Digestion and Absorption

Syllabus Unit: Human Physiology

Why it matters

Chapter overview

The digestive system breaks down complex food into absorbable monomers and moves them across the gut wall into blood and lymph. NEET tests this topic as a tight chain: the anatomy of the alimentary canal and its glands, the specific enzymes that act at each step, the mechanism by which the products cross the intestinal epithelium, and the common clinical disorders that arise when any link in that chain fails.

Biology Syllabus Human Physiology 20%–22% weightage

Live Animation1 / 4

1MouthMastication + amylase

2StomachChurning + pepsin, HCl

3Small IntestineDigestion + absorption

4Large IntestineWater absorption

1Digestive System Anatomy

A single continuous tube from mouth to anus, supported by three accessory glands that pour digestive secretions into it.

Learning Objectives

Sequence the parts of the human alimentary canal from mouth to anus.
Name the three pairs of salivary glands and the enzyme saliva contains.
Identify the three accessory digestive glands and the secretion each contributes.
Describe the four histological layers of the gut wall (mucosa, submucosa, muscularis, serosa).

Key Terms

Alimentary CanalThe continuous digestive tube: mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum, anus.

VilliFinger-like projections of the small intestine mucosa that vastly increase surface area for absorption.

ChymeThe semi-fluid, acidic mass of partly digested food released from the stomach into the duodenum.

SphincterA ring of muscle (e.g. pyloric, ileocaecal) that controls one-way passage of contents between gut regions.

The human digestive system has two parts: the **alimentary canal**, a single ~9 m tube running mouth to anus, and the **accessory glands** — salivary glands, liver and pancreas — that secrete digestive juices into it without food ever passing through them.

The alimentary canal, in order

Mouth (buccal cavity) — teeth and tongue for mastication; opens into the pharynx.
Oesophagus — a muscular tube that moves food to the stomach by peristalsis; no digestion occurs here.
Stomach — a J-shaped bag in the upper left abdomen; secretes gastric juice and churns food into chyme.
Small intestine (duodenum → jejunum → ileum) — the main site of digestion and absorption, ~6 m long with villi.
Large intestine (caecum → colon → rectum) — absorbs water and electrolytes, forms and stores faeces.

The accessory glands

Salivary glands — three pairs (parotid, sub-mandibular, sub-lingual); secrete saliva containing salivary amylase (ptyalin) that starts starch digestion.
Liver — the largest gland in the body; hepatocytes secrete bile, stored and concentrated in the gallbladder before release into the duodenum.
Pancreas — a heterocrine gland; its exocrine acinar cells secrete pancreatic juice (enzymes for all three food classes) into the duodenum via the pancreatic duct.

Key PointThe common bile duct from the liver/gallbladder and the pancreatic duct usually join and open together into the duodenum at the hepatopancreatic ampulla (of Vater), guarded by the sphincter of Oddi.

Gut wall: four layers, same order throughout

Mucosa — innermost; secretory and absorptive epithelium, folded into villi in the small intestine.
Sub-mucosa — connective tissue with blood vessels, lymphatics and nerves.
Muscularis — inner circular and outer longitudinal smooth muscle producing peristalsis.
Serosa — outermost thin membrane covering the gut.

Common MistakeNEET trap: the ileo-caecal valve prevents backflow from the large intestine into the small intestine — direction of flow questions are common.