We have the venuels very tiny veins and then we have the capillaries. Now the capillaries are the blood vessels, very tiny blood vessels that connect the arteries to our veins. And in the capillaries this is where we have the exchange of nutrients for the waste products taking place within our organs and within our tissues. Now most of the time within our organs we have a single system of capillaries but sometimes in certain organs we have a portal system and that simply means we have two consecutive network of capillaries that are next to one another. Now we have several portal systems in our body as we'll discuss in the next several lectures. We have one in our brain, we also have one in the kidneys and our small intestine.