The organs of digestion fall into two groups.

• Oral cavity — intake & mechanical breakdown
• Pharynx — shared passage for food and air
• Esophagus — the muscular delivery chute
• Stomach — mixing vat & protein digestion
• Small intestine — most digestion & absorption
• Large intestine — water recovery & waste
• Anus — the controlled exit

• Teeth and tongue
• Salivary glands
• Liver, gallbladder & pancreas

From the esophagus onward, the tract wall is built from the same four concentric layers, or tunics. Knowing them once means understanding the entire tube:

• Mucosa — innermost lining; secretes & absorbs
• Submucosa — connective tissue; vessels & nerves
• Muscularis — smooth muscle; mixes & propels
• Serosa — the protective outer wrap

The Mouth — Mechanical Headquarters

Chewing (mastication) is the body's most powerful mechanical tool. By fracturing food into smaller pieces, it dramatically increases the surface area available for enzymes to work on later. Your 32 adult teeth are specialized for the task:

• Incisors — chisel-edged, for cutting
• Canines — pointed, for tearing
• Premolars & molars — broad, for grinding

The tongue repositions food and mixes it with saliva, produced by three pairs of salivary glands — the parotid, submandibular, and sublingual. Saliva moistens food into a swallowable mass and delivers salivary amylase, an enzyme that begins breaking down starch on contact.

The Swallow & the Chute

Swallowing (deglutition) begins as a voluntary act and finishes as an involuntary reflex you couldn't stop if you tried. The tongue pushes the food mass — now a bolus — into the pharynx. There, a flap of cartilage called the epiglottis folds down to seal the airway, ensuring food heads toward the stomach rather than the lungs.

The bolus then enters the esophagus, which doesn't rely on gravity. Instead, rhythmic waves of muscular contraction called peristalsis squeeze the bolus downward. At the bottom, the lower esophageal sphincter opens to admit food, then closes to keep corrosive stomach contents from washing back up.

The stomach is a muscular reservoir that stores food, churns it, and launches protein digestion. Its folds, called rugae, flatten out to let the organ expand after a large meal.

Microscopic gastric pits release hydrochloric acid, which makes the stomach intensely acidic, and pepsin, the enzyme that dismantles proteins. A thick mucus coating shields the stomach's own lining from this acidic environment. The end product is chyme — a soupy, partially digested mixture released gradually into the small intestine.

The Small Intestine — Absorption Engine

Despite its name, the small intestine is the longest part of the tract and the site of most digestion and absorption. It has three regions:

• Duodenum — receives chyme, bile & pancreatic juice
• Jejunum — the primary absorption zone
• Ileum — completes absorption

Its real genius is surface area. The lining is folded into ridges (plicae circulares), carpeted with fingerlike villi, and each villus cell is topped with microscopic microvilli — the "brush border." Simple sugars and amino acids pass into blood capillaries, while digested fats enter lymphatic vessels called lacteals.

The Large Intestine — Recovery & Removal

What enters the large intestine is mostly water and indigestible residue. Its main job is to reabsorb water and electrolytes, compacting the remainder into formed waste. It runs from the cecum (with its attached appendix) through the four segments of the colon — ascending, transverse, descending, and sigmoid — and finishes at the rectum. Its muscular wall gathers into pouches called haustra, drawn up by ribbon-like muscle bands, the teniae coli.