Heart, blood vessels and circulation circuits

Vessels that carry blood from the heart to organs are arteries, those that bring it to the heart are veins, and the finest vessels between them are capillaries. Artery walls are thick, elastic and muscular – they withstand high pressure. Vein walls are thinner and pressure is low; valves in large veins prevent backflow. A capillary wall is one cell thick, and through it oxygen, nutrients and carbon dioxide are exchanged between blood and tissue. Blood circulates in two circuits. The systemic circuit starts in the left ventricle with the aorta, goes through arteries and capillaries to body organs, gives up oxygen, takes carbon dioxide and returns via the venae cavae to the right atrium. The pulmonary circuit starts in the right ventricle with the pulmonary artery; in lung capillaries the blood gives up carbon dioxide, becomes rich in oxygen and arrives in the left atrium via the pulmonary veins. Usually arterial blood is oxygen-rich and venous blood oxygen-poor; only in the pulmonary artery and vein is this reversed.