Question

Explain the concept of net filtration pressure. Describe the two main forces that drive the direction of fluid flow as you move across a given capillary. How do these forces change from the arterial end to the venous end of the capillary?

Answer 1

Net filtration pressure is the total pressure gradient which causes fluids to filter across a capillary.

it depends on

a: capillary hydrostatic pressure which tends to move fluid out of the capillary.(CHP)

b: capillary oncotic pressure which tends to move fluid into the capillary(COP)

By the time blood is in capillary, bp has fallen to 35mmHg. It slows down further within capillary to reach 18mmHg on the venous end. This contributes to the CHP. Oncotic pressure is due to the solutes in blood and tends to remain constant over the length of capillary i.e, 25mmHg. Net filtration pressure is the difference betwwen CHP and COP which changes from 10mmHg(pushing water out) on arterial side to -7mmHg(sucking water in, sign specifies direction of fluid flow) on venous side