Digestive System

I. Digestion : breaking down of food materials for use by body cells

A.Overview of the digestive process

1. Ingestion:

2. Secretion:

3. Mixing & propulsion:

4. Digestion:

a. mechanical digestion

i. chewing

ii. churning

b. chemical digestion

5. Absorption:

6. Defecation:

B. Layers of the GI Tract

four layers

1. mucosa: innermost layer

composed of three layers

a. epithelium

b. lamina propria

c. muscularis mucosae

2. Submucosa

enteric nervous system

3. muscularis

two layers- inner circular fibers & outer longitudinal fibers

sphincters

4. serosa: outermost layer

visceral peritoneum

II. GI Tract: Overview

Long tube, approximately 30 feet in length

Sphincters control movement from one area to the next

Cardiac

Pyloric

Ileocecal

Anal

Accessory structures: do not contain ingested food but are necessary for digestive processes

Examples: teeth, liver

A. Mouth & Associated Structures

tastebuds : gustatory receptors

teeth

dentin

four types of teeth

1. incisors

2. canines

3. premolars & molars

baby/milk/deciduous teeth:

permanent teeth:

tongue

papillae

i. filiform papillae

ii. fungiform papillae

salivary glands: secrete saliva

a. three pairs open into oral cavity

i. parotid glands

ii. sublingual glands

iii. submandibular glands

b. composition of saliva (pH 6.35-6.85)

i. 99.5% water

ii. 0.5% solutes:

a. salts

b. mucin

c. lysozyme

d. salivary amylase

f. lingual lipase:

c. functions of saliva

physiology of digestion in the mouth

a. mechanical digestion

mastication

bolus

b. chemical digestion: breakdown of starches

B. pharynx

C. esophagus

peristalsis

heartburn

D. Stomach

rugae

has 3 layers of muscle (not just 2)

mucosa is composed of simple columnar epithelial cells

a. cells extend down into the lamina propria to form gastric pits

b. three kinds of exocrine gland cells in the pits

mucous neck cells

chief (zymogenic) cells - pepsinogen

parietal cells

physiology of digestion and absorption in the stomach

a. mechanical digestion

chyme

b. chemical digestion

hydrochloric acid

pepsin

regulation of gastric secretion - three phases

a. cephalic phase

b. gastric phase

c. intestinal phase

E. accessory structures emptying into small intestine; three organs (pancreas, liver, gall bladder) important in chemical digestion

1. pancreas

pancreatic juice: water, salts, sodium bicarbonate, several enzymes (trypsin: protein digestion, ones used in nucleic acid digestion)

2. liver

hepatocytes

bile -

contains water, bile salts, bile acids, cholesterol, bile pigments, several ions, and lecithin (a phospholipid)

bilirubin

stercobilin

functions of the liver

i. maintenance of blood glucose levels

ii. fat storage and steroid synthesis

iii. deamination of amino acids

iv. removal of drugs and hormones

v. storage of vitamins and glycogen

vi. removal of damaged RBCs

3. gall bladder: storage of bile

hepatic duct

cystic duct

common bile duct

F. small intestine

Important site of absorption

i. composed of 3 sections

a. duodenum

b. jejunum

c. ileum

ii. circular folds

iii. structure of mucosa :

a. villi

b. microvilli

c. glands

physiology of digestion

a. mechanical digestion - 2 types

i. segmentations

ii. migrating motility complexes ( MMCs):

b. chemical digestion:

i. carbohydrate digestion

ii. protein digestion

iii. lipid digestion

iv. nucleic acid digestion:

physiology of absorption

a. carbohydrate absorption

b. protein absorption

c. lipid absorption

micelles

G. large intestine

1. anatomy

a. ileocecal valve

b. appendix

c. colon

i. ascending

ii. transverse

iii. descending

iv. sigmoid

d. rectum

e. anus

2. mucosa layer differs from small intestine

3. muscularis

haustra

4. physiology of digestion

a. mechanical digestion

haustral churning

mass peristalsis

b. chemical digestion

performed by bacteria

B vitamins and vitamin K

5. water absorption and feces formation

6. defecation