Digestion, Absorption, and Metabolism

Definitions• Digestion:

– Mechanical and chemical process in which food is broken down in the GI tract, releasing many nutrients in forms the body can use

• Absorption:

– Process in which released nutrients are taken into the cells lining the GI tract

• Metabolism: – The sum of the body processes (catabolic and

anabolic) involved in converting food necessary for energy, tissue building, and metabolic controls

Digestion: Basic Principles• Principle of change

– The body cannot use food as it is eaten.

– Food must be changed into simpler substances to be absorbed and then used by cells to sustain life.

• Principle of wholeness

– The parts of the digestive process comprise a continuous whole.

– Food components travel through the gastrointestinal (GI) system until they are delivered to cells or excreted.

The Gastrointestinal System• Tube that extends from the mouth to the anus in which the

movement of muscles and release of hormones and enzymes digest food (also called the alimentary canal or gastrointestinal tract)

• The major parts of the gastrointestinal system:– Mouth and Salivary glands– Pharynx– Esophagus– Stomach– Small Intestine– Large Intestine– Rectum– Accessory organs: liver, gallbladder, pancreas

Functions of the GI System• Motility:

–Movement through the GI tract

• Digestion: –Mechanical and Chemical

• Secretion and Absorption: –Into the GI tract (secretion) or into blood

(absorption)

• Storage • Elimination

Digestion: Mechanical and Chemical Changes

• Through a balanced system of mechanical and chemical digestion, food is broken down into smaller substances and the nutrients are released for biologic use.

• Mechanical and chemical actions make up the digestive process.

• Food must undergo these changes to be delivered to cells.

Mechanical Digestion• Breaking down of food and moving it along the

GI tract

• Mastication:

– Process of chewing and biting food in the mouth

– First step in digestion

• GI motility:

– Beginning in the mouth, muscles and nerves in the GI tract coordinate their actions to provide motility, an automatic response to the presence of food.

Mechanical Digestion• Layers of the GI tract provide two types of

movement: tonic contraction and peristalsis

• Tonic contraction:– Ensures continuous passage of the food

mass and valve control along the way

• Peristalsis:– Periodic muscle contraction and relaxation

which are rhythmic waves that mix the food mass and move it forward

Mechanical Digestion• Nerves

– The intramural nerve plexus is the network of nerves in the GI wall extending from the esophagus to the anus

– These nerves do three things:

• Control muscle tone in the wall

• Regulate the rate and intensity of alternating muscle contractions.

• Coordinate all the various movements

• Problems with the nerves in the digestive tract can cause problems with the digestive process.

Chemical Digestion• Substances that break down food:

– Digestive Enzymes: specific proteins that break down nutrients

– Hydrochloric Acid (HCL): acid made by the stomach that breaks down proteins in the foods we eat and lowers pH and total alkalinity

• HCL activates digestive enzymes that break down food into small particles for absorption– Buffer Ions: produces correct pH for

enzyme activity

Chemical Digestion• Substances that break down food:

– Mucus: A clear liquid made by the intestines. Lubricates and protects the GI tract tissues and helps mix the food mass

– Water and Electrolytes: Carry and circulate the products of digestion through the tract and into the tissues. Major electrolytes in the body include sodium, potassium, magnesium, calcium, chloride, and phosphate.

– Bile: A digestive fluid made by the liver and stored in the gallbladder which divides fat into smaller pieces to assist fat enzymes

Digestion of a Meal• You consumed the following for lunch at

McDonald’s: – Big Mac (542 calories, 279 fat calories and 52%

fat)– Small French Fries (211calories, 45 fat calories

and 47% fat calories)– Small Chocolate Milk Shake (380 calories, 81 fat

calories and 21% fat calories )– Cherry pie (298 calories, 162 fat calories and 54%

fat)• That is a whopping 1,431 calories, 567 fat calories

and 40% fat. – That is an artery clogging meal. I hope you remain

awake to learn how it digests!

Digestion in the Mouth and Esophagus• Mechanical Digestion

– Mastication (biting and chewing) breaks down food– Tongue pushes bolus (food mass) to pharynx

(throat)– Food is swallowed and passes down esophagus

(long hollow tube from pharynx to stomach) by peristaltic waves controlled by nerves

– Gastroesophageal sphincter at stomach entrance relaxes, allowing food to enter, then constricts to retain food.

• If the gastroesophageal sphincter is not working properly, acid and mixed food seep back into esophagus, resulting in heartburn

Digestion in the Mouth and Esophagus• Chemical Digestion

– Saliva: mixture of water, protein, and salts Saliva: mixture of water, protein, and salts that makes food easy to swallow that makes food easy to swallow

– Salivary glands (parotid gland, sublingual gland, and submandibular gland) secrete material containing salivary amylase or ptyalin (enzyme that breaks down carbohydrates)

– Ebner’s glands at the back of the tongue secrete a lingual lipase (enzyme that breaks down fat)

Digestion in the Mouth and Esophagus

• Chemical digestion–Salivary glands also secrete a mucous

material to lubricate and bind food particles, facilitating the swallowing of the food bolus.

–Secretions from the mucous glands in the esophagus help move food toward the stomach.

The Stomach• Regions of the stomach:

– Fundus: upper portion of the stomach

– Pyloric region (pyloric sphincter): opening or valve

– Duodenum: first part of the small intestine

• Functions:

– Store food

– Initiate digestion of proteins

– Kill bacteria with the strong acidity (low pH of the gastric juice)

– Make chyme

The Stomach

Stomach Digestion• Mechanical Digestion

– Under sphincter control, the food enters the upper portion of the stomach as individual bolus lumps

– Stomach muscles knead, store, mix, and propel the food mass forward

– By the time the food mass reaches the lower portion of the stomach, it is a semiliquid acid/food mix called chyme

– Chyme is released slowly into the first section of the small intestine (duodenum) by the pyloric valve

Stomach Digestion• Chemical Digestion: three types of gastric

secretions

– Hydrochloric Acid: Parietal cells in the stomach lining secrete acid to promote gastric enzyme activity

– Mucus: Secretions protect the stomach lining from the erosive effect of the acid and also bind and mix the food mass and help move it along.

– Enzymes: Pepsinogen is secreted by stomach cells and activated by acid to become pepsin, a protein-splitting enzyme

Stomach Digestion• Various sensations and emotions

stimulate the nerve impulses that trigger these secretions

• Anger and stress can increase secretions

• Sadness and fear can decrease secretions and diminish blood flow

• Do you feel more or less hungry when you are:

–Stressed, sad, or angry,

Stomach Digestion• High-calorie foods—especially fatty foods

—empty out of the stomach at a slower rate; thus the feeling of fullness lasts longer.

• Caloric density influences the rate in which the stomach empties

• Problems with valves and sphincters can cause reflux and discomfort

Small Intestine• About 20 feet in length• Small in diameter compared to large

intestine• Fills most of the abdomen• Functions:

–Peristalsis–Protection against infection–Hormone secretion (secretin and

cholecystokinin)–Majority of food digestion

Small Intestine• Three regions of the small intestine:

– Duodenum: upper region-connecting stomach to ileum• Absorbs carbohydrates, lipids, amino acids,

Ca2+, iron– Jejunum: middle and largest region

• Absorbs carbohydrates, lipids, amino acids, Ca2+, iron

– Ileum: lower region

• Absorbs bile salts, vitamin B12, water and electrolytes

 

Small Intestine

Small Intestine Digestion• Mechanical Digestion

• Under nerve control, intestinal muscles produce several types of movements as follows:

– Peristaltic waves slowly push food mass forward

– Pendular movements sweep back and forth

– Segmentation rings chop food mass into successive soft lumps and mix them with secretions

– Longitudinal rotation rolls food in a spiral motion, exposing new surfaces for absorption

Small Intestine Digestion• The small intestine together with accessory

organs (pancreas, liver, and gallbladder) secrete enzymes

• Pancreatic enzymes– Carbohydrate: Pancreatic amylase converts

starch to maltose and sucrose– Protein: Trypsin and chymotrypsin split large

protein molecules into small peptide fragments and eventually into single amino acids

– Fat: Pancreatic lipase converts fat to glycerides and fatty acids

Small Intestine Digestion• Intestinal enzymes

– Carbohydrate: Disaccharides (maltose, lactose, and sucrose) converted into monosaccharides (glucose, galactose, and fructose)

– Protein: Enterokinase activates trypsinogen from the pancreas to split the following:• polypeptides dipeptides amino acids

– Fat: Intestinal lipase splits fat into glycerides and fatty acids

Small Intestine Digestion• Mucus is important because the contents

entering from the stomach are highly acidic and it protects the mucosal lining from irritation and erosion

• Bile: emulsifying agent produced by the liver and stored in the gallbladder which breaks fat into smaller pieces so it can be broken down easier

• Hormone secretin keeps the environment alkaline at a pH of 8 so pancreatic enzymes can work.

• Hormone cholecystokinin triggers release of bile from the gallbladder

What Happened to the McDonald’s Meal?

• Carbohydrates in bun, fries, and cherry pie reduced to simple sugars (glucose, fructose, galactose)

• Proteins in hamburger patty and milk shake become amino acids

• Lipids in burger, fries, milk shake and cherry pie become fatty acids and glycerides

• Lettuce is reduced to cellulose or fiber• Any vitamins and minerals: liberated from food• Excess calories are stored as fat in adipose tissue

 

Bioavailability

• Bioavailability refers to:– Amount of nutrient present in the GI tract

– Competition between nutrients for absorption

– Form in which the nutrient is present

• All nutrients present in a food are not absorbed because of differing bioavailability.– This is considered when determining dietary

intake standards

Absorption in the Small Intestine• The inner surface of the small intestine is 600

times larger than its outer area, facilitating the delivery of nutrients into circulation

• Three absorbing structures– Mucosal Folds: surface of small intestine piles into

folds– Villi: small, finger-like projections cover the mucosal

folds, increasing the area of exposed intestinal surface

– Microvilli: smaller projections cover each villi (look like bristles on a brush)

Absorption in the Small Intestine

• Each villus can receive protein and carbohydrate material as well as receive fat materials in its lymph vessels

• These lymph vessels are called lacteal

• If the entire surface of the small intestine were spread out, it would be half the size of a basketball court

Large Intestine• 4 to 5 feet in length

• Populated by bacteria which synthesize Vitamin K

• Consists of the cecum and colon

• Primary function is to reabsorb water from the stool and eliminate the remaining relatively dry waste

• Food and drink consumption and intestinal secretions result in approximately 5 gallons

(19 L) of fluid being placed in the large intestine daily

Large Intestine• Water is taken up by the large intestine

– Most water in chyme is absorbed in the first half of the colon

– Only about 100 mL of water is left in the large intestine after the digestion process to form and eliminate feces

• Dietary fiber is not digested– Contributes bulk to food mass and gas– What foods high in dietary fiber is associated

with gas?– Helps form feces

Transport• The vascular system (blood circulation) is

responsible for supplying the body with nutrients, oxygen, and other vital substances

– Veins and arteries

– Transports waste, such as carbon dioxide and nitrogen, to lungs and kidneys for removal

• Water-soluble products of digestion can be absorbed into the vascular system.

• Nutrients travel to the liver by the portal circulation, then to cells throughout the body

Transport• Lymphatic system

–Route for fatty materials, which are not water soluble

–Fat molecules pass into lymph vessels in villi

Metabolism

Energy Storage: Glycogenesis

• Anabolic process of converting extra glucose into glycogen

• Glycogen is stored in the liver and muscles for quick energy to be used at a later time

Energy Storage: Lipogenesis• When glycogen reserves are full,

additional excess energy from carbohydrates, fat, or protein is stored as fat in adipose tissue

• Lipogenesis: The building up of triglycerides for storage in adipose tissue

Energy Storage: Gluconeogenesis

• Conversion of amino acids to glucose

• Excess protein is not stored as muscle but is further broken down–Nitrogen unit is removed

–Remaining carbon chain can be converted to glucose (if needed) or to fat for storage

Metabolic Pathways of Excess Energy

Alcohol• Alcohol is a food-7 kcal/gram• Cannot be stored in the body-toxic• Absorbed without chemical digestion• Alcohol metabolism is first because a byproduct of

alcohol is acetaldehyde-which destroys healthy tissue

• Liver detoxifies alcohol and uses remaining to produce fatty acids-triglycerides stored in liver

• Single drinking binge can accumulate fat in the liver• Can lead to liver disease• Blood alcohol levels peak after one drink in 30-45

minutes 12oz of beer, 5 oz of wine, 1.5 oz of 80 proof alcohol

• www.health.org

Discussion Questions

1. What is the difference between mechanical and chemical digestion?

2. What functions do the following perform:– Stomach– Small Intestine– Pancreas– Gallbladder– Liver– Large Intestine