Describe the structure of arteries, veins and capillaries and understand their roles.
The artery is carrying blood in high blood pressure. The walls of the artery is fairly thick, containing muscles. The lumen would be narrow which carries the blood. The muscle contracts and we can see this 'maintains' the blood pressure.
The capillary is the sight of exchange. The capillary walls are very thin (one cell thick) this capillary will have a diameter of a red blood cells.
The vein returns the blood to the heart under low pressure. So although the veins are large, the vein will have a large lumen so the pressure is kept low (low resistance). We can also find that they have valves to stop blood from flowing down.