Final answer:
The capillary hydrostatic pressure (CHP) is the force that acts to move fluid out of the blood and into the tissue space. This is counteracted by osmotic pressure which draws fluid back into the capillaries, and the balance of these pressures is regulated by the net filtration pressure.
Step-by-step explanation:
The force that acts to move fluid out of the blood and into the tissue space is called the capillary hydrostatic pressure (CHP). CHP is generated by the heart's pumping force and is the primary force driving fluid out of the capillaries and into the tissues. This movement of fluids is part of a process known as filtration, which is balanced by osmotic pressure that draws fluid back into the capillaries. The net filtration pressure (NFP), which is the difference between the capillary hydrostatic pressure and the blood colloidal osmotic pressure, determines the direction and amount of fluid flow.
However, in the tissues, the hydrostatic pressure in the interstitial fluid increases, known as the interstitial fluid hydrostatic pressure (IFHP), but is typically lower than the CHP due to the lymphatic system which absorbs excess fluid. The balance between hydrostatic and osmotic pressures ensures that there is an equilibrium between the fluid entering and leaving the tissue spaces, preventing issues like swelling.