Cardiovascular System Health Article

The arteries and arterioles

Blood leaving the heart from either the left or right ventricle enter a network of vessels called the arteries. Arteries are highly elastic vessels, having flexible fibers in their structure and a relatively thick layer of smooth muscle. Larger arteries have three layers—the inner (intima), the middle (media), and the outer (adventitia). Blood flows through the central opening, known as the lumen, which is lined with endothelial cells. The layers of the blood vessels interact to exert major control over blood pressure and where the blood flows. The adventitia contains the nervous control and blood vessels for the arteries, the media contains smooth muscles, and the endothelial layer of the intima is important for sensing environmental changes.

The aorta, the largest artery, branches directly off the left ventricle, and is especially elastic because of the addition of cardiac muscle cells in the area where it branches off the heart. The elastic qualities of arteries are important so that they can expand to receive the blood volume under high pressure, and contract to continue forcing the blood into the rest of the circulatory system. The elasticity of the arteries is a significant component of the blood pressure during diastole, when the ventricles of the heart relax.

From the left ventricle the coronary arteries, which supply blood to the heart itself, emerge from the aorta. Then the aorta makes a large U-turn in the chest, eventually becoming the abdominal artery. Major branches to the head (carotid arteries), arms (axillary arteries), and legs (femoral arteries) come off this one vessel. The flow of blood in the arteries is pulsile, increasing and decreasing with each heartbeat, about 70 times per minute. The flow of blood in the branch arteries accounts for the pulse that can be felt in the wrists and neck.

The other major artery, the pulmonary artery, carries blood from the right ventricle to the lungs. Although the systemic arteries carry oxygenated blood, the arteries of the pulmonary system carry deoxygenated blood to the lungs. A vessel is called an artery because it carries blood away from the heart, not because the blood it carries contains oxygen.

As arteries move away from the heart, they branch into smaller vessels called arterioles. Arterioles are structurally similar to arteries and play an important role in directing blood to the parts of the body needing it most, such as muscles under stress.

The veins and the venules

The major veins of the body are collectively called the venae cavae. The superior vena cava takes in blood

from the arms through the axillary veins, from the head through the jugular veins, and from the heart through the coronary veins. The inferior vena cava collects the blood from the legs from the femoral veins and from the abdomen from the hepatic, portal, and renal veins, among others. Both the superior and inferior venae cavae empty into the right atrium.

The pulmonary vein brings blood oxygenated in the lungs back to the left atrium, so it can be pumped to cells throughout the body. As with arteries, veins are not so named because the vessel carries deoxygenated blood, but by their role in bringing blood back to the heart.

Veins have the same three structural layers as arteries but the layers contain less elastic tissue and muscle components, making the walls thinner and six to ten times more expandable. The blood pressure in veins is lower than in the arteries, so to keep the blood flowing to the heart there are one-way valves that prevent backflow. Additionally, the action of the muscles in the legs help to return the blood to the heart, a mechanism called the venous pump.

As veins move farther from the heart they branch into smaller structures known as venules. The venules end in very thin blood vessels known as the capillaries.