Chapter 56: Introduction to the Gastrointestinal System

The continuous GI tract, including the esophagus, stomach, small intestine, and large intestine, is supported by accessory organs such as the pancreas, liver, and gallbladder. The tube is composed of four layers: the mucosa, the muscularis mucosa, the nerve plexus, and the adventitia. The GI system’s four major activities are secretion, absorption, digestion, and motility. Secretions begin with saliva in the mouth. The stomach produces digestive enzymes and hydrochloric acid (HCl) from parietal cells, which is stimulated by gastrin release, parasympathetic stimulation, and histamine-2 (H2) receptors. Digestion continues as the food bolus, now called chyme, enters the small intestine, where secretin stimulates the pancreas to release sodium bicarbonate (to neutralize the acid) and pancreatic enzymes (like trypsin and amylases). If fat is present, the gallbladder contracts to release bile, which is essential for breaking down fats, into the small intestine. Most absorption of water and nutrients, facilitated by the vast surface area of the small intestine, occurs here, with the absorbed products flowing into the portal system and then to the liver for filtering and processing. Motility is maintained by an inherent Basic Electrical Rhythm (BER) controlled by the local nerve plexus. The primary movement in the esophagus is peristalsis, while the small intestine relies on segmentation—a contracting and relaxing motion that ensures thorough mixing and maximum absorption. Overall GI activity is modulated by the autonomic nervous system: the sympathetic system slows activity, while the parasympathetic system increases muscle tone, secretions, and contractions. GI function is also controlled by several reflexes, including local reflexes (such as the gastrocolic reflex and the ileogastric reflex) that maintain homeostasis, and two central reflexes mediated by the medulla. Swallowing is a complex, timed reflex that moves the bolus into the esophagus. Vomiting is a complex, protective reflex initiated by the Chemoreceptor Trigger Zone (CTZ) or, in immature or injured brains, the more primitive emetic zone. The CTZ can be stimulated by various factors including increased intracranial pressure, tactile stimulation, inner ear signals, or exposure to chemical agents and toxins.