Digestive and Urinary Systems

Anatomy and Physiology of the Digestive System

The digestive system performs four fundamental functions: ingesting food, breaking it down mechanically and chemically, absorbing nutrients, and eliminating solid waste. This journey begins in the oral cavity, where chemical digestion is initiated through the action of salivary amylase, an enzyme that starts breaking down starches. Once food is chewed into a bolus, it enters the esophagus. The esophagus propels this food bolus downward toward the stomach using coordinated, involuntary muscular contractions known as peristalsis, much like squeezing a flexible tube to slide contents forward. At the junction of the esophagus and stomach, the lower esophageal sphincter acts as a muscular valve that prevents the retrograde flow of acidic stomach contents back into the esophagus.

Inside the stomach, food is converted into chyme, which is an acidic, semi-fluid mixture of partially digested food and gastric secretions. The stomach facilitates this transformation by secreting hydrochloric acid and pepsinogen, which work together to initiate the chemical digestion of proteins. In addition to these digestive secretions, specialized parietal cells located within the gastric mucosa produce intrinsic factor. Intrinsic factor is a vital substance required for the intestinal absorption of vitamin B12. Once the stomach completes this initial phase of digestion, chyme passes through to the small intestine.

The small intestine is divided into three consecutive anatomical sections: the duodenum, the jejunum, and the ileum. The vast majority of all chemical digestion and nutrient absorption takes place within the small intestine. To maximize absorption, the internal mucosal wall is lined with microscopic projections called villi and microvilli. You can think of villi and microvilli like the dense, plush loops on a terrycloth towel; they dramatically expand the available surface area to facilitate the uptake of digested nutrients into blood and lymph capillaries.

Digestion within the small intestine relies heavily on contributions from accessory organs. The liver produces bile, a substance that emulsifies dietary fats to assist in their digestion and absorption. The gallbladder stores and concentrates this bile, later releasing it into the duodenum via the common bile duct when fatty chyme arrives. Simultaneously, the exocrine pancreas secretes digestive enzymes directly into the duodenum, including lipase, amylase, and proteases. Material remaining after small-intestinal digestion moves into the large intestine, which includes the cecum, colon, rectum, and anal canal. The large intestine absorbs water and electrolytes from these remaining indigestible materials and compacts the residues into feces. Furthermore, normal bacterial flora residing within the large intestine synthesize beneficial compounds, specifically vitamin K and select B vitamins.