Name the 3 types of blood vessels in the human circulatory system and discuss how they are adapted to carry out their functions.

The three types of blood vessels in the human circulatory system are arteries, veins and capillaries. Arteries are the vessels that transport blood away from the heart and are therefore adapted to the transport of blood under high pressure through the presence of thick, elastic walls and a narrow lumen which allows for a small blood passage. Secondly, veins are the structures that carry blood to the heart under low pressure, and hence have thinner walls and a larger lumen to allow for a larger blood passage. For the same reason, they also contain valves to prevent backflow and ensure that blood flows in only one direction. Finally, capillaries are the vessels that deliver nutrients and oxygen to body tissues and remove waste products. Since capillaries are the sites for exchange of oxygen, carbon dioxide and nutrients, in order to facilitate diffusion in and out of their structures, they are only one cell thick and have thin, selectively permeable walls.