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Bio 169 Digestive System Study Guide
Spring Semester 2004

Recommended Reading: Martini, Chapter 24

Overall view of digestion: digestive system = a tube + accessory structures

Gastrointestinal Tract (also called: GI tract, "alimentary canal", "the gut")

a tube from mouth to anus with muscular rings between organs (sphincters)

Ingestion

Movement of food (by skeletal and smooth muscle)

peristalsis -- wave-like contraction

Digestion

mechanical (breaking into small particles to create large surface area)

chemical (hydrolysis of large molecules to small ones by enzymes)

--> chyme

Absorption

transport of food molecules, ions, and water from outside of body (lumen) across epithelial cell membranes to inside of body

Defecation

Accessory Structures

teeth

tongue

salivary glands

liver

gall bladder

pancreas

 

Histology -- Standard Plan of the Gut Tube (4 layers)

mucosa

epithelium

stratified squamous in mouth, esophagus, and anus

why? protection from friction

simple columnar everywhere else

secretion of enzymes, mucus, fluids (into lumen)

secretion of gut hormones (into blood)

absorption of metabolites, ions, water (active and passive membrane transport)

lamina propria

muscularis mucosa

submucosa

areolar connective tissue

binds mucosa to muscularis

rich blood supply

contains an intrinsic nervous system

Meissner’s plexus (sensory; controls secretion)

muscularis

upper esophagus, external anal sphincter are skeletal muscle

elsewhere is smooth muscle

contains an intrinsic nervous system:

myenteric plexus (controls motility)

serosa - simple squamous epithelium

visceral peritoneum (covers organs)

parietal peritoneum (lines peritoneal cavity)

peritoneal cavity

ascites

retroperitoneal space

mesentery

suspends organs

contains blood vessels, nerves, lymphatics

falciform ligament

suspends liver

greater omentum

fatty apron over viscera

Oral Cavity - digestion begins here

Teeth

root, neck, crown

pulp cavity (blood vessels, nerves)

apical foramen

enamel on crown - hardest material in the body

Salivary Glands

three pairs - parotid, submandibular, sublingual - secrete 2 kinds of saliva

secretes watery fluid (serous) and/or viscous fluid (mucous)

watery fluid is bicarbonate-rich solution with enzymes

enzymes: amylase (digests starch), lysosyme (digests bacterial cell wall)

viscous fluid is mucus-containing

 

functions:

mucus lubricates food mass to help swallowing

suppress bacterial growth (lysosyme)

initiates digestion of starch (amylase)

cleans mouth and teeth

secretion stimulated by parasympathetic, inhibited by sympathetics

mastication (mechanical digestion)

tongue (also: source of lingual lipase), teeth and tongue form a bolus (=ball) of food

Esophagus - "swallowing tube"

crosses diaphragm through diaphragmatic hiatus (what is "hiatal hernia"?)

epithelium - stratified squamous

mucus glands in submucosa

muscularis

skeletal muscle (top 1/3)

smooth m (lower 2/3)

two sphincters:

upper esophageal sphincter (=pharyngeal muscles)

lower esophageal sphincter

deglutition (3 phases: voluntary (or buccal) --> pharyngeal --> esophageal)

Stomach

cardia

fundus

body

greater curvature

lesser curvature

pylorus

rugae

epithelium - gastric pits leading to branched glands

chief cells - secrete enzymes - exocrine

goblet cells - secrete mucus - exocrine

parietal cells secrete HCl and intrinsic factor (for B-12 uptake) - exocrine

G cells - secrete gastrin - endocrine secretion

muscularis is three layers (longitudinal, circular, oblique)

powerful mixing waves provide mechanical digestion (twisting movement)

sphincters: cardiac (also called, lower esophageal) and pyloric

serosa (continuous with visceral peritoneum)

enzymes:

pepsinogen --> pepsin (conversion caused by acid or pepsin)

breaks proteins to protein fragments (polypeptides)

lipase (gastric lipase not very important in adults)

HCl function: denatures protein, provides low pH for pepsin, kills microbes

regulation of secretion and motility of stomach - 3 phases

cephalic phase (smell, sight, and taste of food)

gastric phase (distention and rising pH stimulate through reflex and gastrin)

intestinal phase (inhibition by secretin, CCK, and other gut hormones)

secretion of HCl - vagus nerve (parasympathetic) and gastrin and histamine from mast cells

peptic ulcer - often cured by antibiotics to eliminate Helicobacter pylori infection; also, histamine receptor blockers (e.g, Zantac) or treated by vagotomy in refractory cases

secretion of enzymes - stimulated by gastrin

Pancreas

head, body, tail

posterior to stomach

duct(s) empty secretions into duodenum

ampulla of Vater (common bile duct enters here too)

compound gland - both exocrine and endocrine

exocrine organized as clusters - acini

pancreatic juice is enzymes + fluid (about 1.5 l/day)

pancreatic enzymes

trypsin, chymotrypsin (as inactive precursors, trypsinogen and chymotrypsinogen)

peptidases

amylase (test for in blood as a sign of pancreatitis)

lipase

nucleases

pancreatic fluid is a dilute bicarbonate solution - neutralizes stomach HCl

regulation of secretion by gut hormones

secretin --> bicarbonate-rich fluid

cholecystokinin (CCK) --> enzymes

Liver

covered with a capsule of dense connective tissue

divided into two major lobes (R and L) by falciform ligament

histology - basic unit is lobule

portal veins and hepatic arteries at edges

hepatic portal vein drains the intestine; carries absorbed nutrients

Kuppfer cells in sinusoids (phagocytic)

bile ducts at edges

= portal triad (portal vein, hepatic artery, bile duct)

central vein in middle

sheets of hepatocytes form sinusoids

liver function: makes bile, blood proteins (e.g., albumen), and stores glycogen and fat

also: removal of drugs and hormones, bilirubin from breakdown of RBC’s

bile (about 1 liter/day) emulsifies fat, activates lipase

hepatic ducts --> cystic duct --> concentrated and stored in gall bladder

released in response to CCK into common bile duct

common bile duct enters duodenum at ampulla of Vater (jaundice if bile flow is blocked)

Small Intestine

three parts: duodenum, jejunum, ileum

plica are folds in wall - increase surface area

villi of mucosa project about 1 mm into lumen - gives velvety feel to musosa

microvilli on cells also increase surface area (called a brush border)

lacteal (lymphatic vessel) projects into villus

absorption of fat

secretions: intestinal juice is dilute bicarb solution + mucus (about 2l/day)

some enzymes bound to brush border membranes:

maltase, sucrase , lactase, other hydrolases; convert disaccharides (e.g., maltose) to monosaccharides (e.g., glucose)

motility: peristalsis and segmentation

 

absorption

simple diffusion

e.g., fatty acids from trigyceride hydrolysis, ethanol

facillitated diffusion

e.g., glycerol, fructose

primary active transport

directly uses ATP

e.g., sodium ion

coupled co-transport (secondary active transport)

uses sodium gradient to transport glucose, amino acids

triglycerides are resynthesized in mucosal cells and secreted as fatty droplets (chylomicrons)

enter lacteals - part of lymphatic system which enters the venous circulation

Large Intestine

ileocaecal sphincter (valve)

ascending, transverse, descending, sigmoid colon, rectum

epithelium has pits which lead to straight glands

secrete mucus

function: actively absorb ions and passively absorb water

diarrhea due to inadequate water absorption or excess water secretion by small or large intestine

--> dehydration if extensive/prolonged

defecation

stimulated by feces in rectum

partly involuntary (relax internal anal sphincter -- under control of paraympathetics)

partly voluntary (contract muscles of abdominal wall)


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Last updated 13 April 2004