How does the digested food particles go into the body cells? In this lesson, we will learn about the third stage of the digestive system, absorption. To check the previous lesson on digestive system, click here.
Absorption is the process whereby digested food substances are absorbed into the body cells.
Products of digestion such as simple sugars, amino acids, fatty acids, glycerol are absorbed throughout the small intestine especially the ileum. The absorbed nutrients pass through the small intestine into the bloodstream. Water and mineral salts are absorbed by the small intestine and the colon. Most of the water, about 90%, is absorbed in the colon. But how does the small intestine is adapted to perform this function?
Adaptations of Small Intestine:
There factors which determine the rate of absorption;
The surface area of the small intestine ---- the larger the surface area, the greater rate of absorption.
The membrane that separates the food substances from the blood capillaries --- the thinner membrane will allow the food molecules to pass through it in a shorter time.
The concentration gradient of individual substances.
Factor 1:
The inner surface of the small intestine is folded. These folding bear numerous finger-like projection called villi. The epithelial cells of these villi, have in turn numerous microvilli to increase the surface area.
Factor 2:
The villi have thin walls and cell membranes.
Factor 3:
The small intestine is long enough (6 meters) to provide sufficient time for absorption. In each villus is a lymphatic capillary which is responsible for transporting fats while the blood capillaries transport sugars and amino acids. The continual transport of digested food substances maintain the concentration gradient for the absorption of digested food substances.
How does absorption take place in the intestines (ileum)?
Glucose and amino acids are absorbed by diffusion into the blood. However active transport is also required for the absorption. When the concentration of glucose in the lumen of the small intestine is low than the concentration of glucose in the blood capillaries the with the active transport, food is absorbed into the blood from the lumen. Glycerol and fatty acids diffuse into the epithelium. Here they combine to form minute fat globules which enter the lacteals (lymphatic capillaries).
What happens to the indigested matter?
The undigested matter (fiber, bile salts, and other substances) are stored temporarily in the rectum, before it is discharged as faces through the anus. The removal of undigested matter from the body is called egestion.
In the next lesson we will learn about assimilation of these absorbed food substances.
*All pictures are taken from google*
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