heart and blood vessel disorders
Biology of the Blood Vessels
Introduction
The Biology of the Heart and blood vessels constitute the cardiovascular (circulatory) system. The The blood vessels consist of Arteries Arterioles Capillaries Venules Veins All blood is carried in these vessels. Blood travels from the heart in arteries, which branch into smaller and smaller vessels, eventually becoming arterioles. Arterioles connect with even smaller blood vessels called capillaries. Through the thin walls of the capillaries, oxygen and nutrients pass from blood into tissues, and waste products pass from tissues into blood. From the capillaries, blood passes into venules, then into veins to return to the heart. Arteries and arterioles have relatively thick muscular walls because blood pressure in them is high and because they must adjust their diameter to maintain blood pressure and to control blood flow. Veins and venules have much thinner, less muscular walls than arteries and arterioles, largely because the pressure in veins and venules is much lower. Veins may dilate to accommodate increased blood volume. If a blood vessel breaks, tears, or is cut, blood leaks out, causing bleeding. Blood may flow out of the body, as external bleeding, or it may flow into the spaces around organs or directly into organs, as internal bleeding. 