Chapter 36: Structure and Function of the GI System

The GI tract is structurally organized into the upper (esophagus and stomach), middle (small intestine), and lower (large intestine) segments, all defined by four distinct tissue layers, including the protective inner mucosa. Motility, necessary to propel food and fluid along the tract, is controlled by a complex interplay between the intrinsic pacemaker-generated slow-wave activity, the Enteric Nervous System (ENS) (which includes the myenteric and submucosal plexuses), and the Autonomic Nervous System (ANS); generally, parasympathetic stimulation increases movement, while sympathetic stimulation is inhibitory. The chapter highlights crucial hormonal regulation, detailing the actions of gastrin, cholecystokinin (CCK), secretin, ghrelin (the appetite stimulant), and the incretins (GLP-1 and GIP), which collectively modulate secretion rates, gastric emptying, and nutrient utilization. The tract produces vast daily secretions—such as saliva, gastric juices (containing HCl and pepsinogen from parietal and chief cells, respectively), and alkaline intestinal fluid—essential for lubrication and chemical breakdown, while the mucosal barrier safeguards the lining from highly acidic contents. Digestion and absorption primarily occur in the vast surface area of the small intestine, where brush border enzymes complete the breakdown of carbohydrates into monosaccharides and proteins into amino acids, utilizing processes often linked to sodium transport. Fats are first emulsified by bile, then hydrolyzed by pancreatic lipase, and absorbed via micelles before being packaged into water-soluble chylomicrons for transport through the lymphatics. Finally, the GI system maintains a powerful immune barrier through structures like Gut-Associated Lymphoid Tissue (GALT), Peyer patches, and specialized Microfold (M) cells, which sample antigens from the lumen, along with a diverse community of intestinal microflora that provides metabolic support, synthesizes vitamins, and offers crucial resistance against invading pathogens.