Blood Vessels

Blood leaving the left side of the heart is loaded with oxygen from the lungs. When it leaves the left ventricle, the blood passes into a large blood vessel known as the aorta. The aorta is the first of a series of blood vessels that carry the blood on its round trip through the body and back to the heart.   As blood flows through the circulatory system, it moves through three types of blood vessels—arteries, capillaries, and veins.

Arteries Large vessels that carry blood from the heart to the tissues of the body are called arteries. Arteries are the superhighways of the circulatory system. Except for the pulmonary arteries, all arteries carry oxygen-rich blood. Arteries have thick walls that help them withstand the powerful pressure produced when the heart contracts and pushes blood into the arteries. The figure at right shows that the walls contain connective tissue, smooth muscle, and endothelium. The elastic connective tissue allows an artery to expand under pressure. Contractions of the smooth muscle regulate the diameter of an artery.

Blood Vessels

Capillaries The smallest of the blood vessels are the capillaries. Capillaries are the side streets and alleys of the circulatory system. The walls of capillaries are only one cell thick, and most are so narrow that blood cells must pass through them in single file. The real work of the circulatory system—bringing nutrients and oxygen to the tissues and absorbing carbon dioxide and other waste products from them—is done in the capillaries.

Veins Once blood has passed through the capillary system, it must be returned to the heart. This is the job of the veins. As with arteries, the walls of veins contain connective tissue and smooth muscle. Large veins, such as those shown in the leg in the figure below, contain valves that keep blood moving toward the heart. Many veins are located near and between skeletal muscles. When you exercise, contracting these muscles helps force blood through the veins. Blood flow through the veins of the arms and legs often occurs against the force of gravity. Exercise helps to keep blood from accumulating in the limbs and stretching the veins out of shape. If the walls around the veins weaken from lack of activity, the valves can weaken. This causes blood to pool in the veins, producing a condition known as varicose veins.

Veins Contraction of skeletal muscles helps move blood in veins toward the heart.