Digestive systems and human digestion

Definition and purpose of digestion

Digestion is the breakdown of large, insoluble food compounds into small, water-soluble components that can be absorbed into the blood plasma. It is a form of catabolism and is commonly divided into mechanical digestion (physical breakdown of food into smaller pieces) and chemical digestion (enzymatic breakdown into usable molecules). Mechanical digestion begins in the mouth with chewing (mastication) and continues in the small intestine via segmentation; chemical digestion relies on enzymes that convert food into small compounds the body can use. In humans, digestion starts in the mouth where saliva (including salivary amylase) begins carbohydrate digestion and helps form a bolus. The bolus moves through the esophagus to the stomach, where gastric juice (notably hydrochloric acid and pepsin) initiates protein digestion while mucus and bicarbonates protect the stomach lining. As partially digested food (chyme) enters the duodenum, it mixes with pancreatic enzymes and bile, and digestion continues mainly in the small intestine, where most nutrient absorption occurs. Digestion is also regulated by digestive hormones and depends strongly on pH: enzymes function best within specific pH ranges, the stomach is highly acidic for protein denaturation and defense, and the duodenum is neutralized to activate and protect enzyme activity. Across organisms, digestion can occur internally (within a controlled digestive tract) or externally (enzymes secreted into the environment). Different animals and microbes have specialized structures and strategies—such as phagosomes for intracellular digestion, gastrovascular cavities in some animals, and specialized organs like crops or ruminant stomach chambers—to improve access to nutrients and absorption efficiency.

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Mechanical versus chemical digestion

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