So when the blood leaves the arteries, it goes through the artrioles (this is obviously the arteriole end of the capillary due to the high pressure, the blood is forced out of the capillaries as tissue fluid. This is the hydrostatic pressure.
To go back into the capillaries, I am not 100% sure about this but the venule end of the capillary has a much lower hydrostatic pressure than the arteriole end. Due to the blood and tissue fluid having more solutes in them they have a lower water potential than in the capillaries (theyve also lost alot of water) so they go down the water potential gradient back into the capillaries. This is the oncotic pressure.
Like I said Im not sure if its 100% true, pls correct me if it isnt.