CAPILLARY EXCHANGE AND

EDEMA

Understanding swelling of part(s) of the body…

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Capillary Exchange There are approximately 10 billion

capillaries in the body. The major means by which nutrients and waste products are exchanged is by the process of diffusion..

In addition, a small amount of fluid moves out of the capillary at the arteriolar end, and most, but not all, of that fluid reenters the capillary at its venous end.

At the arterial end of the capillary, blood pressure and a small negative pressure in the interstitial spaces move fluid from the capillary.

The small negative pressure exists in the interstitial spaces because fluid moves into lymph capillaries when tissues are compressed during movement.

When the tissues are no longer compressed, the volume of the tissue in creases slightly, and the interstitial pressure decreases

. One-way valves in the lymphatic vessels prevent the fluid from passing back from the lymph capillaries into the interstitial spaces.

Fluid is attracted into the capillary by osmosis. The concentration of small solute molecules is approximately the same in blood and in the interstitial fluid.

The concentration of proteins in the interstitial fluid, however, is much lower than in the plasma, and the proteins in the plasma are too large to pass through the wall of the capillary.

The osmotic pressure caused by the plasma proteins is called the blood colloid osmotic pressure, and it is much higher than the osmotic pressure of the interstitial fluid.

Edema is an abnormal accumulation of interstitial fluid. Edema has many causes. The underlying problem in all types of edema is a distur bance in the normal balance between hydrostatic and os motic forces at the capillary level.

1. When a capillary is damaged, 2. In acute starvation, 3. Edema results from an increase in arterial blood

pressure, Edema can also result from problems with other systems, such as the blockage of lymphatic vessels or impaired urine formation:

If the lymphatic vessels in a region become blocked, the volume of interstitial fluid will continue to rise.

If the kidneys are unable to produce urine but the individual continues to drink liquids, the blood volume will rise.

Venous tone

This is a continual state of partial contraction of the veins as a result of sympathetic stimulation.

Increased sympathetic stimulation increases venous tone by causing constriction of the veins, which forces the large venous volume to flow toward the heart.

Consequently, venous return and preload will in crease, causing an increase in cardiac output.

Conversely, decreased sympathetic stimulation decreases venous tone, allow ing veins to relax and dilate.

As the veins fill with blood, venous return to the heart, preload, and cardiac output decrease.

The periodic muscular compression of veins The periodic muscular compression of veins forces blood to flow through them toward the forces blood to flow through them toward the heart more rapidly. heart more rapidly.

The valves in the veins prevent flow away The valves in the veins prevent flow away from the heart so that when veins are from the heart so that when veins are compressed, blood is forced to flow toward compressed, blood is forced to flow toward the heart. the heart.

The combination of arterial dilation and The combination of arterial dilation and compression of the veins by muscular compression of the veins by muscular movements during exercise causes blood to movements during exercise causes blood to return to the heart more rapidly than under return to the heart more rapidly than under conditions of rest.conditions of rest.