Hydrostatic and Colloid Osmotic Pressures
- Hydrostatic pressure pushes fluid out. Think of a water hose with holes. The water pressure in the hose pushes the water out of the holes.
- Colloid osmotic pressure keeps fluid in. Think of pieces of sponge floating inside the water hose that keep the water from leaking out as fast.
Hydrostatic pressure is the pushing power of circulating blood and fluid, i.e. fluid pushes itself through the normal leaks in the small spaces between the thin cells of the capillary walls. Then the fluid makes its way into the cells/tissues outside of the capillaries.
Colloid osmotic pressure is the opposite. It is exerted by the protein albumin. Albumin is normally too large to pass through the capillary walls, so it stays inside the capillaries. The structure of albumin is such that it pulls fluid towards itself, like a sponge would pull water. This effect of albumin is colloid osmotic pressure. This keeps too much fluid from leaving the capillaries.
Capillary hydrostatic pressure and plasma colloid osmotic pressure are the major pressures that determine filtration and absorption of fluid by capillaries.