Digestion

Core Concepts

Mechanical vs. chemical digestion

Mechanical digestion is physical breakdown: teeth tear and grind, the stomach churns, and the small intestine's contractions mix contents. It increases surface area so chemicals can work faster — think of shredding paper before burning it. Chemical digestion uses enzymes and acids to break molecular bonds: starches into sugars, proteins into amino acids, fats into fatty acids and monoglycerides. The two proceed side by side at every stage.

The mouth: where it begins

Digestion begins at the first bite. Teeth (mastication) grind food; the tongue positions and tastes it; saliva moistens the food into a soft mass called a bolus and begins starch breakdown through salivary amylase. Poorly chewed food asks the stomach and intestines to do extra work downstream.

The esophagus: a one-way expressway

Swallowing moves the bolus through the pharynx into the esophagus; the epiglottis closes over the airway to keep food out of the lungs. Waves of muscle contraction called peristalsis push the bolus downward. At the bottom, the lower esophageal sphincter (LES) opens to admit food and closes afterward; when it relaxes at the wrong time, stomach contents reflux into the esophagus — the familiar "heartburn."

The stomach: storage, mixing, and protein breakdown

The stomach is a muscular reservoir that stores a meal, churns it into a soupy mixture called chyme, and begins protein digestion. Glands release hydrochloric acid, which kills many microbes, denatures proteins, and activates pepsinogen into pepsin — the enzyme that splits proteins. The stomach also produces intrinsic factor, needed later for vitamin B12 absorption. Contrary to popular belief, the stomach does very little absorbing: water, alcohol, and a few small molecules pass through its walls, but most nutrients are absorbed downstream.

The small intestine: the main event

The small intestine — duodenum, jejunum, then ileum — is where digestion is completed and absorption mostly occurs. The duodenum receives bile (from the liver via the gallbladder), which emulsifies fat into tiny droplets so enzymes can attack it, and pancreatic juice, containing enzymes for starch, protein, and fat plus bicarbonate to neutralize acidic chyme. The intestinal wall is lined with finger-like villi and microscopic microvilli, multiplying surface area enormously; rhythmic segmentation contractions keep contents in contact with the absorbing surface. End products — monosaccharides, amino acids, fatty acids, and monoglycerides — cross into the blood or the lymphatic system.

Accessory organs: liver, gallbladder, pancreas

These three organs are not part of the tube but are essential to it: the liver produces bile; the gallbladder stores and concentrates it and releases it when fat arrives; the pancreas produces digestive enzymes and bicarbonate. Each gets full treatment in topic 3 — the key idea here is that digestion depends on secretions arriving from outside the tract.

The large intestine: water recovery and waste

By the time chyme reaches the large intestine (cecum, colon, rectum), most nutrients are absorbed; what remains is fiber, water, electrolytes, and unabsorbed material. The colon reabsorbs water and electrolytes, forming stool, and hosts trillions of microorganisms (the gut microbiota) that ferment fiber, producing gases, short-chain fatty acids, and some vitamin K and B vitamins. Slow peristalsis moves contents toward the rectum; when the rectum stretches, the defecation reflex signals the need to eliminate, and the person decides when and where.

Common Confusions