Exchange of Fluid Volume Through the Capillary Membrane
Now that the different factors affecting fluid movement through the capillary membrane have been discussed, it is possible to put all these together to see how the capillary system maintains normal fluid volume distribution between the plasma and the intertitial fluid.
The average capillary pressure at the arterial ends of the capillaries is 15 to 25 mm Hg greater than at the venous ends. Because of this difference, fluid “filters” out of the capillaries at their arterial ends, but at their venous ends fluid is reabsorbed back into the capillar- ies. Thus, a small amount of fluid actually “flows” through the tissues from the arterial ends of the capillaries to the venous ends. The dynamics of this flow are as follows.
Analysis of the Forces Causing Filtration at the Arterial End of the Capillary. The approximate average forces operative at the arterial end of the capillary that cause movement through the capillary membrane are shown as follows:
Thus, the summation of forces at the arterial end of the capillary shows a net filtration pressure of 13 mm Hg, tending to move fluid outward through the capil-lary pores.
This 13 mm Hg filtration pressure causes, on the average, about 1/200 of the plasma in the flowing blood to filter out of the arterial ends of the capillaries into the interstitial spaces each time the blood passes through the capillaries.
Analysis of Reabsorption at the Venous End of the Capillary.
The low blood pressure at the venous end of the cap-illary changes the balance of forces in favor of absorp-tion as follows:
Thus, the force that causes fluid to move into the capillary, 28 mm Hg, is greater than that opposing reabsorption, 21 mm Hg. The difference, 7 mm Hg, is the net reabsorption pressure at the venous ends of the capillaries. This reabsorption pressure is considerably less than the filtration pressure at the capillary arterial ends, but remember that the venous capillaries are more numerous and more permeable than the arterial capillaries, so that less reabsorption pressure is required to cause inward movement of fluid.
The reabsorption pressure causes about nine tenths of the fluid that has filtered out of the arterial ends of the capillaries to be reabsorbed at the venous ends.
The remaining one tenth flows into the lymph vessels and returns to the circulating blood.