Absorption in the small intestine

By the time food reaches the main part of the small intestine the large food molecules have been digested into their smaller nutrient molecules by enzymes.

starch sugar

fat fatty acids and glycerol

protein amino acids

These smaller molecules are called the products of digestion. They are now small enough to pass through the tiny holes in the wall of the small intestine into the bloodstream (this is absorption). Other nutrient molecules such as vitamins and minerals, which are small molecules already and do not need to be digested, also pass into the bloodstream.

The inner wall of the small intestine has many tiny projections called villi. They increase the rate at which nutrients are absorbed into the bloodstream. To help speed up absorption they have the following features:

• large surface area - there are so many of them
• thin wall - made of a single layer of cells
• capillary network - each villus has its own blood supply to absorb nutrients.

Any materials not absorbed into the bloodstream remain in the small intestine and are eventually pushed into the large intestine.

Villi in the small intestine

Large intestine

The material entering the large intestine cannot be broken down by the digestive enzymes and so can provide no further nutrients. It consists mainly of vegetable fibre, cellulose and a large amount of water from the digestive juices which were poured onto the food earlier in the digestive system.

Most of the water is absorbed through the wall of the large intestine back into the bloodstream for use by the body. This leaves a semi-solid waste called faeces. Faeces are stored in the rectum before being expelled from the body through the anus (this is egestion).