Digestive System

Digestion includes six basic processes: 1 ingestion 5. absorption2. secretion 6. defecation3. mixing and propulsion, 4. mechanical and chemical digestion,

Mechanical digestion consists of movements of the gastrointestinal tract that aid chemical digestion.

_ Chemical digestion is a series of catabolic (hydrolysis) reactions that break down larger carbohydrate, lipid, and protein food molecules into smaller molecules that are usable by the body cells.

Figure 14.1Slide 14.1Copyright © 2001 Benjamin Cummings, an imprint of Addison Wesley Longman, Inc.

Digestive System

Digestive System

_ The breaking down of larger food molecules into smaller molecules is called digestion.

_ The passage of these smaller molecules into blood and lymph is termed absorption.

_ The organs that collectively perform digestion and absorption compose the digestive system and are usually divided into two main groups: those composing the gastrointestinal (GI) tract and accessory structures.

_ The accessory structures include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

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Structure of the Wall

The GI tract is composed of four layers or tunics: 1. Mucosa - The mucosa, forming the inner lining of the tract, is supplied

with a rich network of blood vessels, nerve fibers and endocrine cells.

2. Submucosa Connective tissue that lies just below the mucosa. It contains blood vessels and nerves

3. Muscularis Contains two layers of muscle tissue. Important for peristalsis, mixing of food with digestive juices, and the mechanical \ breakdown of food

4. Serosa - Outermost covering, composed of visceral peritoneum.

It is a continuous tube extending from the mouth to the anus. The loops

of the digestive tract are anchored to the posterior wall of the abdominal

cavity by a large double fold of peritoneal tissue called the mesentary.

Figure 14.1Slide 14.1Copyright © 2001 Benjamin Cummings, an imprint of Addison Wesley Longman, Inc.

Digestive System

Digestive OrgansMouth/Oral Cavity

PharynxEsophagusStomach

Small intestine Duodenum

Jejunum Ileum

Large intestineRectumAnus

The Oral Cavity

Roof of the mouth Hard Palate- anterior roof of the mouth Soft Palate- posterior roof of the mouth

Sides of the mouth Cheeks- lateral walls of the oral cavity

Floor of the mouth Tongue/ functions

SwallowingChewingSpeechTaste

Bitter, sour, sweet and salt

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Buccal Cavity

lips

gullet – throat

Uvula

Hard Palate

Soft Palate

Tongue

Palatine Tonsils

The Oral Cavity_ Teeth

l Functions• Divide the food into smaller pieces

– Mastication- chewing• Two sets of teeth

– Deciduous teeth- baby or milk teeth» Total = 20» Start to erupt 6 months of age and are

complete by 2 years» They loosen and fall out between ages 6 and

12.

The Oral Cavity

_ Teeth - continuedl Permanent teeth

• Total = 32• All permanent teeth but the third molars

have erupted by the end of adolescence• Third molars emerge between 17 and 25• They often fail to erupt and are said to be

impacted

The Oral Cavity

_ Types of teethl Incisors- bitingl Canine- tearingl Premolars (bicuspids)- grindingl Molars- grinding

Figur 14.4ASlide 14.6ACopyright © 2001 Benjamin Cummings, an imprint of Addison Wesley Longman, Inc.

The Mouth: Teeth

Wisdom Teeth

Molars

_ Crown- upper portion of the tooth that is visible. It is made up of calcium phosphate, the biologically manufactured substance

_ Neck- boundary between the root and crown

_ Root- portion of the tooth that is embedded in the jawbone

Tooth Terms_ Enamel- covers the crown, very

hard_ Dentin- bulk of the tooth, very

similar to bone but does not have cells

_ Gingiva- gums_ Pulp cavity- contains

connective tissue, blood vessels and nerve fibers. Supplies nutrients to tooth tissues and provides tooth sensations

_ Root canal- where pulp cavity extends into the root. Provides route for blood vessels and nerves

Pharynx

Pharynx- 3 Divisions

_ Nasopharynx- behind the nose and palate

_ Oropharynx- behind the mouth to the soft palate, above the hyoid bone

_ Laryngopharynx- hyoid bone to the esophagus

Pharynx

_ Uvula- a small muscular flap of tissue that hand from the soft palate and prevent food from entering the nasal cavity during swallowing by closing off the posterior nares

_ Tonsils- part of the lymphatic systeml Palatine tonsils- at the posterior end of the oral

cavity are masses of lymphoid tissuel Lingual tonsils- covers the base of the tongue

Pharynx and Tonsils

Salivary GlandsSaliva- secreted in response to sight, smell or thought

of food, or the presence of food in the mouth

There are three pairs of salivary glands: 1. parotid Largest gland, lies anterior to each ear

2. submandibular empty their secretions into the floor of the mouth

3. sublingual glands Smallest gland, located on the floor of the mouth inferior to the tongue. Empty their secretions

into the floor of the mouth

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Salivary Glands

Salivary Glandsi

Saliva

Saliva contents Water Salivary amylase- alkaline secretion, digests starch Amount per day- 1000 to 2000 mL

Function of saliva Prepares food for swallowing- mucus Prevents mouth from drying out Starts the breakdown of starch

Swallowing

Deglutition

Bolus

Involuntary nervous response

Epiglottis

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the act of swallowing

the ball of food

Starts as a voluntary action but shortly becomesan involuntary action

the covering of the opening to the windpipe

animation 1

Esophagus_ Location: pharynx to the stomach_ Length: 10 inches_ Histology: stratified squamous epithelium

l Mucous glands- secrete mucous

_ Muscles: Upper 1/3 = skeletal muscle_ Lower 1/3 = smooth muscle_ Middle 1/3 = mixture of both_ Function: carries food from the mouth to the

stomach_ Peristalsis: involuntary muscular contractions that

move food along the esophagus

Figure 14.3ASlide 14.4Copyright © 2001 Benjamin Cummings, an imprint of Addison Wesley Longman, Inc.

Motility: Peristalsis

Stomach

Stomach Location: left side and high up in the abdomen. It is “J” shaped Functions:

Temporary storage of ingested food Mechanical breakdown of ingested food Breakdown of chemical bonds in food items by acids and

enzymes Production of intrinsic factor needed for vitamin B12 absorption

Can store 4 liters of material Food remains in stomach for 2-6 hours Four divisions of the stomach

Cardia- connects the esophagus to the stomach Fundus- bulge of the stomach superior to the cardia Body- large area between the fundus and the curve of the “J” Pylorus- connects the stomach to the small intestine

Man Eating

Stomach

Figure 14.7Slide 14.9ACopyright © 2001 Benjamin Cummings, an imprint of Addison Wesley Longman, Inc.

Structure of the Stomach Wall

Stomach Sphincter Muscles

_ Cardiac sphincter- from the esophagus

_ Pyloric sphincter- connects to the small intestine

Stomach Rugae

Rugae

_ Rugael Folds of the stomach,

they smooth out and disappear as the stomach becomes distended from food

Muscles of the Stomach

_ Longitudinal muscles

_ Circular muscles

_ Inner oblique muscles

_ These muscles add strength and assist in the mixing and churning of food

Stomach Muscles

Physiology of the Digestive System

_ Stomachl Mechanical digestion - churning or turning the food and

mixes it until it becomes chyme (semi-fluid mixture of food)

l Histology- simple columnar epithelium with mucous cells

Gastric SecretionsThree types of gastric glands

1. Mucus cells

2. Chief cells

Secretion - mucus- watery substance

Function - Protects stomach lining

Secretion - Pepsinogen (inactive form of pepsin); made active in the presence of HCl

Function - begins digestion of proteins into polypeptides

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McGraw-Hill

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Gastric Secretions - cont.

3. Parietal cells

Secretion - HCl & intrinsic factor (pH .9-1.5)

Function - Kills bacteria, absorption of vitamin B12,

softens meat & bones, activates pepsinogen, HCl, pepsin, proteins and shorter chain amino acids

4. Intrinsic factorVitamin B12

Function- essential for the normal development of

red blood cells

Malfunction of intrinsic factor = pernicious anemia

Hormones that control Gastric Secretions

_ Hormone: Gastrin_ Location of hormone: pyloric region of stomach_ When released: when solid food enters the

stomach_ Where does it go? To the blood stream back to

the stomach_ What does it do? Stimulates the glands to

produce gastric juice

Regulation of Gastric SecretionsThree Phases

1. Cephalic phase- parasympathetic reflex (30-50%)

2. Gastric phase (40-50%)

3. Intestinal phase (5%)

Anticipate food, medulla oblongata, taste, smell, sight And thought of food

Food in the stomach, stimulates more gastrin whichin turn stimulates more gastric juices

Chyme entering the small intestine triggers this phase. This inhibits gastric juices and more gastrin is secreted

Stomach Absorption- absorbs only water,certain salts and lipid soluble drugs and also alcohol.

Small Intestine_ Diameter: 4 cm (1.6 in) to 2.5 cm (1 in.)_ Length: 20 feet_ Functions:

l Complete the digestion of foodl Absorb the end products of digestion into the blood and

lymphl Secrete hormones that help control the production of

pancreatic juice, bile and intestinal juicel Control the amount of fluid and electrolytes lost from

the body

Small Intestine

3 Division of the Small Intestine

_ Duodenum: (latin for 12)l Length: 10 inchesl Function: receives chyme from the stomach

and digestive secretions from the pancreas and liver

_ Jejunum: l Length: 8 feetl Function: bulk of chemical digestion and

nutrient absorption

Duodenum

Gallstones

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Duodenum valve

Bile

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Jejunum

Jejunum

3 Division of the Small Intestine

_ Ileuml Length: 12 feetl Function: longest section, ends at the ileocecal

valve (a sphincter valve) which controls the flow of material from the ileum into the cecum, the first part of the large intestine.

Ileum

Villi and the Small Intestine

_ Three parts of the villi:l Arteriolel Venulel Lymph vessel (lacteal)- absorbs fatty acids

_ Villi area coveredl Larger than the size of a tennis court

_ Functionl Increase surface area for absorption (600 times)

Villi and the Small Intestine

_ Goblet cellsl Mucous secreting cells

_ Digestive enzymes in the microvillil Peptidase- proteinsl Maltase, sucrase, lactase- disaccharidesl Lipase- lipids

Figure 14.9Slide 14.10ACopyright © 2001 Benjamin Cummings, an imprint of Addison Wesley Longman, Inc.

The Wall of the Small Intestine

VilliMicrovilli

Hormones secreted by Intestinal Cells

_ 1. Secretinl Stimulates the pancreas to secrete a fluid high

in bicarbonate ions that help neutralize chyme

_ CCK (cholecystokinin)l Stimulates the release of bile from the

gallbladder and the secretion of digestive enzymes from the pancreas

3 solutions in the small intestine

_ The following solution mix with chyme from the stomach in the small intestine:

l Bile from the liverl Pancreatic juice from the pancreasl Intestinal juice from glands in the wall of the

intestines

Digestion in the Small Intestine

_ Bilel Produced in the liverl Stored in the gallbladderl Contains: water, ions, cholesterol, sodium bicarbonate

ions (NO ENZYMES)• These act as emulsifying agents- which lower the surface

tension of fats, forming small droplets

l Green color comes from broken down hemoglobinl Bilirubin- main end product produced by the

decomposition of hemoglobin

Digestion in the Small Intestine

_ Bile- cont.l Travels to the small intestine by the bile ductl Large amounts of bile are released from the

gallbladder in the presence of fatty acidsl The hormone that causes the gallbladder to

contract and eject bile is CCK (cholecystokinin)l The stimulus for the release of CCK is fatty

acids and amino acids in the duodenum

Digestion in the Small Intestine

_ Bile- cont.l If the gallbladder is removed, there is no

storage of bile so it is constantly secreted into the small intestine by the liver

l Gallstones are hard deposits of cholesterol and or calcium

GallbladderThe gallbladder is a sac located on the posterior surface of the

liver.

The gallbladder stores and concentrates bile.

Cholecystokinin (CCK) stimulates the release of bile into the common bile duct.

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Gallstones

Pancreas

Pancreas

Pancreas Location: left side behind the stomach Two types of glands:

Exocrine gland- secretes materials onto the surface or into a cavity

Digestive enzymes (do most of the work in the SI)Amylase- starch to maltoseTrypsin- proteinsLipase- fatNucleases- nucleic acids

Sodium bicarbonate ionsNeutralizes the chymepH of the contents of the small intestine are slightly

alkaline- 7.5-8

Pancreas_ Endocrine gland- secretes materials into the blood

l Regulates blood sugar levels• Insulin• Glucagon

l Hormone secreting cells of the pancreas• Beta cells- insulin• Alpha cells- glucagon• Delta cells- somastatin- inhibits the release

of both insulin and glucagon

Pancreas_ Pancreatic secretion control

l Sightl Soundl Food in the mouthl Hormones

• Secretin• Secreted from the intestinal mucosa• When the intestinal content is acidic, secretin aacts

as a chemical messenger causing the pancreas to secrete enzymes in response. As a result, the alkalinity of the pancreatic juices neutralizes the acid content in the SI

Liver_ Location: right hypochondriac and epigastric abdominopelvic

regions_ Materials carried to the liver from the SI for storage

l Vitaminsl Mineralsl Nutrients

_ Functionsl Secretes bilel Synthesizes bile saltsl Sythesizes plasma proteinsl Storage of glucose, iron, vitaminsl Detoxification of toxic compoundsl Carbohydrate, lipid and proteins metabolisml Filters damaged RBC’s, bacteria and other materials from the

blood

Liver i

Liver

_ Liver secretions for digestionl Bile- stored in the gallbladder

• Water, bicarbonate ions, bilirubin, cholesterol, lipids

• Collectively known as bile salts

l Emulsification• Creates tiny droplets of fat (which coats the lipid to

increase surface area for the lipid enzyme to attack

Large Intestine_ AKA: Colon

_ Length: 5 feet

_ 3 divisions:l Cecum- vermiform appendix- vestigal organl Colon

• Ascending colon

• Transverse colon

• Descending colon

– Sigmoid colon- S-shaped curvel Rectum- thick muscular layerl Anal canal

• Internal sphincter valve- smooth muscle

• External sphincter valve- skeletal muscle (voluntary)

Figure 14.12Slide 14.13A

Large Intestine

Large Intestine

_ Functions:l Large quantities of water are reabsorbedl Storage of wastel NO digestion of food material

_ Solid waste of the the large intestine: feces_ Most of the material that enters the large intestine

is undigested fiber_ Millions of bacteria inhabit the large intestine and

are carried out of the body with the feces

Large Intestine_ Bacteria feed on undigested material in the

large intestine and release gas as a waste product

_ They also synthesize vitamin K and and certain B vitamins which are absorbed into the bloodstream

_ The reddish brown color of feces comes from bilirubin from a breakdown of hemoglobin