The cardiovascular system part of the circulatory system is a closed network of blood vessels through which the blood keeps circulating due to the action of the heart. The blood vessels that carry the oxygenated blood away from the heart to the body organs are called the arteries. The blood vessels that collect the deoxygenated blood and bring it back to the heart are called the veins.

The blood vessels are thick near the heart and divide into smaller arteries and finally into capillaries that are only one cell layer thick. Just to compare, the largest arteries called the aorta and the largest vein called the vena cava are each about an inch in diameter!