Gastrointestinal Physiology Overview

Gastrointestinal Physiology

• The digestive system's function is accomplished by four basic processes: secretion, motility, digestion, and absorption.
• Digestion breaks down complex molecules into simpler ones for easier absorption. It relies on digestive juices and intestinal wall motility.
• Waste products (undigested food) are excreted.
• The gut wall has lymphoid tissue that plays a role in immune responses.
• The alimentary canal stretches from mouth to anus.
• Accessory digestive glands (salivary glands, liver, gallbladder, exocrine pancreas) secrete substances into the alimentary canal.
• Glands within the stomach and small intestine walls also contribute to digestion.

Learning Objectives

• Students should be able to describe the general functions of the digestive system, the basic digestive processes, and the regulation of gut functions.
• Students should be able to identify the functions of saliva.
• Students should be able to analyze the stomach's functions and protective mechanisms.
• Students should be able to determine the role of pancreatic juice and bile in digestion.
• Students should be able to summarize the digestion and absorption of carbohydrates, proteins, and fats.
• Students should understand the functions of the large intestine.
• Students should have a basic understanding of gut hormones.

Digestive System Components

• The digestive system comprises the alimentary canal and digestive glands.
• Alimentary canal extends from mouth to anus.
• Digestive glands (salivary glands, liver, gallbladder, exocrine pancreas) deliver secretions into the alimentary canal.
• Glands within the stomach and small intestine walls contribute to digestion.

Basic Digestive Processes

• Secretion: Digestive glands release enzymes and other substances into the alimentary canal.
• Motility: Muscle contractions move food through the digestive tract.
• Digestion: Breaking down complex food molecules into simpler, absorbable components.
• Absorption: Movement of absorbed nutrients into the bloodstream.

Gut Secretions

• Digestive glands located along the alimentary canal and accessory glands (salivary glands, pancreas, liver) produce digestive juices containing water, electrolytes, mucus, and enzymes.

Gut Motility

• Smooth muscle contractions move food through the digestive tract.
• Types of gut motility
  • Propulsive movement (peristalsis): Pushes food forward through the digestive tract at a regulated speed for proper digestion and absorption.
  • Mixing movement (segmentation): Mixes food with digestive juices.

Digestion & Absorption

• Digestion is performed by enzymes that break down food molecules.
• Absorption usually occurs in the small intestine.
• Undigested and unabsorbed materials are excreted via defecation.

Control of Gut Functions

• Neural Regulation
  • Enteric nervous system (the brain of the gut): A network of neurons within the gut wall, it controls the gut independently of the autonomic nervous system.
  • Autonomic nerves regulate gut activity by modulating the enteric nervous system.
    • Sympathetic and parasympathetic nerves.
• Hormonal Regulation
  • Endocrine cells in the gut mucosa secrete hormones that circulate in the blood.
  • These hormones influence gut motility and secretion.

The Stomach

• Stores food.
• Mixes food with gastric secretions to form chyme.
• Secretes gastric juice that includes HCl, enzymes (e.g., pepsin), and the intrinsic factor.
• HCl activates pepsin, essential for protein digestion; absorbs iron and calcium; sterilizes ingested food.
• Piecemeal evacuation: releases chyme into the small intestine at the appropriate rate for digestion and absorption.
• Protective mechanisms include a thick, alkaline mucus layer that protects the stomach lining from the acidic environment.
• Pepsinogen is secreted in its inactive form and activates into pepsin by HCl.
• Mucosal barrier turnover is rapid.

The Pancreas

• Pancreatic juice helps digest carbohydrates, proteins, and triglycerides.
• Contains digestive enzymes (amylase, trypsin, chymotrypsin, carboxypeptidase, lipase).
• Contains sodium bicarbonate to neutralize acidic chyme.
• Hormonal regulation:
  • Secretin: Released in response to acidic chyme in the duodenum.
  • Cholecystokinin (CCK): Released in response to fat and protein in the duodenum.

The Liver

• Synthesizes and secretes bile.
• Stores bile in the gallbladder between meals.
• Releases bile into the duodenum during meals.
• Functions:
  • Secretory (bile formation): Bile salts aid fat digestion and absorption.
  • Synthesis of plasma proteins.
  • Metabolic functions (carbohydrate, protein, and fat metabolism).
  • Blood storage and filtration.
    • Accommodates blood from the digestive system during meals.
    • Filters blood, removing bacteria and old red blood cells.
  • Detoxification: Removes toxins and hormones from the blood.

Functions of Bile

• Neutralizes HCl in the duodenum through its bicarbonate content.
• Aids fat digestion and absorption by emulsifying fat globules.
• Bile salts break down large fat globules into smaller ones, increasing the surface area available for enzyme action (pancreatic lipase).

The Small Intestine

• Receives chyme, mixes it with intestinal juice, pancreatic juice, and bile.
• Completes digestion of carbohydrates and proteins.
• Absorbs digested food.

The Large Intestine

• Absorbs water and electrolytes.
• Forms and eliminates feces.
• Synthesizes certain vitamins (e.g., Vitamin K).

Swallowing (Deglutition)

• Mechanism that transports food from the mouth to the stomach through the pharynx and esophagus.
• Swallowing involves voluntary and involuntary phases:
  • Buccal phase (voluntary): Tongue pushes food back into the pharynx.
  • Pharyngeal phase (involuntary): Stimulates pressure receptors triggering swallowing reflex.
  • Esophageal phase (involuntary): Peristaltic waves conduct food to the stomach.

Salivary Glands

• Secrete saliva, which contains salivary amylase to begin carbohydrate digestion.
• Moistens the buccal cavity for articulation.
• Contains antibacterial agents (enzymes, ions, immunoglobulins, lactoferrin)
• Maintains the oral pH at 7.
• Buffers acidity to avoid tooth enamel damage by Ca2+ loss.