Exchange in the Gut
Exchange in the Gut is VERY important.
The food you eat is broken down in your Gut, and it forms simple sugars such as Glucose, Amino Acids, Fatty Acids and Glycerol.
But these aren't any good if they stay in your gut, they would just be passed out of your body into your Faeces
Exchange in the Gut 2
The molecules from your food have to be made available to your cells. In cells they provide fuel for respiration and the building blocks of all the tissues in your body.
To do this, they need to get out of your small intestine, and into the bloodstream.
This happens via a combination of Active Transport and Diffusion
Exchange in the Gut 3
Food needs to be broken down into a soluble form during digestion, as Diffusion can only take place in when molecules are dissolved in water.
The digested food molecules are then small enough to pass through the walls of the Small Intestine into the blood vessels
They move in this direction as there is a very high concentration of these food molecules in the Small Intestine and a low concentration in the blood vessels, so it is moving along a Steep Concentration Gradient
Exchange in the Gut 4
The lining of the Small Intestine is folded into thousands of tiny finger like projections, called Villi
These greatly increase the uptake of digested food molecules by diffusion, as only a certain number of food molecules can diffuse over a given surface area of gut lining at any one time
Like the lungs, the lining of the Small Intestine has a great bloody supply, which can carry away all the digested food molecules to the rest of the body, as soon as it has diffused across. This maintains a Steep Concentration Gradient at all times.
Exchange in the Gut
Diffusion is not the only way digested food molecules leave the gut into the blood. As the food moves along the Small Intestine and the time between the last meal gets longer, the concentration gradient gets lower and lower, until there is less food molecules in the gut than the blood
So that none of the food molecules are wasted, they are moved into the blood vessels by Active Transport