Discuss the functions and structures of the 3 types of blood vessels.

The 3 types of blood vessels in the body are arteries, veins and capillaries. Arteries carry oxygenated blood away from the heart to the rest of the body. Arteries need to maintain a high blood pressure in order to get this blood around the body, so they have thick muscular walls and smaller internal lumens. Veins, however, carry deoxygenated blood back into the heart from around the body. Veins carry blood at a lower pressure, so they have thin walls and larger internal lumens. This low blood pressure means that blood can sometimes flow backwards, so veins have valves to prevent this from happening. Lastly, capillaries are the microscopic vessels found in the muscles and lungs in which gas exchange takes place. Gas exchange enables oxygen in the blood to pass through the capillary walls into the tissues while carbon dioxide passes from the tissues into the blood. In order for gas exchange to be efficient, the capillary walls must be very thin (one cell thick) and they must have a very low blood pressure.