What are the functions of blood vessels? How do materials get into and out of our blood vessels? Information on blood vessels.
Blood carries food and oxygen to every living cell in the body and brings back nitrogen wastes, carbon dioxide, and other materials given out by the cells. Of course, all these materials must be in some form that dissolves before they can get into the blood or out of it. Digested food is absorbed into the blood through the linings of the small intestine and the stomach. Oxygen gets into the blood and carbon dioxide gets out of it through the lungs.
If you pour some salt or sugar into a little water, it dissolves. Af ter a while, all the water tastes salty or sweet. So the salt or sugar has spread through the water in some way. According to the molecular theory, this is what happens. When a material dissolves, each molecule separates from other molecules of the same kind. At first, the molecules are fairly close together. But as they keep bouncing about, they move into the spaces between the water molecules and spread through the water. When a material spreads from one place to another by the movement of the separate molecules, we say that the material diffuses. Only dissolved materials can move by diffusion.
Without diffusion, your cells could not stay alive. Blood carries dissolved materials to and from your cells. These dissolved materials diffuse into and out .of the blood vessels. However, the larger blood vessels have such thick walls that diffusion would be much too slow. So the dissolved materials diffuse between the blood and the cells through the capillaries, which have very thin walls. In fact, the walls of a capillary are onlyone cell thick.
Dissolved materials always diffuse from where there is more of them toward where there is less. Food and oxygen are being used up in the cells, while the blood flowing through the capillaries nearby contains more food and oxygen. So the food and oxygen in the blood diffuse through the walls of the capillaries into the jellylike material called protoplasm of which the cells are made. Protoplasm is not a liquid, but materials can dissolve in it much as they do in blood. In a similar way, nitrogen wastes, carbon dioxide, and other materials formed in the cells diffuse from the protoplasm through the walls of the capillaries into the blood.