The movement of fluids between cellular compartments is regulated by osmotic and hydrostatic forces.
Hydrostatic pressure is the force exerted by a fluid against a wall which causes movement of fluid between compartments. This pressure is important for exchanging plasma and nutrients between capillaries and surrounding tissues and also in the nephrons (kidneys) where ensures proper filtering of the blood to form urine. Fluid also moves between compartments along an osmotic gradient (the difference in concentration of solutes on one side of the cell membrane to that on the other side). Water constantly moves into and out of fluid compartments via osmotic gradient.