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
Meissners 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 RBCs
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