Digestion & Absorption

The human digestive system transforms complex food into absorbable monomers through a coordinated sequence of mechanical and chemical processes occurring along a continuous alimentary canal. This canal begins at the mouth, where teeth mechanically break food down while salivary amylase (from parotid, sublingual, and submandibular glands) initiates starch hydrolysis at pH 6.8, converting it to maltose. The bolus then moves through the pharynx and oesophagus via peristalsis into the stomach.

The stomach is a J-shaped muscular organ with rugae (mucosal folds) that allow expansion. Gastric glands secrete hydrochloric acid, which serves three critical functions: it kills ingested bacteria, activates pepsinogen into its active form pepsin, and maintains an acidic pH of 1.5-2.0 optimal for protein hydrolysis. Pepsin cleaves proteins into large peptides. The mucus lining prevents autodigestion of the stomach wall. The resulting semi-digested mass, chyme, enters the duodenum through the pyloric sphincter.

The small intestine is the principal site of both digestion and absorption. In the duodenum, two critical secretions arrive: pancreatic juice and bile. The pancreas delivers a suite of proenzymes — trypsinogen and chymotrypsinogen — which are activated by enterokinase (an intestinal enzyme that converts trypsinogen to trypsin; trypsin then activates chymotrypsinogen). The pancreas also contributes pancreatic lipase, amylase, and nucleases. Bile, produced by the liver and stored in the gallbladder, contains bile salts that emulsify fats into smaller droplets, vastly increasing the surface area for lipase action. Crucially, bile contains no digestive enzymes — it only performs emulsification, a distinction frequently tested in NEET.

The jejunum and ileum possess villi and microvilli (brush border) that amplify the absorptive surface area roughly 600-fold. Brush border enzymes such as maltase, sucrase, lactase, aminopeptidase, and dipeptidases complete digestion to monomers. Absorption occurs by multiple mechanisms: glucose and amino acids are absorbed via active transport into blood capillaries; fatty acids and glycerol are absorbed into lacteals (lymphatic capillaries) and reach the bloodstream via the thoracic duct. Water absorption predominantly occurs in the large intestine via osmosis.

The large intestine features haustra (sacculations) and houses symbiotic bacteria that synthesize certain B vitamins and vitamin K. It absorbs remaining water and minerals, compacts undigested residue into faeces, and expels it through the rectum and anus via defecation.

Hormonal regulation is elegant: gastrin (from stomach G-cells) stimulates gastric juice secretion; secretin (from duodenal S-cells) triggers pancreatic bicarbonate release to neutralize acid; cholecystokinin (CCK, from duodenal I-cells) stimulates bile release and pancreatic enzyme secretion; and gastric inhibitory peptide (GIP) inhibits gastric secretion when fats enter the duodenum.

The key testable concept is the complete enzyme-substrate mapping at each digestive site and the distinction that bile emulsifies fats but is not enzymatic.