# 56 Absorption, small intestine and significance of villi

Absorption is the movement of digested food molecules through the wall of the intestine into the blood or lymph. 

Digestion is completed in the small intestine. By now, most carbohydrates have been broken down to simple sugar, proteins to amino acids, and fats to fatty acids and glycerol. These molecules are small enough to pass through the wall of the small intestine and into the blood. This is called absorption. The small intestine is especially adapted to allow absorption to take place very efficiently. 

It has a very rich blood supply. Digested food molecules are small enough to pass through the wall of the intestine into the bloodstream. Water, mineral salts and vitamins are also absorbed in the small instestine. 

The small intestine absorbs 5-10 dm³ of water each day. How ever, the colon absorbs much less water and salt than the small intestine, generally around 0,3–0,5 dm³ per day.

Significance of Villii 

Villi are finger like projections that increase the surface area for absorption. If a section of small intestine was turned inside out, its surface would be kike a carpet. Inside each villus are:

- Blood capillaries: absorb amino acids and glucose.

- Lacteals: absorb fatty acids and glycerol.

Food molecules are absorbed:

 - mainly by diffusion.

 - or by active transport.

Epithelial cells contain mitochondria to provide energy for absorption against the concentration gradient.

Role of the hepatic portal vein 

The hepatic portal vein transports absorbed food from the small intestine to the liver. After a meal, the blood in this vein contains very high concentrations of glucose and amino acids, as well as vitamins and minerals. The liver reduces levels backs to normal.