So if we block the semipermeable membrane with some type of impermeable barrier, for example, our hand, we will be applying a pressure that will ultimately stop osmosis from taking place. So the pressure that must be applied to stop the natural flow of water down its osmotic potential gradient is called osmotic pressure. So notice that the water tries to move this way, but our osmotic pressure points in the opposite direction because what it does is it ultimately stops the water from actually moving across this membrane. Now, what about hydrostatic pressure? What exactly is hydrostatic pressure? Well, hydrostatic pressure is slightly easier to actually explain because hydrostatic pressure is nothing more than fluid pressure.