Cardiovascular Physiology Concepts
                                    Richard E. Klabunde, Ph.D.

Hydrostatic and Oncotic Pressures

There are two hydrostatic and two oncotic pressures that affect transcapillary fluid exchange. Click on the following links to learn more about these pressures:

    capillary hydrostatic pressure

    tissue (interstitial) hydrostatic pressure

    capillary (plasma) oncotic pressure

    tissue (interstitial) oncotic pressure.

Capillary Hydrostatic Pressure (P_C )

This pressure drives fluid out of the capillary (i.e., filtration), and is highest at the arteriolar end of the capillary and lowest at the venular end. Depending upon the organ, the pressure may drop along the length of the capillary (axial pressure gradient) by 15-30 mmHg. The axial gradient favors filtration at the arteriolar end (where P_C is greatest) and reabsorption at the venular end of the capillary (where P_C is the lowest). The average capillary hydrostatic pressure is determined by arterial and venous pressures (P_A and P_V), and by the ratio of post-to-precapillary resistances (R_V/R_A). An increase in either arterial or venous pressure will increase capillary pressure; however, a given change in P_A is only about one-fifth as effective in changing P_C as the same absolute change in P_V. Because venous resistance is relatively low, changes in P_V are readily transmitted back to the capillary, and conversely, because arterial resistance is relatively high, changes in P_A are poorly transmitted downstream to the capillary. Therefore, P_C is much more influenced by changes in P_V than by changes in P_A. Furthermore, P_C is increased by precapillary vasodilation (particularly by arteriolar dilation); precapillary vasoconstriction decreases P_C.  Venous constriction increases P_C, whereas venous dilation decreases P_C.

Tissue (Interstitial) Hydrostatic Pressure (P_T)

This pressure is determined by the interstitial fluid volume and by the compliance of the tissue, which is related to the ability of the tissue volume to increase.  Normally, P_T is near zero. In some tissues it is slightly subatmospheric, whereas in others it is slightly positive. Tissue compliance is generally low; that is, small increases in tissue volume as occurs during states of enhanced filtration or lymphatic blockage result in dramatic increases in P_T. The rise in P_T that occurs with increased interstitial fluid volume decreases the hydrostatic gradient across the capillary thereby limiting filtration.

Capillary Plasma Oncotic Pressure (P_C )

Because the capillary barrier is readily permeable to ions, the osmotic pressure within the capillary is principally determined by plasma proteins that are relatively impermeable. Therefore, instead of speaking of "osmotic" pressure, this pressure is referred to as the "oncotic" pressure or "colloid osmotic" pressure because it is generated by colloids. Albumin generates about 70% of the oncotic pressure. This pressure is typically 25-30 mmHg. The oncotic pressure increases along the length of the capillary, particularly in capillaries having high net filtration (e.g., in renal glomerular capillaries), because the filtering fluid leaves behind proteins leading to an increase in protein concentration.

Normally, when oncotic pressure is measured, it is measured across a semipermeable membrane – i.e., a membrane that is permeable to fluid and electrolytes but not to large protein molecules. In most capillaries, however, the wall (primarily endothelium) does have a finite permeability to proteins. The actual permeability to protein depends upon the type of capillary as well as the nature of the protein (size, shape, charge).  Because of this finite permeability, the actual oncotic pressure generated across the capillary membrane is less than that calculated from the protein concentration. The effects of finite protein permeability on the physiological oncotic pressure can be determined knowing the reflection coefficient (s ) of the capillary wall.  If the capillary is impermeable to protein then s = 1.  If the capillary is freely permeable to protein, then s = 0. Continuous capillaries have a high s (>0.9), whereas discontinuous capillaries which are very "leaky" to proteins have a very low s .  In the latter case, plasma and tissue oncotic pressures may have a negligible influence on the net driving force.

Tissue (interstitial) Oncotic Pressure (P_T)

The oncotic pressure of the interstitial fluid depends on the interstitial protein concentration and the reflection coefficient of the capillary wall. The more permeable the capillary barrier is to proteins, the higher the interstitial oncotic pressure. This pressure is also determined by the amount of fluid filtration into the interstitium. For example, increased capillary filtration decreases interstitial protein concentration and reduces the oncotic pressure. This effect of filtration on protein concentration also serves as a mechanism to limit capillary filtration. In a "typical" tissue, tissue oncotic pressure is about 5 mmHg (i.e., much lower than capillary plasma oncotic pressure).

Revised 04/05/2007

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.

© 1999-2007 Richard E. Klabunde, all rights reserved.