Arteries are the passageways through which the blood circulates, the largest of which is the aorta. The aorta branches off the heart and divides into many smaller arteries called arterioles, which have muscular walls that adjust their diameter to increase or decrease blood flow to a particular body area. Arteries and arterioles carry oxygenated blood from the lungs to the rest of the body.
Capillaries are thin walled, highly branched vessels that feed the tissues and collect wastes and carbon dioxide to be carried back to the lungs, liver, or kidney for elimination. Capillaries empty into the venules, which in turn drain into the veins that lead back to the heart, which in turn pumps deoxygenated blood to the lungs to pick up more oxygen, and then back to the heart once again.
Arteries and veins have the same basic structure as they are made up of three layers of tissues: 1) elastic tissue called tunica intima; 2) smooth muscle, tunica media; 3) connective tissue, tunica adventitia. The walls of arteries contain smooth muscle fibre that contract and relax under the instructions of the sympathetic nervous system. Arterioles are also under the control of the sympathetic nervous system, as they constrict and dialate to regulate blood flow. Capillary walls are only one cell thick, which allows exchanges of material between the contents of the capillary and the surrounding tissue.