Digestion is the breaking down of complex food components, such as starch, proteins and fats, into simpler forms that the body can absorb and use. In humans it happens in the alimentary canal, from the mouth to the anus.

What are the parts of the human digestive system?

The alimentary canal (mouth, food pipe or oesophagus, stomach, small intestine, large intestine, rectum and anus) and two glands linked to it, the liver and the pancreas, which send their juices into the small intestine.

How do birds digest food without teeth?

Birds have a muscular chamber called the gizzard. Its walls contract and relax to grind the food, often helped by small stones, or grit, that the bird swallows.

Nutrition and Digestion in Animals

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Why food must be digested

Food gives animals the energy and materials for all their life processes. But most food is made of complex components, carbohydrates, proteins and fats, that the body cannot use directly. Digestion breaks them into simpler forms. In humans this happens step by step in the alimentary canal, a long tube that begins at the mouth and ends at the anus.

The journey of food from the mouth to the anus, with the work done at each stage and the liver and pancreas joining the small intestine — Mouth to anus

In the mouth

The teeth crush and chew food into fine pieces; this is mechanical digestion. Saliva moistens the food so it is easy to swallow and contains a juice that breaks starch into sugar. Thinking of your favourite food makes more saliva flow, and chewing a piece of chapati for a minute makes it taste sweet.

You can test this (Activity 9.1). Put plain boiled rice in test tube A and well-chewed boiled rice in test tube B, add water and a few drops of iodine solution. Iodine turns starch blue-black.

Test tube A with boiled rice turns blue-black; test tube B with chewed rice shows little or no blue-black — The action of saliva on starch

Tube A turns blue-black, but tube B stays almost unchanged, because saliva has already broken its starch into sugar. Chewing longer leaves even less starch.

Down to the stomach

The tongue pushes the soft food into the food pipe (oesophagus). Its walls contract and relax in a wave that moves the food down; the same kind of movement pushes food along the whole canal. The stomach churns the food and mixes it with a secretion of digestive juice, which breaks down proteins, acid, which helps and kills many harmful bacteria, and mucus, which protects the stomach lining. The food becomes a semi-liquid mass. Much of this was learnt after 1822, when Dr William Beaumont watched digestion through a gunshot wound in Alexis St. Martin’s stomach that never fully closed.

The small intestine

Though called small, it is about 6 m long, the longest part of the canal. Three juices act here: bile from the liver, which is mildly basic, neutralises the acid and breaks fats into tiny droplets; pancreatic juice, also basic, which works on carbohydrates, proteins and fats; and the juice of the intestine wall, which completes digestion. The inner lining has thousands of finger-like projections that give a huge surface for absorption, the passing of nutrients into the blood. In celiac disease, gluten, a protein in wheat, barley and rye, damages this lining; millets such as jowar, bajra and ragi contain no gluten and are good substitutes.

The large intestine

It is about 1.5 m long but wider than the small intestine. It absorbs water and some salts, leaving semi-solid stool, which is stored in the rectum and passed out through the anus, a process called egestion. Fibre-rich food such as fruits, vegetables and whole grains keeps it working well, and helpful bacteria living there break down fibre; fermented foods such as curd and buttermilk support them.

Cows and birds

A ruminant’s steps: chew a little, partial digestion, back to the mouth, chewed again, swallowed again; a bird’s gizzard grinds food with grit — Other ways to digest

Cows and buffaloes chew grass partly and swallow it. After partial digestion in the stomach, the food returns to the mouth and is chewed slowly again, rumination, before going on; a cow chews for about 8 hours a day. Birds have no teeth; the walls of their gizzard grind food, often with swallowed grit.

Why it matters

Knowing how digestion works explains healthy habits: chewing well, eating fibre, brushing and cleaning the tongue, and eating at regular times without overeating. Continue with respiration in animals. Answers to the textbook questions are in the NCERT solutions for Chapter 9; see all of Chapter 9 notes.

Nutrition and digestion in animals