Chapter 37: Structure and Function of the Digestive System

The gastrointestinal tract functions as an integrated organ system composed of four distinct tissue layers—the mucosa, submucosa, muscularis, and serosa—each contributing to digestion and absorption while remaining under control of intrinsic enteric plexuses and extrinsic autonomic innervation. The digestive process begins in the oral cavity, where mastication breaks down food mechanically while salivary enzymes initiate chemical digestion; the coordinated act of swallowing propels the food bolus through the esophagus via primary and secondary peristalsis, with sphincters preventing reflux and aspiration. The stomach serves as a temporary reservoir where hydrochloric acid and pepsinogen begin protein denaturation and digestion, while the protective mucus layer and prostaglandins shield the gastric mucosa from acid damage; gastric motility is regulated through cephalic, gastric, and intestinal phases controlled by vagal signaling, gastrin secretion, and other hormones. The small intestine—consisting of the duodenum, jejunum, and ileum—is the primary site of nutrient absorption, where villi and microvilli dramatically increase surface area and specialized enterocytes absorb monosaccharides, amino acids, and lipids through active transport, passive diffusion, and emulsification-mediated mechanisms. The large intestine completes the digestive process by absorbing water and electrolytes to form feces, which are propelled toward the rectum through segmental contractions and the gastrocolic reflex before coordinated sphincter action enables defecation. Accessory organs including the liver, gallbladder, and pancreas provide essential secretions and metabolic support: the liver synthesizes bile for lipid emulsification, detoxifies harmful substances, produces clotting factors, and maintains carbohydrate and protein homeostasis; the pancreas supplies alkaline secretions and digestive enzymes while regulating blood glucose through endocrine hormones; the gallbladder stores and concentrates bile for release during fat digestion. The intestinal microbiota contributes to digestion by metabolizing bile components and producing vitamins while supporting immune defense through IgA secretion and pathogen exclusion. Age-related physiological changes including diminished salivation, reduced gastric acid secretion, altered intestinal motility, weakened microbial populations, and compromised nutrient absorption significantly impact digestive efficiency and increase vulnerability to malabsorption and disease in older adults.