Because ultimately, it's a difference in pressure that allows the movement of our fluid from point A to point B. And the two types of pressures we have discussed are hydrostatic pressure as well as osmotic pressure. So let's begin with hydrostatic pressure. So as our blood plasma, as our fluid moves along our blood vessel, along the capillary, that fluid exerts a force and it pushes against the walls of the capillary. And this is what we call the hydrostatic pressure. Hydro simply means water, because our blood plasma is predominantly water.