What are Capillaries Anyway?

The capillaries are the network of very thin walled vessels that allow for the rapid exchange of molecules (i.e. molecules of oxygen, molecules of nutrients, molecules of sodium). They are situated where the arteries and the veins come together.

  • This part of the circulatory system receives blood from the heart through the arteries, which reduce down to the smaller arterioles and then to the even smaller metaarteriles.

It is here where molecules are exchanged between the blood and the interstitial fluid. The interstitial fluid is the liquid that has been forced by pressure and has gathered in the tissue spaces, and surrounds the cells of the tissues of the body.

Most of the bodies cells are within 0.02 mm (a very short distance) of a capillary. Because they are so close and because they are only one cell thick, molecules can quickly move by diffusion between them and the cell.

They do not contain any smooth muscle as does the rest of the circulatory vascular system. However there are, in some cases, small precapillary sphincters at the beginning of the capillaries that do contain smooth muscle and which help determine blood flow through them.

  • Contraction and relaxation of these sphincters causes a pumping motion and allows for a sporadic flow of blood into these thin walled vessels.

When we increase our activity the way we do when we exercise, vasodialation of these sphincters will greatly increase the blood flow to the muscles.

The capillary wall is made up of endothelial cells. These flat thin cells join at their edges. Small pores called clefts allow for the passage of molecules in and out of these vessels. Because there are many more capillaries than arteries, the surface area is very large and the pores that the molecules go in and out of are numerous.

With this increased surface area the velocity of the blood is also greatly reduced allowing diffusion to work properly. For example, the average velocity of blood flow in these vessels is about 1 cm/second compared to 40 cm/second at the aorta.

In addition, the size and number of these pores vary greatly from tissue to tissue. In the brain, for example, they may not contain any pores at all, while the kidney tissues contain relatively large pores so that large molecules can pass easily through the pores.

There are three ways that molecules cross these walls.

Diffusion. (See transport diagram below) The largest amount of molecular exchange occurs at these pores by diffusion. (Diffusion means to spread out or permeate.)

Water flows freely in both directions and there is little net gain or loss of water by this method in the blood. 

However, there is a principle called "concentration gradients." Substances within a liquid in a dissolved state tend to disperse themselves evenly throughout the liquid. That means that heavier concentrations of a substance within a fluid will disperse themselves throughout the fluid until there is equal concentrations all over.

This is called osmosis. Therefore, oxygen, glucose, other nutrients, minerals and hormones coming from the arterial side (arteries) of the circulatory system in heavier concentrations will pass through the membrane dissolved in water and will move towards the cell where there is less of a concentration of that particular substance. This is diffusion of lipid-insoluble substances. Diffusion of lipid-soluble substances will also pass through the pores or clefts by osmosis.

These substances then enter the cell and are utilized. Metabolism of these substances within the cell results in the production of carbon dioxide and other metabolic wastes, which are then removed from the cell. Because of the concentration (conc.) gradient of wastes, they are moved toward the capillary where they are dispersed into the venous system (veins) and are carried away.

Relationship of a capillary and a cell
  • Pinocytosos. (See transport diagram above) This process allows the movement of large lipid-insoluble proteins that are too large to pass through the pores by diffusion to be able to move from the capillary to the cell. This actually accounts for a very small portion of the overall exchange and is done by vesicles that pinch off from the plasma membrane, engulf the molecule, and then transport it through the vessel wall to the interstitial fluid.
  • Bulk Flow and Oncotic Pressure. (See transport diagram above) In addition to a concentration gradient, there is also a pressure gradient. The pressure, though very small because of the increased area, assists in the movement of the fluids from the into the interstitial areas. This accounts for a facilitation of movement from the arterial side (arteries) towards the cell. The other principle called "oncotic pressure" occurs as a result of a limited permeability of plasma proteins. Because of this, most plasma proteins cannot leave the vessel and therefore create a concentration gradient that favors fluid movement back into the capillary. The oncotic pressure exactly balances the hydrostatic pressure so that there is no net water loss from the blood.
  • It is important that the body be kept healthy for this network to function properly.

    The ability to get the right stuff to the cells and to remove everthing that should be removed, can be altered by:

    • disease
    • heart malfunction
    • circulatory depletion
    • nutrient or hormonal deficiencies
    • excessive waste products in the blood
    • many other factors
    • THUS, in order to do our part in helping our bodies not have to work so hard:

    • We need to eat healthy and properly.
    • We need to include natural things in our diets such as organic foods and herbs.
    • We need to keep our bodies strong and flowing properly through exercise.
    • We need to develop a healthy and happy lifestyle with the elimination of any and all detrimental substances from the body.
    • We need to eliminate stress whenever possible.
    • We need to remove anything that has harmful side effects on the body.

    We could go on and on but the point is well made. We need to do what we need to do to be as healthy as naturally as possible.

    Remember the Italian proverb which states,
    "He who enjoys health is rich, though he knows it not."

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