Absorption is the movement of the small, soluble products of digestion from the gut into the blood and lymph. Most absorption occurs in the small intestine, especially the ileum.
Adaptations of the ileum
The ileum is highly adapted for absorption:
- It is very long, giving time for absorption.
- Its inner wall is folded and covered with millions of finger-like villi (and microvilli), giving a huge surface area.
- Each villus has a thin, one-cell-thick epithelium, so nutrients cross quickly.
- Each villus has a network of blood capillaries and a central lacteal (a lymph vessel).
Key idea
Glucose and amino acids are absorbed into the blood capillaries. Fatty acids and glycerol are absorbed mainly into the lacteal.
How nutrients are absorbed
Some substances move by diffusion down a concentration gradient. Others, such as glucose and amino acids, are absorbed by active transport, which uses energy from respiration to move them even against the gradient. Blood from the capillaries drains into the hepatic portal vein, which carries nutrients to the liver.
The large intestine
Water and some mineral salts that were not absorbed earlier are absorbed in the colon (large intestine). This helps to conserve body water and turns the remaining material into semi-solid faeces.
Absorption and the earlier stages are closely linked. Digestion must first produce small molecules; these are then absorbed across the villi; and only afterwards can the cells assimilate them. If the ileum wall is damaged, fewer nutrients are absorbed even when digestion is normal, which can lead to weakness and poor growth.
Example
After a meal, glucose absorbed by active transport in the ileum travels through the hepatic portal vein to the liver, which controls how much glucose enters the general circulation.
Remember
Villi = large surface area + thin wall + good blood supply. These three features make the ileum ideal for absorption.