Summary
Highlights
The digestive system is a series of hollow organs used to break down and process nutrients and energy. It's a fundamental biological system found in nearly all animals, adapted to their specific feeding behaviors and diets. Examples include the house fly's external digestion and the varied digestive tracts of vertebrates like dogs and cows.
Humans, as omnivores, have a generalized digestive system capable of handling both plants and meat. The key to successful digestion is maximizing surface area, both of the food being digested (through chewing) and of the digestive organs themselves. The small intestine, for instance, has an incredibly vast surface area due to its length, folds, villi, and microvilli.
Digestion begins in the mouth, where salivary amylase starts breaking down starches. Chewing increases the surface area of food, allowing enzymes to work more effectively. Food, now a bolus, travels down the esophagus to the stomach through peristalsis, a wave-like muscular contraction. The epiglottis prevents food from entering the respiratory system.
The stomach acts like a 'scorched earth' digestive machine, churning and mixing food with gastric juice, a powerful cocktail of hydrochloric acid, pepsin, mucus, and water. Hydrochloric acid breaks down food and kills bacteria, while pepsin begins protein breakdown. Mucus protects the stomach lining from its own acids. The result is chyme, a liquidy slop that moves to the small intestine.
The duodenum, the beginning of the small intestine, is where major absorption and secretion occur, including the neutralization of gastric acid by bicarbonate. The small intestine, despite its name, is very long (4.5 to 10.5 meters) and features numerous folds, villi, and microvilli, creating an immense surface area (250 square meters) for efficient nutrient absorption into capillaries. Bile salts from the liver and gallbladder emulsify fats, like those in a milkshake, breaking them into absorbable fatty acids.
After the small intestine, chyme enters the large intestine (cecum). The large intestine's main function is to reabsorb water and bile salts, preventing constant diarrhea. It's wider but shorter than the small intestine. The appendix, once thought vestigial, is now believed to act as a 'safe house' for beneficial gut bacteria, recolonizing the digestive system after illness.
The final stage involves the elimination of waste. Food can spend up to three days in the digestive tract, with much of that time in the large intestine for water reabsorption and preparing the feces. The waste is then expelled through the anal sphincters.