GI Embryology + Physiology
ORAL CAVITY
Function: sensory analysis, mechanical processing (teeth, tongue + palatal surface), lubrication (mucus
+ salivary gland secretions), Slight digestion via salivary amylase
Cephalic Phase (Initial): Involves ingestion, mechanical/chemical digestion in mouth
ESOPHAGUS hollow muscular tube behind the trachea (begins posterior to cricoid cartilage),
innervated by fibers of esophageal plexus. Has longitudinal (lateral) + circular (medial) m. fibers
Location of swallowing: Voluntary (oropharyngeal) phase and Involuntary phase (UES relaxes 
peristalsis  LES relaxes)
Function: Move food to stomach via peristalsis  circular m. behind bolus will contract while longitudinal m. in front of bolus
contracts  pushes/pulls bolus down esoph. – also occurs in stomach and intestine.
GASTRIC
Bolus enters the gastroesophageal junction. Stomach made up of tissue that allows stretch when full and contraction when
empty. Note: different areas have different cell types  secretion of different hormones when triggered by bolus’ arrival.
Gastric Phase: Begins when food enters distended stomach stim. Stretch receptors + peptide sensitive
chemoreceptors
o VAGAL reflexes turned ON: ParaNS is most important for digestion.
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Vagovasal reflex: afferent and efferent fibers of the vagal nerve (CN X) via
dorsal vagal complex in brain  coordination of gastric, intestinal, and
pancreatic secretions req for digestion.
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Allows contraction of muscle and accomodation for large amounts
of food
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Clinically  vagotomy (thanksgiving is NOT fun)
o Some digestion of proteins, lipids, and carbs but NO ABSORPTION!
Gastric Secretions:
1. Hydrochloric Acid: secretion from parietal cell d/t distention of stomach; l/t  [H+] = pH~2;
kills microbes from bolus, denatures protein/ inactivates enzymes, aids digestion of plant walls
and meats, activates pepsin from pepsinogen
a. Parietal cells: Stimulated by Ach, Gastrin + Histamine
b. Beware mechs and OTCs that decr. Acid production = impaired digestion
i. Atropine = Blocks Ach; Omeprazole = blocks H/K-ATPase
2. Gastrin: Peptide hormone secreted by G cells  binds to parietal + ECL (via Cholecystokinin B receptors) cells
a. ECL cells release Histamine  + nearby parietal cells to take up CO2/ H2O (carbonic acid  HCl)  pump out HCl via K/H
ATPase pump.
i. Histamine release regulated by low pH of stomach ie) H2 blockers (Zantac, Cimetidine) work to decr. acid production
b. Clinically: Patient’s on PPI’s have  gastrin levels b/c of no (-) feedback to turn off gastrin secretion  parietal cells increase in
sz despite  acid production.
i. Stop medications = crazy amounts of acid released 2/2 increased parietal cell sz
3. Somatostatin: Decreases acid production and slows down digestion via inhibition of gastrin, secretin, cholecystokinin, etc.
4. Pepsin: Released from chief cells as pepsinogen  pepsin via HCl; Aids in protein break down
SMALL INTESTINE:
90% of absorption thus is required for living (along with liver); main function is absorption of nutrients, salt,
electrolytes, and water. High SA via valvulae, conniventes, villi and microvilli.
Intestinal Phase: Involves secretions and movements
Intestinal Enzymes: Localized to the brush border  sucrose, maltose, lactose, peptidases, nucleases
Duodenum: Absorption of Iron and Folate; Pts with gastric bypass or ulcers will require supplements.
Folic Acid (B9): Pteroylmonoglutamic acid (absorbable form) post-conversion via jejunal enzymes
o Deficiency: D/t decreased intake (tea + toast in elderly; alcoholics), Jejunal disorders (celicac dz, tropical sprue, drug effects), 
requirement in pregnancy
Ileum: Vitamin B12 and Bile salts are absorbed; Patients with Chron’s are deficient + have trouble digesting fats d/t surgical resection of the ileum.
Cobalamin Absorption: Bound to proteins in food  pepsin + low pH releases cobalamin  Parietal cells secrete IF which eventually
binds to B12  absorbed by ileal hepatocyte and cleaved from IF  storage in liver (3-5 years)
o Cobalamin Deficiency: Susceptible to megaloblastic anemia, myelopathies d/t damage to the posterior column of spinal cord 
problems with vibration + proprioception (can feel numbness and tingling in extremities)

Megaloblastic Anemia
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Pernicious 2/2 no IF for b12 absorption.
PANCREAS
Lies posterior to stomach + bound to the posterior wall of cavity.
Endocrine: a and b cells secrete glucagon and insulin respectively.
Exocrine: Acinar and ductal epi cells secrete pancreatic juices  includes pancreatic amylase, protease, lipase and nucleases to produce
intestinal enzymes. Flows from pancreatic duct to duodenum.
o Secretes Bicarb to buffer the gastric acid traveling to SI
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5.
Secretes trypsin, chymotrypsin and carboxypeptidase for protein digestion
ABSORPTION
Water and Electrolytes: 90% water reabsorbed by SI; Na/Cl reabsorbed via transporters l/t passive water movement  No watery poo!
a. SI also secretes electrolytes + water. Simultaneously absorption > secretion  net absorption of water and electrolytes.
i. Response to bacterial toxins, bile salts, hormones  secretory diarrhea
Amino Acids: Via proteolytic degradation with serine proteases from pancreas (trypsin, chymotrypsin, elastase) and carboxyl proteases
from stomach (pepsin)
Dietary Fats: TGs digested in jejunum via pancreatic lipase  FFAs + B-monoglycerol
a. 2/2 Emulsification via bile salts  regenerated into chylomicrons  lymphatics.
b. MCT: Don’t need emulsification; Useful for patients with bile salt deficiency.
Vitamins:
a. Water Soluble: Thiamine, Niacin, Riboflavin, Biotin, Vit C
b. Fat Soluble: A, D, E, K
Calcium: Occurs in SI with Vit D to increase serum levels; Iron: Occur in duodenum
MUCOSAL IMMUNE SYSTEM:
70% in the digestive tract
GALT: Peyers patches (SI), Mesenteric LN, Appendix, Solitary LN (Ileum)
o Peyer’s patches: mucosa  submucosa; follicles responsible for monitoring bacteria population in intestines + prevention of
overgrowth
o Mesenteric LN: Next in line of defense + attaches intestine to abd wall
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Trap and attack bacteria
o Appendix: Maintains microbiome to extent; composed of B cell mediated immune responses and T cells coming from outside of
the thymus