GIT Physiology Overview

GIT Physiology

• The gastrointestinal tract (GIT) is a 5-meter long hollow muscular tube
• Accessory glands, including salivary glands, liver and gall bladder, and pancreas, are also part of the digestive system
• The GIT consists of oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus

Learning Objectives

• List the parts of the GIT and their functions
• Identify the structure of the digestive tract wall
• Describe movements in the GIT
• Describe mastication and swallowing
• List the source and functions of GIT secretions

Structure of the Digestive Tract Wall

• Mucosa: Inner layer with exocrine and endocrine cells
• Submucosa: Connective tissue layer with blood and lymph vessels, and submucosal plexus
• Muscularis Externa: Smooth muscle layer with an inner circular layer and an outer longitudinal layer, with myenteric plexus between layers
• Serosa: Outer protective layer

Regulation of GIT Functions

• Neural (Nervous): Enteric nervous system (local nerve plexuses) and autonomic nerves (sympathetic and parasympathetic) control secretions and motility
• Hormonal: Polypeptides secreted from mucosal endocrine cells (APUD) throughout the GIT act as local hormones, regulating functions

Main Functions of Digestive Tract

• Motility: Propels food from mouth to rectum
• Secretion: Releases digestive juices (e.g., saliva)
• Digestion: Breaks down food into absorbable molecules
• Absorption: Transports nutrients, electrolytes, and water from the GIT to the bloodstream

GIT Motility

• GIT smooth muscle cells have pacemaker cells (pace setters)
• The basic electrical rhythm (BER) are slow wave depolarizations and repolarizations that smooth ms cells produce spontaneously.
• Nerves and hormones influence the BER to reach the necessary firing level to facilitate action potential.
• Smooth muscle cells form functional syncytium, where ions pass and circulate freely between adjacent cells via gap junctions

Motility of the GIT (Mouth)

• Mastication: Chewing food into smaller pieces to aid swallowing
• Swallowing: Transport of food from mouth to stomach. Occurs in 3 phases (buccal, pharyngeal and esophageal)

Motility of the Esophagus

• The esophagus is a 25 cm muscular tube.
• Upper esophageal sphincter prevents air entering GIT
• Lower esophageal sphincter prevents gastric contents re-entering the esophagus
• Peristalsis sweeps food down the esophagus

Motility of the Stomach

• Proximal area (fundus and body) has thin walls and weak contractions for food storage (receptive relaxation)
• Distal area (pylorus) has thick walls and strong peristaltic contractions to move food into the duodenum

Motility of the Small Intestine

• Two main patterns: segmentation mixes contents with enzymes and other secretions, maximizes exposure for absorption and digestion
• Peristalsis propels the contents into the large intestine

Motility of the Large Intestine

• Hahustration: mixing contents
• Mass movement: propels contents through one segment into the next downstream segment
• Defecation: involves involuntary and voluntary reflexes to evacuate colonic contents through the anal canal

Secretions of GIT

• The total secretion volume is approximately 6-8 liters per day arising from specialized cells lining the GI tract
• Pancreas, liver, and gallbladder secrete digestive juices
• GI secretions involve lubricating (water and mucus), protecting (mucus), sterilizing (HCl), neutralizing (HCO3-), and digesting (enzymes)

Secretions of GIT: Saliva

• Three pairs of salivary glands: parotid, sublingual, and submandibular
• Parotid glands secrete serous saliva
• Sublingual and submandibular glands secrete a mix of serous and mucous saliva.
• Salivary glands contribute to antibacterial action, buffering acidic materials, cleaning food remnants.

Gastric Secretions

• Gastric HCL activates pepsinogens into pepsin, enhances pH for pepsin activity. It helps in protein digestion, kills bacteria, and facilitates iron and calcium absorption.
• Intrinsic factor aids vitamin B12 absorption

Pancreatic Secretions

• Pancreas has both endocrine and exocrine functions
• Exocrine secretions (pancreatic juice) include an aqueous component containing HCO3- (neutralizing stomach acid) and an enzymatic component (trypsin, chemotrypsin, lipase, and amylase) for carbohydrate, fat, and protein digestion.

Functions of the Liver

• Metabolic Regulation: Stores absorbed nutrients (vitamins) and releases them as needed.
• Hematological Regulation: Produces plasma proteins and removes old red blood cells.
• Production of Bile: Liver makes bile to aid fat digestion and absorption

Gall Bladder Secretion

• Bile aids fat digestion. Emulsifies fats, exposes more surface area for enzymatic action, forming micelles which are water soluble and aids fat absorption
• Bile has alkali content, important in neutralizing HCl in the duodenum.
• Contains mucin for lubrication

Functions of the Gall Bladder

• Stores and concentrates bile.
• Aids in fat digestion and absorption in the intestine
• Aids absorption of fat-soluble vitamins in the small intestine

This quiz covers the key components and functions of the gastrointestinal tract (GIT), including its structure, movement, and secretions. It aims to enhance understanding of the GIT's anatomy, from the oral cavity to the anus, and the physiological processes involved in digestion.