Lecture 30 Digestive Flashcards
GI Tract
mouth -stratified squamous epithelium pharynx esophagus stomach -simple columnar epithelium small intestine large intestine
Accessory Organs
salivary glands
liver
pancreas
gallbladder
Functions of the Digestive System
digestion
absorption
secretion
motility
digestion
chemical breakdown of food molecules by hydrolysis
absorption
of nutrients, electrolytes and H2O
secretion
mucus, digestive enzymes, acid, bicarbonate, electrolytes
motility
muscular movements of GI tract to mix and propel food
peristalsis - moves material forward
segmental contractions - mix contents
regional specialization (“assembly line”)
ingestion → mechanical breakdown → chemical digestion → absorption → waste processing
GI tract structure: 4-layered tube
mucosa
submucosa
muscularis (externa)
serosa
mucosa
mucosa - epithelium + lamina propria (areolar CT) + muscularis mucosae
submucosa
submucosa - connective tissue, vascular
muscularis (externa)
muscularis (externa) - smooth muscle
serosa
- thin covering membrane (visceral peritoneum)
Mouth, Pharynx and Esophagus
functions:
ingestion, mastication (chewing), deglutition (swallowing)
salivary glands
secrete saliva: H2O, ions, mucus, enzymes: amylase, lipase
amylase
begins chemical digestion of starch → disaccharides
esophagus
swallowing (upper portion), peristalsis (lower portion)
lower esophageal sphincter controls entry into the stomach
Stomach
functions
storage
mechanical breakdown of food → chyme
sterilization
chemical digestion: acid (HCl) and enzymes (pepsin)
stomach structure
mucosa: simple columnar epithelium, gastric glands
- secrete acidic gastric juice (pH 1-2), 1-3 L/day
- mucous cells secrete alkaline mucus to protect stomach epithelium
muscularis: 3 layers thick
- pyloric sphincter controls passage of chyme from stomach to duodenum
acid secretion
parietal cells secrete hydrochloric acid (HCl)
CO2 + H2O H2CO3 H+ + HCO3-
H+ is active transported into the lumen, Cl- follows via diffusion through channels
HCO3- is transported back into ECF (countertransport with Cl-)
chief cells
enzyme secretion
chief cells secrete pepsinogen (inactive), activated at low pH to form pepsin
pepsin digests proteins into smaller peptides
Small Intestine, Liver and Pancreas Functions and SI regions
Small Intestine, Liver and Pancreas
functions: chemical digestion and absorption
SI regions: duodenum, jejunum, ileum
Digestion
duodenum receives chyme from stomach, secretions from liver and pancreas
Liver
processes absorbed nutrients (delivered via hepatic portal vein)
secretes bile, stored in gallbladder
bile salts
bile salts
derived from cholesterol, function to emulsify fats → micelles
bile pigments
(bilirubin, biliverdin) - waste products from hemoglobin breakdown
Pancreas
acinar cells secrete digestive enzymes:
trypsin, chymotrypsin, carboxypeptidase, amylase, lipase
many enzymes are secreted in inactive form (zymogens), activated by trypsin in lumen
duct cells
secrete bicarbonate (NaHCO3) to neutralize acid (pH → 8)
SI (brush border)
enzymes complete digestion
complete digestion of starch
exopeptidase
digests terminal peptide bonds to release amino acids
endopeptidase
digests internal peptide bonds
small intestine
has huge surface area, specialized for absorption
(1) length > 3 meters
(2) circular folds
(3) villi - epithelium (enterocytes and goblet cells) + lamina propria (capillaries and lacteals)
(4) microvilli - “brush border” membrane
transport mechanisms of the small intestine
- Na+, Cl
- , K+ absorbed via active transport and diffusion through channels
- glucose & amino acids - cotransport with Na+ (secondary active transport)
- H2O - via osmosis, follows solute transport
water soluble nutrients
are absorbed into intestinal capillaries → liver (via HPV)
chylomicrons
lipids are formed into chylomicrons and absorbed into lymphatic vessels (lacteals)
Large Intestine
functions
fluid absorption, waste packaging and elimination
- LI absorbs most remaining water and ions from chyme
- intestinal microflora - bacteria in colon, produce some vitamins (K, B12)
- defecation reflex
Neural and Hormonal Control
enteric ns ans GI peptides hormones phases of digestion
. Enteric Nervous System
- submucosal and myenteric plexuses
- local control within the GI tract (short reflex)
Autonomic Nervous System
parasympathetic: vagus nerve - stimulates GI tract motility and secretion (long reflex)
sympathetic division mostly inhibits GI tract
gastrin
secreted by G cells in the gastric glands
- stimulates gastric acid secretion; stimulates gastric motility and mucosal growth
(- acid secretion is also stimulated by histamine secreted by ECL cells in gastric glands)
CCK
(cholecystokinin) - secreted by endocrine cells in intestinal crypts
- stimulates bile release from gallbladder and pancreatic enzyme secretion
secretin
- stimulates bicarbonate secretion by pancreas
GIP
(gastric inhibitory peptide) - stimulates insulin secretion by pancreas;
- GIP, CCK and secretin all inhibit gastric acid secretion
Phases
- Cephalic Phase - sensory stimuli and thoughts of food activate autonomic NS (vagus n.)
- Gastric Phase - vagus n., mechanical & chemical stimuli in stomach stimulate gastric secretion
- Intestinal Phase - arrival of chyme in duodenum triggers SI endocrine and exocrine secretion;
hormonal feedback inhibits gastric acid secretion and slows stomach emptying
