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
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 from the capillaries, 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
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
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.
- 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) of the
capillary towards the cell.
The other principle called "oncotic pressure" occurs as a result of a limited
permeability of plasma proteins out of the capillary. 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
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:
- 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
- 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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