. The infection-fighting cells and antibodies that areproduced by this response enter the bloodstream and help destroy the invading sub-stance. By draining the excess fluid away from the spaces between cells, the lymphaticsystem also prevents the accumulation of fluid from causing swelling.

In the intestine, the lymphatic system functions in the absorption and transportof fat-soluble substances such as cholesterol, long-chain fatty acids, and fat-solublevitamins. These pass from the intestinal mucosa into the lacteals, the smallest ofthe lymph vessels, which drain into larger lymph vessels. Lymph vessels from theintestine and most other organs of the body drain into the thoracic duct, whichempties into the bloodstream near the neck. Therefore, substances that are ab-sorbed via the lymphatic system do not pass through the liver before entering theblood circulation.

The Blood Delivers Absorbed Nutrients to Body Cells 77

Stomach

Hepatic portalvein

Large intestine

Small intestine

Liver

F I G U R E 3 . 1 4

Small water-soluble substances are delivered from the stomachand intestines to the liver via the hepatic portal vein.


Nutrients are transported into the body cellsFor nutrients to enter cells, they must first cross the cell membrane. The cell mem-brane allows some substances, such as water, to pass freely back and forth but blocksthe passage of others. By being selectively permeable, it regulates what can enter andleave cells. Once the nutrients have entered the cell, they can be broken down andused for energy, or they can be used to build the types of structural and regulatorymolecules that are needed by the body.

Mitochondria Cell membrane Nucleus

Lysosomes Ribosomes Cytoplasm Roughendoplasmicreticulum

Selectively permeable Describes amembrane or barrier that will allow somesubstances to pass freely but will restrictthe passage of others.

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Metabolic Reactions Synthesize and Break Down Molecules

Our bodies use nutrients to produce energy and to manufacture substances needed bythe body to grow and maintain itself. The sum of these chemical reactions is calledmetabolism. Each nutrient plays a unique role in metabolism. If a nutrient is missing,the production of important body structures, chemical compounds, or energy may beimpaired. The consequences may be as severe as a life-threatening deficiency disease oras mild as impaired athletic performance.

Many of the reactions of metabolism occur in series known as metabolic pathways.Molecules that enter these pathways are modified at each step of the pathway with thehelp of enzymes. Some of the pathways use energy to build body structures, whereasothers break large molecules into smaller ones, releasing energy. Reactions that synthe-size molecules occur in different cellular organelles than those that break down mole-cules for energy (Figure 3.15). For example, ribosomes are organelles that specialize inthe synthesis of proteins. The mitochondria are organelles responsible for breakingdown molecules to obtain energy.

Cellular respiration breaks down nutrients to produce energyIn the mitochondria, the glucose, fatty acids, and amino acids derived from carbohy-drates, fats, and proteins, respectively, are completely broken down in the presence ofoxygen to produce carbon dioxide, water, and energy. This process is called cellularrespiration. The energy released by cellular respiration is used to make a moleculecalled ATP (adenosine triphosphate). ATP can be thought of as the energy currency of

F I G U R E 3 . 1 5

Structure of a general animal cell. Almostall human cells contain the structuresillustrated here.

Metabolism The sum of all chemicalreactions that occur in living cells.�*

Organelles Cellular organs that carryout specific metabolic functions.�*

Mitochondria The cellular organellesresponsible for generating energy in theform of ATP for cellular activities.

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ATP (adenosine triphosphate) Thehigh-energy molecule used by the body toperform energy-requiring activities.

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Cellular respiration The reactionsthat break down carbohydrates, fats, andproteins in the presence of oxygen toproduce carbon dioxide, water, and energyin the form of ATP.

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the cell. The chemical bonds of ATP are very high in energy, and when they break, theenergy is released and can be used to power body processes, such as pumping blood orcontracting muscles—or it can be used to synthesize new molecules needed to main-tain and repair body tissues.

Carbohydrate, protein, and fat are used for energy or for synthesisThe meal consumed at the beginning of this chapter has now been delivered to thecells. The carbohydrate in the bread has been broken down into glucose. The cellscan use glucose to produce ATP or to synthesize other molecules for immediate useor storage. The protein in the turkey, milk, and bread has been broken down intoamino acids that can be used by cells to synthesize needed body proteins, make glu-cose if it is in short supply, or produce ATP. The fat in the milk, turkey, and mayon-naise has been broken down into fatty acids. These can be used to make ATP or toproduce lipids needed for body function, or they can be stored as body fat for lateruse (Figure 3.16).

Waste Products Must Be Eliminated from the Body 79

T H I N K I N G F O R Y O U R S E L F

1. Does your breakfast stick with you too long?a. Imagine you wake up on a Sunday morning and join

some friends for a large breakfast consisting of a cheeseomelet and sausage (foods high in fat and protein); a croissant with butter (which contains carbohydratebut is also very high in fat); and a small glass of orangejuice. After the meal, you remember that you haveplans to play basketball with a friend in just an hour.If you keep your plans and play basketball, whatproblems might you experience while exercising?

b. Had you remembered your plans for strenuous exercisebefore you had breakfast, what type of meal might you

have selected to ensure that your stomach would emptymore quickly?

2. How do the foods you eat affect digestion? To find out select one day from the food record you kept inChapter 2.a. Which of the foods you ate might slow stomach

emptying if consumed with a meal?b. Which of your choices would stimulate the release

of bile from the gallbladder?c. Which foods would begin their digestion in your

mouth?

Waste Products Must Be Eliminated from the Body

The waste products left over from digestion and metabolism must be removed fromthe body. Substances such as fiber that are not absorbed from the intestine are elimi-nated from the gastrointestinal tract in the feces. The waste products of metabolismare eliminated by the lungs, the skin, and the kidneys. Carbon dioxide, a waste prod-uct produced by cellular respiration, is transported from the cells where it is producedto the lungs by red blood cells. At the lungs, red blood cells release the carbon dioxide,which is then exhaled into the environment. A significant amount of water is also lostfrom the lungs, by evaporation.

Some water, minerals, and byproducts of protein breakdown are lost through theskin in perspiration or sweat. But the kidney is the primary site for the excretion ofwater and the nitrogen-containing byproducts of protein breakdown. These are ex-creted in the urine. The amounts of water and other substances excreted in the urineare regulated so that homeostasis is maintained (see Chapter 9).

GlucoseAmino acidsFatty acids

ENERGY

Muscle contraction, kidneyfunction, and other work

Some usedas fuel:

Some usedas raw

materials:

F I G U R E 3 . 1 6

Nutrients delivered to cells can be used eitherto produce energy or as raw materials for thesynthesis of carbohydrates, fats, and proteinsin the body.


S U M M A R Y

1. The organization of all life begins with the same basicstructure—the atom. Atoms are linked by chemicalbonds to form molecules. The cell is the smallest unitof life. Cells of similar structure and function areorganized into tissues, and tissues into organs andorgan systems.

2. The digestive system is the organ system primarilyresponsible for the movement of nutrients into thebody. The digestive system provides two majorfunctions: digestion and absorption. Digestion is theprocess by which food is broken down into units thatare small enough to be absorbed. Absorption is theprocess by which nutrients are transported into thebody.

3. The gastrointestinal tract consists of a hollow tubethat begins at the mouth and continues through thepharynx, esophagus, stomach, small intestine, largeintestine, and anus. The passage, digestion, andabsorption of food in the lumen of this tube are aidedby the secretion of mucus, enzymes, and hormones.

4. The processes involved in digestion begin in responseto the smell or sight of food and continue as foodenters the digestive tract at the mouth, where it isbroken into smaller pieces by the teeth and mixedwith saliva. From the mouth, food passes through thepharynx and into the esophagus. The rhythmiccontractions of peristalsis propel it down theesophagus to the stomach.

5. The stomach acts as a temporary storage site for food.The muscles of the stomach mix the food into asemiliquid mass called chyme, and gastric juicecontaining hydrochloric acid and pepsin beginsprotein digestion. Stomach emptying is regulated bynervous and hormonal signals from the small intestineand depends on the amount and composition of foodconsumed.

6. The small intestine is the primary site of nutrientdigestion and absorption. The absorptive surface ofthe small intestine is increased by folds and fingerlikeprojections called villi, which are covered with tinyprojections called microvilli.

7. In the small intestine, secretions from the intestineitself as well as from the gallbladder and pancreas aiddigestion and absorption. Bile from the gallbladderhelps make fat available to fat-digesting enzymes bybreaking it into smaller globules. It also facilitates fatabsorption. Pancreatic juice contains digestiveenzymes and bicarbonate to neutralize stomach acid.

8. Absorption across the intestinal mucosa occurs byseveral different processes. Simple diffusion, osmosis,and facilitated diffusion do not require energy but can

move substances only from an area of greaterconcentration to an area of lesser concentration. Activetransport requires energy but can transport substancesfrom an area of lower concentration to an area ofhigher concentration.

9. Components of chyme that are not absorbed in thesmall intestine pass on to the large intestine, wheresome water and nutrients are absorbed. The largeintestine is populated by bacteria that digest some ofthese unabsorbed materials, such as fiber, producingsmall amounts of nutrients and gas. The remainingunabsorbed materials are excreted in feces.

10. If any of the processes of digestion and absorption aredisrupted, health and nutritional status can beaffected. Dietary modification may be required tocompensate for differences in the digestive tract thatoccur at different stages of life.

11. Absorbed nutrients are delivered to the cells of thebody by the cardiovascular system. The heart pumpsblood to the lungs to pick up oxygen and eliminatecarbon dioxide. From the lungs, blood returns to theheart and is pumped to the rest of the body to deliveroxygen and nutrients and remove carbon dioxide andother wastes before returning to the heart. Blood ispumped away from the heart in arteries and returnedto the heart in veins. Exchange of nutrients and gasesoccurs at the smallest blood vessels, the capillaries.

12. The products of carbohydrate and protein digestionand the water-soluble products of fat digestion entercapillaries in the intestinal villi and are transported tothe liver, which serves as a processing center, removingthe absorbed substances for storage, converting theminto other forms, or allowing them to pass unaltered.The liver also protects the body from toxic substancesthat may have been absorbed.

13. The fat-soluble products of digestion enter the lymphsystem via lacteals in the intestinal villi. The nutrientsabsorbed via the lymphatic system enter the bloodcirculation without first passing through the liver.

14. When nutrients reach the cells, they must betransported across cell membranes. Within the cells,some nutrients are broken down to produce energyand others are used to synthesize molecules needed bythe body. The sum of all the chemical reactions of thebody is called metabolism. The reactions thatcompletely break down macronutrients in the presenceof oxygen to produce water, carbon dioxide, andenergy are referred to as cellular respiration.

15. Unabsorbed materials are excreted in the feces. Thewaste products of metabolism are excreted by thelungs, skin, and kidneys.

R E V I E W Q U E S T I O N S

1. What is the smallest unit of plant and animal life?2. Name three organ systems involved in the digestion

and absorption of food.3. How do teeth function in digestion?4. What is the role of saliva in the gastrointestinal tract?5. What is peristalsis?6. List two functions of the stomach.7. What is the role of stomach acid in digestion?8. Where does most digestion and absorption occur?9. How does the structure of the small intestine aid

absorption?

10. What is the cause of most ulcers?11. What products of digestion are transported by the

lymphatic system?12. What path does an amino acid follow from absorption

to delivery to the cell?13. What path does a large fatty acid follow from

absorption to delivery to the cell?14. What is adenosine triphosphate (ATP)?15. List four ways that waste products are eliminated

from the body.

R E F E R E N C E S

1. McManus, T. J. Helicobacter pylori: An emerging infectiousdisease. Nurse Practitioner. 25:42–46, 2000.

2. Rhoades, R., and Pflanzer, R. Human Physiology , 3rd ed.Philadelphia: Saunders College Publishing, 1996.

References 81

the digestive systemmyresource.phoenix.edu/secure/resource/sci241r4/sci241...the digestive system: 3...

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