Water Moves Across CapiIIaryWaIIs Both by Diffusion (Osmosis) and by Bulk Flow
The exchange of water between the capillary plasma and the interstitial fluid merits special consideration for two reasons. First, the forces that govern water movement are more complicated than the simple diffusive forces that affect solute movement.
Second, a particular imbalance in these forces causes an excessive amount of water to accumulate in the interstitial space, which leads to the important clinical sign, edema.As the preceding discussion emphasized, solutes such as oxygen, carbon dioxide, glucose, electrolytes, and fatty acids move between the capillary plasma and the interstitial fluid by diffusion. Water also moves by diffusion; the diffusional movement of water is called osmosis. The physical prerequisites for osmosis are (1) the presence of a Semipermeable membrane (a membrane that is permeable to water but not to specific solutes) and (2) a difference in the total concentration of impermeable solutes on the two sides of the membrane.
The capillary wall constitutes a Semipermeable membrane. Water readily passes through the capillary pores; however, in most organs the capillary walls are impermeable to the plasma proteins. (The molecules of albumin, globulin, and the other plasma proteins are slightly too large to pass through the pores in most capillaries.) The normal concentration of plasma proteins is 7 grams per deciliter (g∕dL) within the capillary plasma but only 0.2 g∕dl. in the interstitial fluid. The higher protein concentration within the capillaries creates an osmotic imbalance. Water molecules tend to move through osmosis from the interstitial fluid into the capillary blood plasma. (Remember, when water moves through osmosis, it moves toward the side of the membrane with the higher concentration of impermeable solute.)
The tendency for water to move through diffusion is quantified by osmotic pressure (see Chapter 1).
The normal osmotic pressure created by the proteins in the plasma is 25 mm Hg; that is, the osmotic effect of the plasma proteins is equivalent to a pressure of 25 mm Hg driving water into the capillaries. The osmotic pressure created by the plasma proteins is also called plasma oncotic pressure or colloid osmotic pressure. (The term colloid is used because the plasma proteins are not in a true solution but rather in a colloidal suspension.)Recall that a low concentration of plasma proteins is normally found in the interstitial fluid. The oncotic pressure created in the interstitial fluid by these proteins is normally only about 1 mm Hg. On balance, the osmotic effect of the capillary fluid is much greater than the osmotic effect of the interstitial fluid, so water would move through osmosis from the interstitial fluid into the capillaries. The movement of water in this direction is called reabsorption. The movement of water in the opposite direction, from the capillary plasma into the interstitial fluid, is called filtration.
The net oncotic pressure difference favors reabsorption. Net oncotic pressure difference is calculated by subtracting the oncotic pressure of interstitial fluid from the oncotic pressure of capillary blood (e.g., 25 mm Hg - 1 mm Hg = 24 mm Hg).
In addition to being affected by diffusional (osmotic) forces, water responds to hydrostatic pressure differences across the capillary wall. Hydrostatic pressure differences cause water to move by bulk flow; in this case the bulk flow occurs through the capillary pores. The hydrostatic pressure within the capillaries (capillary blood pressure) is higher at the arteriolar end of capillaries than at the venous end (see Figure 22-1). However, a representative average capillary hydrostatic pressure would be about 18 mm Hg. Interstitial fluid hydrostatic pressure is normally about -7 mm Hg. (The negative sign simply means that interstitial fluid pressure is lessy but only slightly less, than atmospheric pressure.) The negative interstitial fluid pressure (-7 mm Hg) together with the positive capillary hydrostatic pressure (18 mm Hg) creates a hydrostatic pressure difference of 25 mm Hg across the wall of a typical capillary.
This hydrostatic pressure difference tends to force water out of the capillaries and into the interstitial spaces; that is, the net hydrostatic pressure difference favors filtration.The net hydrostatic pressure difference (which favors filtration) almost balances the net oncotic pressure difference (which favors reabsorption). However, the balance is rarely perfect. Usually, the hydrostatic pressure difference slightly exceeds the oncotic pressure difference, so there is a small, net filtration of water out of the capillaries. This water would simply accumulate in the interstitial spaces and cause swelling there if not for the lymph vessels, which collect excess interstitial fluid and return it to the bloodstream through the subclavian veins (Figure 23-4).
The hydrostatic pressures in the capillaries and in the interstitial fluid are, by convention, always measured relative to atmospheric pressure. Thus, to say that interstitial pressure is normally “negative” does not imply that a vacuum exists but only that the interstitial pressure is slightly below atmospheric pressure. If the interstitial spaces of the body were always pressurized more than atmospheric pressure, all parts of the body would bulge outward. The Subatmospheric interstitial fluid pressure probably accounts for the fact that the skin normally stays snug against the underlying tissue and that some body surfaces normally have a concave shape (e.g., axillary space, orbits of the eyes).