Digestion in human beings

A long tube

A meal of rice, dal and vegetables has big, complex molecules. The body cannot use them as they are. They must be broken into small molecules that dissolve and pass into the blood. This is digestion.

Food passes along the alimentary canal, a long tube from the mouth to the anus. Each part does a job, and glands add juices with enzymes. Enzymes are proteins that speed up the breaking of food.

The mouth

• The teeth crush the food, and the tongue mixes it with saliva.
• Saliva, from the salivary glands, has the enzyme salivary amylase. It breaks starch into a simple sugar.

Chew a piece of bread for a few minutes, and it starts to taste sweet. That is salivary amylase at work.

The oesophagus

The food tube from the mouth to the stomach is the oesophagus. Its walls squeeze in waves, one after another, and push the food along. These waves are called peristalsis. They go on along the whole canal.

The stomach

The stomach is a muscular bag that churns the food. Gastric glands in its wall make three things.

• Hydrochloric acid makes the food acidic, so that pepsin can work. It also kills many germs.
• Pepsin is an enzyme that breaks down proteins.
• Mucus covers the lining, so that the acid does not harm the stomach itself.

A ring of muscle, a sphincter, lets the food leave the stomach a little at a time.

The small intestine

The small intestine is the longest part of the canal, folded many times. Here digestion is finished. It gets juices from the liver and the pancreas, and makes its own.

• Bile from the liver (stored in the gall bladder) has no enzyme. It makes the acidic food alkaline, and breaks fat into tiny drops, as soap does. This is called emulsifying the fat.
• Pancreatic juice has trypsin, for proteins, and lipase, for the emulsified fats.
• Intestinal juice finishes the work. Proteins become amino acids, complex carbohydrates become glucose, and fats become fatty acids and glycerol.

Animals that eat grass, such as cows, have a longer small intestine than animals that eat meat. Grass has cellulose, which takes a long time to digest.

Absorbing the food

The inner wall of the small intestine has millions of finger-like folds called villi (one is a villus). They give a very large surface to take in the digested food. Each villus has many blood vessels. The blood carries the food to every cell, where it gives energy, builds new tissue and repairs old tissue.

You should find that with $20$ villi on each mm², each $0.6$ mm tall, the lining is almost $5$ times the flat wall.

The large intestine

What is not absorbed passes into the large intestine. Its wall takes back more water. The rest is waste. It leaves the body through the anus, and a sphincter there controls when. Removing this undigested waste is called egestion.

Try it in Python

This program follows a meal along the canal, and prints the end product of each kind of food.

Practise with new numbers

Common mistakes

• Thinking bile is an enzyme. It has no enzyme. It makes the food alkaline and breaks fat into drops.
• Thinking digestion happens only in the stomach. It starts in the mouth and ends in the small intestine.
• Thinking the large intestine absorbs most of the food. Food is absorbed by the villi of the small intestine. The large intestine takes back water.
• Mixing up egestion and excretion. Egestion removes undigested food. Excretion removes wastes made by the cells.

Remember these

• Saliva has salivary amylase: starch to sugar.
• The stomach makes hydrochloric acid, pepsin for proteins, and mucus to protect its lining.
• Bile emulsifies fats. Pancreatic juice has trypsin and lipase. Intestinal juice finishes digestion.
• End products: amino acids, glucose, and fatty acids with glycerol.
• Villi give a large surface for absorption. The large intestine takes back water.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which enzyme in the mouth acts on starch, and what does it make?

2. Give two jobs of the hydrochloric acid in the stomach.

3. Bile has no enzyme. What does it do?

4. Why is the small intestine of a cow longer than that of a tiger?

5. How do villi help the small intestine?