The arteries carry blood away from the heart. All arterial vessels, with the exception of the pulmonary artery, carry oxygenated blood, which is blood containing red blood cells that are bound by oxygen. In general, arteries decrease in size as their proximity to the heart decreases. An exception is the coronary arteries, which provide blood to the outer portion of the heart itself. Accordingly, the largest artery is the one leaving the heart – the aorta.
The largest vessels, including the aorta and pulmonary artery, are elastic vessels. The elastic vessels expand when the heart pumps blood into them. The resistance to relax further pushes the blood out into the arterial branches, which are the medium sized arteries branching off into the tissues.
The medium arteries are muscular, containing a layer of smooth muscle in the vessel wall. This muscle layer contracts to push the blood into the peripheral system. The coronary and renal arteries are muscular arteries. The smallest arteries are referred to as arterioles, thin vessels that provide oxygenated blood directly into the tissues and organs. These small vessels are also made of smooth muscle, regulating pressure and flow. The small arterial vessels are the most influential blood vessels because their size and placement in the cardiovascular system determine blood flow into the capillary beds.
The capillary bed is where gas and nutrient exchange occurs between the blood and tissues. This area of tissue contains the smallest blood vessels, the capillaries. Each capillary is one red blood cell-wide, allowing a single blood cell through at a time to exchange oxygen through a thin vessel wall. The capillaries connect the arterial and venous system, returning the deoxygenated blood cells to the heart via the veins.
The venous system carries blood from the tissues to the heart. All veins, with the exception of the pulmonary vein, carry deoxygenated blood. Veins are elastic vessels, but their contraction is somewhat dependent on the muscle system. The skeletal muscles squeeze the venous system during movement, helping push the blood through them. A lack of skeletal muscle movement can result in venous pooling and clots, particularly in larger vessels and at vessel junctions. Blood flow through the venous system also depends on the arterial blood pressure and the integrity of the vessel walls in the system. Backflow in the veins is prevented by valves in the vessels, similar to their use in the heart.
As the venous system approaches the heart, the vessels increase in size. The largest veins are those that empty into the heart - the vena cava. The superior vena cava serves the head, neck, and arms; the inferior vena cava serves the torso and lower extremities and is the largest of the two.