Section 11. Organisation and maintenance of the organisms
Topic 6.3. Human nutrition Topic 6.3.6. Absorption By: Mr.Murad A. A.

Key objectives
To be able to: * Define absorption * Identify the small intestine as the site of absorption * Describe the significance of villi in absorption * Describe the structure of a villus * Describe the roles of villi capillaries and lacteals in absorption * State the role of the hepatic portal vein in the transport of the absorbed food to the liver * Identify the role of the small intestine and the colon in the absorption of water * Describe absorption of digested fats

i. Definition of absorption Movement of digested food molecules and soluble nutrients through the epithelial lining of the intestine into the blood or lymph.

ii. Site of absorption
Throughout the whole of the alimentary canal but mostly in the ileum, as the ileum is structurally adapted to this function in the following ways: * It is fairly long so providing large absorption surface area. * Rich in blood capillaries so providing steep diffusion of digested food into the blood * -------------------------------------------------
Its epithelial lining is one-cell thick so providing short diffusion distance * -------------------------------------------------
Its epithelial lining is folded into villi and micro villi so increasing the internal surface area.

Notice that the ileum epithelial lining is the surface across which absorption takes place. That is why it has been modified as shown below. iii. The structure of the villus:-

* Teacher’s tips: * Notice that the blood capillaries are situated between the muscular wall of the ileum and its epithelial lining * The ileum epithelial lining is modified into villi and microvilli.

iv. Structural adaptations of villi for absorption

* -------------------------------------------------
The outer membrane of the epithelial cells, forming the villi walls, is elongated into microvilli, so increasing surface area for absorption. * ------------------------------------------------- The villi epithelial cells are rich in mitochondria, so ensuring high energy production for the active uptake of glucose Teacher’s tips:- Use these two structural adaptations to explain how villi cells are structurally adapted for absorption:- * Villi are rich in blood capillaries so maintaining a steep concentration gradient between the ileum and the blood needed for, * Efficient absorption of:- 1. Glucose by active transport. 2. Amino acids, minerals and vitamins by diffusion. 3. Water by osmosis.

* Villi have a supply of lacteals (vessels of the lymphatic system) that absorbs fatty acids and glycerol.

Teacher’s tips:-
Notice that digested fats aren’t absorbed directly into the blood, but by the lymphatic system which eventually transports it to the blood circulation.

v. The role of the hepatic vein
It transports the absorbed digested food and soluble nutrients (except fatty acids and glycerol) to the liver, where excess blood glucose is converted and stored as glycogen and excess amino acids are deaminated into urea and glucose, which is also stored in the liver cells as glycogen.

This explains why the blood composition of glucose and amino acids are higher in the hepatic portal vein than in the hepatic vein.