Absorption in the Small Intestine

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.

  • Only small, soluble molecules can pass through the wall of the small intestine

Adaptations for Absorption

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:

  • A thin wall – Just one cell thick
  • Millions of villi – Finger-like structures
  • A network of Capillaries

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.

Illustration of a cross-section of intestinal villi, highlighting its features. The walls are depicted as one cell thick, and within each villus, there's a dense network of capillaries. Surrounding the base are prominent blood vessels. The image uses a colour-coded system to distinguish different parts, with pink for the villus walls, green for capillaries, and red and blue for blood vessels. Labels provide further clarification.

  • Remember, one of these finger-like structures is called a villus

Thin wall

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.

Illustration displaying the detailed structure of the small intestine, starting with an outline of the entire organ, progressing to a close-up cross section showing its intricate circular patterns. Further zoom-ins reveal the presence of folds, known as villi, and the cellular structures on the surface of the villi called microvilli. The microvilli are shown as tiny projections on the epithelial cells. The entire visual journey is connected with arrows, guiding from the larger structure down to the cellular level, with accompanying labels for clarity.

  • On the surface of the villi are microvilli, which further increase the surface area.

Network of capillaries

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.

  • However, do not confuse egestion with excretion