Chapter 15: The Digestion and Absorption of Food

The Digestion and Absorption of Food comprehensively explores the digestive system's structure and function, examining how the gastrointestinal tract mechanically and chemically breaks down food into absorbable molecular units and transports nutrients across the intestinal epithelium into circulation. The anatomical foundation begins with the organization of the GI tract wall into four distinct layers—the mucosa responsible for secretion and absorption, the submucosa containing blood and lymphatic vessels, the muscularis externa enabling coordinated contractions, and the serosa providing protection and reducing friction. The enteric nervous system operates as an independent neural network integrated with autonomic innervation and endocrine signaling to coordinate motility patterns and secretory responses throughout digestion. Motility mechanisms including peristalsis for advancing boluses and segmentation for mixing are regulated by sphincters that control material flow between anatomical regions. The mouth initiates digestion through mastication and salivary amylase secretion, while the stomach serves as a temporary reservoir that mechanically churns food and releases pepsinogen for initial protein breakdown alongside gastric acid and intrinsic factor for cobalamin absorption. The small intestine functions as the primary absorption site, with its extensive villus and microvillus architecture dramatically increasing surface area, while brush-border enzymes complete carbohydrate and protein hydrolysis. Pancreatic enzymes including amylase, lipase, and proteases work synergistically with hepatic bile salts to solubilize and digest macronutrients, with lipids requiring micellar solubilization before enterocyte uptake and reassembly into chylomicrons for lymphatic transport. The large intestine reabsorbs water and electrolytes while harboring microbial communities that ferment remaining carbohydrates and synthesize essential vitamins. Hormonal regulation through gastrin, secretin, cholecystokinin, and glucose-dependent insulinotropic peptide integrates with neural reflexes to precisely time secretion and motor activity. The chapter applies these mechanisms to clinical conditions including lactose intolerance from disaccharidase deficiency, gallstone formation from cholesterol precipitation, peptic ulcer disease from mucosal damage, and celiac disease exemplifying how autoimmune destruction of intestinal villi severely impairs nutrient absorption.