1. Overview
  2. Esophagus and Stomach
  3. Small Intestine
  4. Large Intestine
  5. Liver, Gallbladder and Pancreas
  6. Regulation of the Digestive System
  1. Overview
  1. Esophagus and Stomach




  1. store food
  2. initiate digestion of proteins
  3. kill bacteria with the strong acidity (low pH of the gastric juice)
  4. make chyme
  1. Fundus
  2. body
  3. pyloric region (pyloric sphincter)

Cell Type


Parietal cells

HCl; intrinsic factor

Chief cells


Goblet cells


Enterochromaffin-like (ECL) cells


D cells


G cells


  1. denatures ingested proteins
  2. optimum pH for pepsin activity is 2.0
  3. at pH 2.0, weak pepsinogen enzymes digest each other to form pepsin

H+ transport

  1. H+ is converted to CO2 (blood)
  2. CO2 diffused into parietal cell
  3. CO2 is converted back to H+
  4. H+ is transported into the GI lumen by a H+-K+-ATpase

Cl- transport

  1. Cl- is transported into the parietal by a Cl-/HCO3- transporter
  2. Cl- diffused into the GI lumen via a Cl- channel
  1. Small Intestine

Absorption in the Small Intestine

Caloric content of food is derived mainly from:


Begins as starch (polysaccharide) and then eventually digested into monosacharides for absorption.

Amylase: Starch digestion begins in the mouth (salivary amylase), and then continues in the duodenum (pancreatic amylase). Amylase digestion of starch produces maltose (disaccharide) and maltriose (trisaccharide) and oliosaccharides.

Brush border enzymes: hydrolyze maltose, maltriose, and oligosaccharides, sucrose, lactose to monosaccharides for absorption.

The three absorbable monosaccharides are glucose, galactose, and fructose.

Transport across epithelial layer

  1. Lumen side: Na+ cotransporter with monosaccharides
  2. Blood side: passive diffusion via a transporter


Stomach: Somewhat digested to short-chain polypeptides by pepsin

Duodenum, jejunum: Digested to amino acids, di-peptides, tri-peptides by pancreatic juice enyzmes

Transport across epithelial layer

  1. Lumen side: Na+ cotransporter with amino acids, di-peptides, tri-peptides
  2. Blood side: passive diffusion via a transporter


Absorption of fats takes place in the duodenum and are transported into the lymphatic system.

  1. Fat droplets, mainly comprised of triglyerides are first emulsified by bile salts (see later section for discussion of bile salts). Emulsification makes the fat droplets smaller, making them more easily digested enzymatically.
  2. Pancreatic lipase digests the smaller, emulsified fat droplets into free fatty acids and monoglycerides.
  3. The free fatty acids and monoglycerides form micelles which migrate towards the brush border membrane. The micelles contain bile salts, lecithin, cholesterol and
  4. The free fatty acids and monglycerides leave the micelle and enter the epithelial cell.
  5. Inside the epithelial cell the free fatty acids and monoglycerides combine with protein to form chylomicrons (lipid + proteins).
  6. The chylomicrons are secreted into the lymphatic system.
  1. Large Intestine

Summary of Water transport in GI tract


Amount of water entering

Amount reasborbed

Small Intestine

Ingestion: 1.5 liters

secretions: 7-9 liters

Total: 8.5-10.5 liters

6.5-9 liters

Large Intestine

1.5-2 liters

1.3-1.7 liters


200 ml

  1. Liver, Gallbladder and Pancreas
  1. Liver and Gallbladder


Major functions

  1. production and secretion of bile
  2. detoxication of blood
  3. secretion and storage of glucose
  4. production of albumin

Liver clears substances via the bile duct in a similar manner to the way the kidney clears substances into the nephron.


Production and secretion of bile

Gallbladder stores bile. Bile entering gallbladder is controlled by the sphincter of Odii.

  1. Pancreas

Endocrine versus exocrine function:

Pancreatic juice contains:


  1. Control of the Digestive System

Autonomic Branch

Effect on GI system

­ parasympathetic

­ motility, open valves

­ sympathetic

¯ motility, close valves

Three Phases in Control of Gasric Function

  1. Cephalic Phase
  2. Gastric Phase
  3. Intestinal Phase
  1. Cephalic Phase:

Regulation by the vagus nerve: lasts approximately 30 minutes.

The vagus nerve is activated by sight, smell, taste of food.

Activation of the vagus nerve:

  1. indirectly causes the parietal cells to secrete HCl
  2. directly stimulates chief cells to secreate pepsinogen to digest proteins


  1. Gastric Phase

Stimulated by

  1. distension of the stomach (i.e. amount of chyme)
  2. chemical nature of the chyme

The goal of this phase is to release acid and proteolytic enzymes into the stomach.

Feedback loops

  1. Vagus nerve and amino acids in the stomach lumen stimulate gastrin release by G-cells
  2. Gastrin stimulates histamine release by ECL cells
  3. Histamine stimulates HCl secretion by parietal cells.


Stimulus for gastric phase

  1. Intestinal phase

Inhibition of gastric activity due to:

Control of Intestine

  1. gastric inhibition
  2. pancreatic secretion
  3. bile secretion