Human Alimentary Canal: Structure & Function

The alimentary canal (gut) is one continuous, muscular tube running from the mouth to the anus. Food moves in one direction and passes through five main jobs:

Process	What it means	Where it mainly happens
Ingestion	Taking food into the mouth	Mouth
Digestion	Breaking large insoluble food molecules into small soluble ones	Mouth, stomach, small intestine
Absorption	Soluble food passing into the blood	Small intestine (ileum)
Water absorption	Water passing from the gut into the blood	Large intestine (colon)
Egestion	Removing undigested food (faeces)	Rectum → anus

The pancreas is not part of the tube — it is a gland that sits nearby and adds digestive enzymes to the small intestine.

Mouth (buccal cavity)

• Ingestion — food is taken into the body here.
• Teeth chew food into smaller pieces — this is mechanical digestion (increases surface area).
• Saliva moistens food and contains an enzyme that starts the chemical digestion of starch.
• The tongue rolls food into a soft ball (a bolus) ready to swallow.

Oesophagus (gullet)

• A muscular tube connecting the mouth to the stomach.
• No digestion happens here.
• Food is squeezed downwards by waves of muscle contraction called peristalsis.

Stomach

• A muscular bag that churns food and mixes it with gastric juice.
• Makes hydrochloric acid, which provides the acidic pH that stomach enzymes need and kills many harmful microbes in food.
• Makes protease enzymes that begin digesting proteins.
• Food is held here for a few hours, then released slowly into the small intestine.

Exam tip. The oesophagus is a common trap — its job is transport by peristalsis, not digestion. Save "acid kills microbes" for the stomach.

The small intestine is the longest part of the canal and has two named regions:

Duodenum (first part)

• Digestion is completed here.
• Receives enzymes from the pancreas and bile that help break down food fully.
• Carbohydrates, proteins and fats are broken into small soluble molecules.

Ileum (second part)

• This is where absorption happens.
• The soluble products of digestion (e.g. glucose, amino acids) pass through the wall into the blood.
• The ileum is long and its inner wall is folded into tiny finger-like villi, giving a huge surface area for fast absorption.

Pancreas

• A gland lying near the stomach (not part of the food tube).
• Makes digestive enzymes (to break down carbohydrates, proteins and fats).
• Releases these enzymes into the duodenum.

Remember the split. Duodenum = digest, Ileum = absorb. Both are parts of the small intestine — water absorption is the job of the large intestine.

By now most useful food has been absorbed. What remains is mostly water and undigested material (e.g. fibre). The large intestine finishes the job:

Colon

• Absorbs water from the remaining material back into the blood.
• This makes the contents more solid, forming faeces.

Rectum

• Stores the faeces.

Anus

• Faeces are removed from the body through the anus. This removal of undigested food is called egestion.

Egestion is NOT excretion.

• Egestion = removing undigested food (faeces) that never entered the body's cells — the colon and rectum do this.
• Excretion = removing waste made by the body's own reactions (e.g. carbon dioxide, urea) — different organs do this.

Exam tip. If you write "the colon excretes faeces," you lose the mark. Faeces are egested, because the food was never part of a metabolic reaction.