Some absorption occurs in the mouth itself, some in the stomach but most absorption occurs in the intestine.
1. In Mouth
Minute quantities of water, water-soluble vitamins and simple sugars like glucose (as in honey) are absorbed in the mouth.
2. In Stomach
Water, glucose, ethanol (alcohol), certain minerals, vitamins and certain drugs may be absorbed into the cells lining the stomach. This absorption occurs by osmosis, diffusion (down the concentration gradient) and active transport (against concentration gradient).
3. Small Intestine
Most absorption of digested food occurs in small intestine. For this, the small intestine is adapted in many ways:
- It is very long and therefore provides more surface area for absorption.
- Many folds in its wall called villi (singular, villus) further increase the surface area of absorption.
- Single cell eptithelial lining further reduces the distance between the food and underlying blood vessels.
- The epithelial cells have microvilli which are projections of plasma membrane to further increase the absorptive surface.
- It is narrow for slow movement of nutrients allowing absorption.
Products absorbed into the blood capillaries of the villi are amino acids and monosaccharides (glucose, fructose, galactose).
Products absorbed into the lacteals (lymph vessels) of the villi are fatty acids and glycerol.
Nutrients absorbed into the blood is carried by veins into the liver, and the nutrients absorbed by the lacteals (small lymph vessels) enters the lymphatic system.
4. Large Intestine
Most of the water present in the food is absorbed in the colon by diffusion. Some mineral ions are absorbed by the colon through active transport.