escapes from the blood vessels into the tissue fluid. Colloid cannot diffuse back into the
blood vessels, but it can enter the lymphatics and be carried away. If it were not for the
lymphatics, colloid would accumulate in the tissue fluid and begin to hold water. The
lymphatics and lymphatic fluid, therefore, help control the volume of tissue fluid.
d. Edema. A swelling of tissue due to an excess of tissue fluid is called edema.
The cells and structures within the tissue are greatly spread apart in this condition. The
amount of stretching from within is different in various tissues. Tissue edema is rather
selflimiting in that when a certain point is reached, the hydrostatic pressure of the fluid
in the stretched tissue is almost as great as that of the capillaries. Because of this, the
formation of tissue fluid is almost stopped. As the tissue swells, the lymphatic vessels
are pulled apart to keep them open. Some of the causes of edema are as follows:
(1) Increased hydrostatic pressure in blood capillaries. A buildup of
hydrostatic pressure in the capillaries is almost always due to some obstruction to the
free drainage of blood into the veins and back to the heart.
(2) Lymphatic obstruction. Since some of the tissue fluid is drained off by
the lymphatics, any obstruction to the lymphatic system would lead to an increase in
(3) Insufficient colloid in the blood. The fact that fluid is absorbed at the
venous end of capillaries depends on the increased osmotic pressure of the blood.
Since the increase of osmotic pressure in the blood is due to the colloid content, any
depletion of the colloids would result in a lower osmotic pressure. The colloids of the
blood are proteins. Protein starvation would deplete the colloids and result in edema. A
draining off of proteins also occurs in certain diseases of the kidneys and proteins are
passed out in the urine. Large amounts of protein can also be lost by their seeping
away from large injured areas.
(4) Increased permeability of blood capillary endothelium. Endothelial
membranes are composed of living tissues. Because they are living, they keep colloids
from diffusing into the tissue fluid. If capillaries become injured, they will permit colloids
to escape into the tissue fluid. When colloids leak out of the capillaries, they raise the
osmotic pressure of the tissue fluid until it becomes the same as that in the capillaries.
When they become equal, tissue fluid is not returned at the venous ends of the
capillaries and tissue fluid builds up. If capillary injury is over a large area, a condition
develops known as surgical shock. There is only a certain amount of fluid in the vessels
of the circulatory system. It is possible, with a massive injury, to deplete the circulatory
system to the point where it can no longer function. As the plasma continues to escape,
there is less and less fluid, with the result that the chambers of the heart do not fill
properly between heart contractions. The result is collapse of the circulatory system