Now, because of the limited size of these pores, we basically prevent the movement of larger things such as red blood cells across the wall of the capillary. So we see that the wall of the capillary is in fact a semipermeable membrane. So now we know how this movement actually takes place. It takes place through these tiny pores and holes we call junctions and slits that exist between our endothelial cells of the capillary wall. Now, the next question is what causes this movement of blood plasma across the wall of the capillary? And in what direction does this fluid actually move?