GIT Physiology Overview

The Digestive System is comprised of a long, muscular tube called the gastrointestinal tract (GIT) and accessory glands like the salivary glands, liver, gallbladder, and pancreas.

The GIT, about 5 meters long, is a continuous tube starting at the mouth and ending at the anus. It is responsible for ingesting and digesting food, absorbing nutrients, and eliminating waste.

Accessory glands produce and release essential secretions that aid in digestion, such as saliva, bile, and pancreatic juice.

The GIT is comprised of the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus.

The mucosa is the innermost layer, lining the lumen. It contains epithelial cells that secrete substances or act as hormones.

The submucosa is a connective tissue layer beneath the mucosa, rich in blood and lymph vessels. It contains the submucous plexus, a network of nerves.

The muscularis externa is the main smooth muscle layer with inner circular and outer longitudinal layers. It contains the myenteric plexus, a network of nerves facilitating muscle movement.

The serosa is the outermost protective layer of the digestive tract wall.

The enteric nervous system (ENS) is a complex, independent network of neurons within the digestive tract. It controls local functions like secretion and motility.

Meissner's plexus (Submucosal nerve plexus) in the ENS regulates GIT secretions.

Auerbach's plexus (Myenteric nerve plexus) in the ENS regulates GIT motility.

Autonomic nerves, part of the peripheral nervous system, also influence GIT functions. The sympathetic nervous system generally inhibits GIT activity, while the parasympathetic system (vagal, sacral) stimulates it.

GIT hormones are polypeptide messengers produced by special mucosal endocrine cells called APUD cells. These hormones act locally or on distant targets, regulating various GIT functions.

The main functions of the digestive tract include motility, secretion, digestion, and absorption.

Motility refers to the coordinated muscular movements that propel food through the GIT, mixing it with digestive fluids.

Secretions, released by various glands and cells lining the digestive tract, have important roles in lubrication, protection, sterilization, neutralization, and digestion.

Digestion involves the breakdown of food into smaller, absorbable molecules by mechanical and chemical means.

Absorption is the primary function of the GIT, where nutrients, water, and electrolytes are taken up from the digestive tract into the bloodstream.

Pacemaker cells, located between the muscular layers of the GIT, generate spontaneous electrical activity called slow waves or Basic Electric Rhythm (BER).

The Basic Electric Rhythm (BER) is a wave-like pattern of electrical activity that sets the pace for smooth muscle contractions in the GIT. It provides the rhythmic basis for motility.

Nerves and hormones can stimulate the BER to reach the firing level, triggering action potentials and resulting in smooth muscle contractions.

Smooth muscle cells in the GIT are connected by gap junctions, allowing ions to flow freely between them, creating a functional syncytium. This enables coordinated muscle contractions.

Chewing, or mastication, mechanically breaks down food into smaller pieces, increasing surface area for easier swallowing and digestion.

Swallowing is a complex process that moves food from the mouth through the pharynx and esophagus to the stomach. Three distinct phases are involved: buccal, pharyngeal, and esophageal.

The upper esophageal sphincter prevents air from entering the GIT during breathing. This is like a gatekeeper that stops food from going the wrong way.

The lower esophageal sphincter prevents gastric contents from refluxing back into the esophagus, acting as a barrier between the stomach and esophagus.

Esophageal peristalsis is a wave-like muscular contraction that moves food down the esophagus into the stomach. It's like a wave pushing the food along.

The proximal stomach (fundus and body) is responsible for storing food and its thin wall allows for receptive relaxation, accommodating incoming food.

The distal stomach (pylorus) is responsible for mixing and emptying food into the duodenum. It has a thicker wall that allows for stronger contractions for mixing and expulsion.

Segmentation, a rhythmic contraction of the small intestines, mixes food with digestive juices and increases contact with the intestinal wall for better digestion and absorption.

Peristalsis in the small intestine propels food towards the large intestine for further processing and waste elimination.

Haustration, the segmental contractions of the large intestine, mixes the contents and facilitates water absorption. Think of it as a gentle churning motion.

Mass movement is a strong peristaltic wave that propels the contents of the large intestine towards the rectum for defecation.

Defecation, or bowel movement, involves both involuntary reflexes and voluntary control to empty the rectum. This process helps regulate waste elimination.