- The small intestine is the region where digested food is absorbed.
- Most absorption happens in the ileum. This is the longest part of the small intestine and is between 2-4 metres long.
- The small intestine has a large internal surface area for absorption to happen quickly and efficiently.
- Glucose and amino acids pass into the bloodstream.
- Fatty acids and glycerol pass into the lacteals connected to the lymphatic system.
- Water is absorbed in both the small intestine and the colon, but the most absorption of water happens in the small intestine.
The ileum is efficient in the absorption of digested food:
- It is fairly long and presents a large absorbing surface to the digested food.
- Its internal surface is greatly increased by circular folds bearing thousands of tiny projections called villi. These villi are about 0.55mm long and may be finger-like or flattened in shape.
- The lining epithelium is very thin and the fluids can pass rapidly through it. The outer membrane of each epithelial cell has microvilli, which increase by 20 times the exposed surface of the cell.
- There is a dense network of blood capillaries (tiny blood vessels) in each villus.
- Network of blood capillaries – transports glucose and amino acids away from the small intestine in the blood
- Internal structure called a lacteal – transports fatty acids and glycerol away from the small intestine in the lymph
- The hepatic portal vein transports absorbed food from the small intestine to the liver.
- Digested and undigested foods have different outcomes once they have passed through the alimentary canal (gut).
Assimilation is the movement of digested food molecules into the cells of the body where they are used.
- Glucose is used in respiration to provide energy.
- Amino acids are used to build new proteins.
- Fats are built into cell membranes and other cell structure and is also an important source of energy for cell metabolism.
The small intestine absorbs most of the water in the contents of the gut. By the time the contents reach the end of the small intestine, most of the digested food has also been absorbed.
The remaining material consists of:
- bacteria (living and dead)
- cells from the lining of the gut
- indigestible substances – such as cellulose from plant cell walls
The colon is the first part of the large intestine. It absorbs most of the remaining water. This leaves semi-solid waste material called faeces. The faeces are stored in the rectum, the last part of the large intestine. Egestion happens when these faeces pass out of the body through the anus.