Gastrointestinal Physiology Overview

Overview of Gastrointestinal Physiology

• The gastrointestinal (GI) system is responsible for the ingestion, digestion, and absorption of nutrients, as well as the elimination of waste products.
• The GI system consists of the mouth, esophagus, stomach, small intestine, and large intestine.

Motility and Movement

• Peristalsis: rhythmic, wave-like muscle contractions that propel food through the GI system
• Segmentation: localized, intermittent contractions that mix food with digestive enzymes
• Mass movement: strong, coordinated contractions that propel food through the large intestine

Digestion and Absorption

• Mechanical digestion: breakdown of food into smaller particles through chewing and grinding
• Chemical digestion: breakdown of nutrients into smaller molecules through enzymatic reactions
• Absorption: uptake of nutrients into the bloodstream through the walls of the small intestine
• Nutrient absorption sites:
  • Carbohydrates: small intestine
  • Proteins: small intestine
  • Fats: small intestine and liver

Gastric Function

• Gastric acid: secreted by parietal cells, pH 1.5-2.5, denatures proteins and activates pepsin
• Pepsin: enzyme that breaks down proteins into smaller peptides
• Gastric mixing: contractions of the stomach mix food with gastric acid and pepsin

Small Intestine Function

• Brush border enzymes: enzymes embedded in the microvilli of enterocytes, break down carbohydrates and proteins
• Pancreatic juice: alkaline fluid containing digestive enzymes, secreted by the pancreas
• Bile: emulsifies fats, secreted by the liver and stored in the gallbladder

Large Intestine Function

• Water absorption: reabsorption of water and electrolytes from the colon
• Electrolyte absorption: reabsorption of sodium, potassium, and chloride from the colon
• Feces formation: remaining waste products are formed into feces

Hormonal Regulation

• Gastrin: stimulates gastric acid secretion and gastric motility
• Cholecystokinin (CCK): stimulates pancreatic juice secretion and gallbladder contraction
• Secretin: stimulates pancreatic juice secretion and bicarbonate production
• Motilin: stimulates gastric motility and hormone secretion