In fact, it's so thin that it only consists of a single endothelial layer. And between the endothelial cells of our capillary wall, we have very tiny junctions, very tiny slits. And these slits and junctions act as pores and allow the movement of the fluid, the blood plasma, along with the molecules dissolved inside the blood plasma across the wall of the capillary. 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.