Oncotic pressure, or colloid osmotic pressure, is a form of osmotic pressure exerted by proteins, notably albumin,
in a blood vessel's plasma (blood/liquid) that usually tends to pull water into the circulatory system.
It is the opposing force to hydrostatic pressure.

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.