Hydrostatic pressure is generated by the systolic force of the heart. It pushes water out of the capillaries.
The water potential is created due to the ability of small solutes to pass through the walls of capillaries. This buildup of solutes induces osmosis. The water passes from a high concentration (of water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach an equilibrium. The osmotic pressure drives water back into the vessels. Because the blood in the capillaries is constantly flowing, equilibrium is never reached.
The balance between the two forces differs at different points on the capillaries. At the ARTERIAL END of a vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement (see net flux) favors water and other solutes being passed into the tissue fluid. At the VENOUS END, the osmotic pressure is greater, so the net movement favors substances being passed back into the capillary. This difference is created by the direction of the flow of blood and the imbalance in solutes created by the net movement of water favoring the tissue fluid.
