The Human Digestive System

The alimentary canal

The alimentary canal (or digestive tract) is a long, continuous tube that runs from the mouth to the anus. Food passes through it in one direction, pushed along by peristalsis, waves of muscle contraction in the wall of the canal, and is broken down step by step as it travels through each organ.

Organs and their roles

Associated digestive glands

• The salivary glands in the mouth produce saliva containing salivary amylase, which begins to digest starch.
• The liver produces bile, which emulsifies fats into smaller droplets so lipase can act on them faster.
• The gall bladder stores bile from the liver and releases it into the small intestine when needed.
• The pancreas produces pancreatic juice, containing amylase, protease and lipase, which is released into the small intestine to digest carbohydrates, proteins and fats.

How it is examined

Questions on this standard often give a diagram of the alimentary canal and ask you to name each labelled organ or gland, describe the path food takes from the mouth to the anus, or match each organ to its main function. You should be able to distinguish organs that are part of the canal itself from glands that add digestive juices into it.

How food is moved through the gut

Food does not fall through the gut under gravity; it is pushed along by peristalsis. The wall of the alimentary canal has two muscle layers, circular muscle and longitudinal muscle, that contract in waves behind the food, squeezing it forward in one direction from the mouth towards the anus.

This is why a person can swallow and move food along even while lying down or standing on their head: peristalsis, not gravity, does the work, and the one-way waves stop food from slipping backwards.

Rings of muscle called sphincters control where food stops and starts, for example at the exit of the stomach. Along the way, glands add digestive juices and the walls absorb the products, so each region has its own job: the stomach stores and churns, the small intestine digests and absorbs, and the large intestine reclaims water.

The five stages of nutrition

Nutrition in humans is described in five stages. Ingestion is taking food into the mouth.

Digestion is breaking it down, both physically and chemically. Absorption is the passage of the small, soluble products through the gut wall into the blood and lymph, mostly at the small intestine.

Assimilation is the use of those products by body cells for energy, growth and repair.

The final stage is egestion, removing undigested material as faeces through the anus. Egestion is not the same as excretion: egested material has never entered the body's cells, while excretion removes waste made by the cells themselves, such as urea.

Listing all five stages in order, ingestion, digestion, absorption, assimilation, egestion, is a quick way to secure the opening marks of a structured question on nutrition.

Common mistakes to avoid

Worked exam-style question

Question. A diagram shows the human alimentary canal with organs labelled P (stomach), Q (small intestine) and R (large intestine), plus a gland S (pancreas). (a) State one function of P.

(b) Explain why Q is described as the main site of absorption. (c) Is S part of the alimentary canal?

Give a reason.

Model answer. (a) P, the stomach, churns food into a semi-liquid and begins protein digestion using pepsin and hydrochloric acid. (b) Q, the small intestine, is where chemical digestion is completed and where digested food is absorbed into the blood and lymph; it is long and its lining is folded into villi, giving a large surface area for absorption.

(c) No, S, the pancreas, is a gland, not part of the canal, because food does not pass through it; it releases pancreatic juice into the small intestine.

Practice question

Try it. Arrange these organs in the order food passes through them, then state which one absorbs most water: small intestine, mouth, large intestine, stomach, oesophagus.

Exam tip

Key terms

• Digestion, breakdown of food into absorbable molecules along the canal.
• Peristalsis, waves of muscle contraction that push food forward.
• Villus, a finger-like fold that gives the small intestine a large surface area.
• Enzyme, a catalyst in the digestive juices that breaks down food.
• Assimilation, the use of absorbed nutrients by the body's cells.

Source:SRC-DSKP-EN

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