living membrane that separates blood from the tissues. This semipermeable membrane
permits the passage of water and crystalloids (see note below) from the blood, but does
not permit the plasma proteins to pass through. Since most of the cells of the body lie
outside of blood vessels, there must be a means of providing all of the cells with food
materials and relieving them of their waste materials. The walls of the arteries are too
thick to allow materials to diffuse through them. Arteries carry blood to the capillaries,
and the thin walls of the capillaries allow water, food materials, and oxygen to nourish
the individual cells. The relationship between the intercellular substance and the tissue
fluid is different in different parts of the body. At some sites, where the intercellular
substance is a sol, the tissue fluid is the medium in which the colloidal amorphous
substance is dispersed. At other sites the amorphous substance exists as rigid gels.
A crystalloid is a substance whose particles are small enough to pass through
A colloid is similar to a solution; the suspended particles are too large to pass
through an animal membrane but they are still so small that they do not settle
A sol is a colloid system in which the particles are suspended in a liquid.
b. Formation of Tissue Fluid. Arteries are under a great deal of hydrostatic
pressure, which would be an ideal means for pushing fluid out through the cellular wall.
However, the walls of the arteries are so very thick, because they are under so much
pressure, that no material can diffuse through them. In fact, these walls are so thick
that the outer side must be fed by fluid from an outside source. Arteries feed into
arterial capillaries. The walls of these capillaries are so very thin that the hydrostatic
pressure within their arterial ends is sufficient to drive fluid out through the endothelium.
The arterial ends of capillaries are the most important source of tissue fluid in the body.
c. Absorption. If tissue fluid were only produced and not absorbed by some
mechanism, the body would swell enormously. There are two mechanisms that
together absorb tissue fluid at the same rate that it is produced.
(1) Venous ends of capillaries. The hydrostatic pressure of the capillaries
must be great enough to force fluid out when another factor is seeking to draw tissue
fluid back into the capillary. The attraction that blood has for tissue fluid is due to the
fact that the osmotic pressure of blood is slightly higher than that of the tissue fluid.
Hydrostatic pressure is greater at the arterial side of the capillary so fluid is forced out at
this point. At the venous end of the capillary, the osmotic pressure of the blood is
greater than the hydrostatic pressure of the capillary; thus, tissue fluid is absorbed.
(2) Lymphatics. That portion of tissue fluid that is not absorbed by blood
capillaries is collected by the lymphatics, and once it has gained entrance here, it is
known as lymph. These lymphatic vessels drain into larger vessels and eventually into
two main trunks that return all the lymph into large veins near the heart. A little colloid