What is the difference between oncotic and hydrostatic pressure?
This is a tricky topic for many people!
Essentially, hydrostatic pressure pushes out while osmotic pressure pulls in. In the capillaries hydrostatic pressure increases filtration by pushing fluid and solute OUT of the capillaries, while capillary oncotic pressure (also known as colloid osmotic pressure) pulls fluid into the capillaries and/or prevents fluid from leaving. Hydrostatic pressure is based on the pressure exerted by the blood pushing against the walls of the capillaries, while oncotic pressure exists because of the proteins - like albumin, globulins and fibrinogen - that do not leave the capillary and draw water.
The same forces also act on the interstitial fluid.