Blood vessels are a set of tube-like structures which carry blood around the body. The three types of blood vessels are arteries, veins and capillaries.
An artery is a blood vessel that carries blood away from the heart.
Arteries have a thick muscular wall which contains a lot of elastic tissue. This allows the artery to expand a little as the blood passes through (you feel this as a pulse where the artery is close to the skin surface), but the muscle and elastic allow it to keep its shape to maintain the high pressure needed for efficient blood flow. A small lumen (central hole) also helps to maintain the high pressure.
Pulmonary artery Edit
The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, where the blood is oxygenated.
A vein is a blood vessel that carries blood towards the heart.
Veins have thin walls and a large lumen. This allows blood to flow back to the heart at low pressure. Veins also contain valves at regular intervals - these close as the blood flows past to prevent blood flowing backwards. Not having these valves would cause problems as there is very little pressure to force the blood back to the heart.
Veins are usually situated between muscles so that when they contract, the blood is pushed along. Smaller venules join together to make veins.
The vena cava is the largest vein, carrying oxygenated blood to the right atrium of the heart. The superior vena cava carries blood from the upper body whereas blood from the lower body is transported to the heart by the inferior vena cava.
Pulmonary vein Edit
The pulmonary vein carries oxygenated blood from the lungs, where oxygen enters, to the left atrium of the heart.
Subclavian vein Edit
The subclavian vein is a paired vein that joins on to the superior vena cava.
A capillary is the smallest of all the blood vessels which carries blood close to every cell in the body, and connects the arteries and veins.
Capillary walls are permeable and one cell thick to allow for easy diffusion of substances such as oxygen, carbon dioxide and glucose. The lumen is only just wide enough to allow red blood cells to pass through. Blood pressure in the capillaries is very low.