Probably the best known of the circulatory system organs is the heart. This fist-sized organ weighs less than a pound, but it carries a lot of responsibility in the human body. The Texas Heart Institute explains that the heart beats approximately 100,000 times each day and pumps about 2,000 gallons of blood on a daily basis. The heart has several components that help control the flow of blood within the heart and throughout the body.
The heart has upper and lower chambers known as the atria and ventricles. The atria fill with blood when it returns to the heart from the lungs and other tissues. The ventricles send blood to the lungs and other organs of the body. The valves of the heart control the direction of blood flow. The mitral valve and tricuspid valve make sure blood flows from the atria to the ventricles. The aortic valve and pulmonary valve keep blood from backing up into the heart as it travels to other areas of the body.
Arteries are the blood vessels that carry blood away from the heart. These vessels consist of three layers: the tunica adventitia, tunica intima and tunica media. The tunica adventitia consists of connective tissue and nerves. The tunica intima is the thinnest layer of an artery. It consists of endothelial cells connected by a matrix of polysaccharides and surrounded by connective tissue. The thickest layer of an artery, the tunica media, consists of connective tissue, elastic fibers and polysaccharides. The aorta is the largest artery in the body and has several branches. These branches include the carotid artery, renal artery and subclavian artery. Arterioles are the smallest veins in the body.
Veins transport blood from the other organs and tissues back to the heart. There are four major types of veins: deep veins, pulmonary veins, systemic veins and superficial veins. Deep veins are located within muscle tissue and usually have a corresponding artery nearby. Pulmonary veins take oxygenated blood from the lungs to the left atrium. Systemic veins take deoxygenated blood from the organs and tissues to the right atrium. Superficial veins do not have a corresponding nearby artery, as they appear close to the surface of the skin. Venules are the smallest veins in the body. They branch into the larger veins that transport blood to the largest veins in the body.
Capillaries are the smallest blood vessels in the body. They transport blood between the veins and arteries. The thin walls of these vessels also exchange waste products, carbon dioxide and nutrients. Precapillary sphincters allow the blood to flow to the capillary beds. When these sphincters close, blood does not flow through the capillary beds. Capillaries work with other circulatory system organs and tissues to keep the cardiovascular system working properly.
Texas Heart Institute: Heart Anatomy
KidsHealth From Nemours: Your Heart & Circulatory System
The Franklin Institute: Blood Vessels