During digestion, enzymes break down large food molecules into smaller molecules. For example, pepsin is a protease enzyme that starts the digestion of proteins in the stomach.
After being broken down, the smaller molecules are absorbed into the bloodstream in the small intestine.
Most of the digestive process takes place in the small intestine. All the nutrients from the food we consume pass through the wall of the small intestine and into the bloodstream. The walls of the small intestine produce digestive juices that contain enzymes. These enzymes are biological catalysts that break food down into smaller molecules.
Some food molecules are too large to pass through the wall of the small intestine. These food molecules are broken into smaller molecules by enzymes. Once broken down, the molecules will be small enough to pass through the wall of the small intestine and into the bloodstream.
For complete digestion and absorption of nutrients, there must be a large enough area in contact with the nutrients.
The small intestine is very long so it provides a large surface area for the absorption of digested food molecules. Also, the small intestine’s inner wall has adaptations that cause substances to pass across it at a faster rate.
These adaptations are:
The diagram below shows how the villi in the small intestine help increase the surface area for absorption. Remember that each individual finger-like structure is called a villus.
The small intestine has a wall that is just one cell thick, and the villi also have a thin membrane. This means there is a short diffusion path across the intestinal wall.
Having a thin wall ensures that digestive molecules are absorbed before they pass out of the body. If the interior wall of the small intestine was thick, this would not be the case.
The interior wall of the small intestine is covered in millions of finger-like structures called villi. The villi give the small intestine a large surface area.
There is a network of capillaries running through the villi, which gives them a good blood supply. These capillaries carry away the products of digestion that have been absorbed.
The bloodstream quickly removes the products of digestion. This maintains a concentration gradient between the contents of the small intestine and the bloodstream. An increase in the concentration gradient increases the rate of diffusion across the intestinal wall.
Any molecules that cannot be absorbed by diffusion are absorbed across by active transport, which requires energy.
The small intestine absorbs most of the water back into the bloodstream in the large intestine. Then the colon, which is a part of the large intestine, absorbs most of the remaining water. This leaves behind a semi-solid substance called faeces, which is stored in the rectum, the last part of the large intestine.
When going to the toilet, the faeces are expelled from the body through the anus. We call this process egestion.