Question

How do hydrostatic & osmotic pressure combine to determine whether fluid enters or leaves capillaries?

Answer 1

Volumes of a fluid move from an area of higher pressure from a capillary bed to an area of lower pressure in the tissues through filtration. The movement of fluid from an area of higher pressure from the tissues into an area of lower pressure in the capillaries is called reabsorption. There are two types of pressure interact to drive each of these movements: hydrostatic pressure and osmotic pressure.

Hydrostatic pressure : The primary force which has driven the fluid in between the capillaries and tissues is hydrostatic pressure that can be defined as the pressure of any fluid enclosed in a space. Blood hydrostatic pressure (BHP) is the force exerted by the blood confined within the blood vessel and the pressure exerted by blood against the wall of a capillary is called capillary hydrostatic pressure (CHP). CHP is the force which driven fluid out of capillaries and into the tissues.

Osmotic pressure : The net pressure that drives reabsorption. One is the movement of fluid from the interstitial fluid back into the capillaries is called osmotic pressure and whereas hydrostatic pressure forces fluid out of the capillary and osmotic pressure draws fluid back in. Osmotic pressure is determined by osmotic concentration gradients which are the difference in the solute-to-water concentrations in the blood and tissue fluid. A region higher in solute concentration (which is lower in water concentration) draws water across a semipermeable membrane from a region higher in water concentration (which is lower in solute concentration).