The heart and blood vessels are organs that make up the circulatory system. The human body has a closed circulatory system, meaning blood pumped by the heart is contained within blood vessels. The heart is an organ that consists mostly of cardiac muscle tissue, called myocardium. The myocardium is made up of several cells, each called a myocardial cell, which is a contractile cell found in the heart muscle tissue. When these cells contract, blood is pumped out of the heart. When these cells relax, the heart fills with blood. The right and left sides of the heart operate as two separate pumps. These two sides are separated by a wall of myocardium, which prevents the mixing of oxygen-rich and oxygen-poor blood. The right side of the heart is responsible for receiving blood from the body and delivering it to the lungs. The left side of the heart receives blood from the lungs and delivers it to the body. The myocardium on the left side of the heart is much thicker and stronger because it generates high blood pressure while working against a greater resistance from the systemic blood vessels, the blood vessels found throughout the body except in the lungs. Because the right side of the heart is pumping blood to the lungs, which are in close proximity to the heart, there is much less resistance, and therefore the right myocardium is much thinner.
The heart contains four inner chambers, two on each side. The left and right sides of the heart each have an upper chamber and a lower chamber. Blood enters the heart at the upper chambers, each called an atrium, and when the atrial walls contract, blood is pushed into the ventricles. When the walls of the lower chambers, each called a ventricle, contract, blood is ejected out of the heart. The atria are separated by the interatrial septum, and the ventricles are separated by the interventricular septum.
Four major blood vessels, found at the upper region of the heart, carry blood to and from the heart. Each side of the heart is associated with an artery and a vein. An artery is a blood vessel that transports blood away from the heart, and a vein is a blood vessel that carries blood back to the heart. Blood enters the right side of the heart through the superior and inferior vena cavae. The superior vena cava is the vein that carries blood from the upper body, and the inferior vena cava carries blood from the lower body. Blood exits the right side of the heart via the pulmonary trunk, which branches to the right and left pulmonary arteries. On the left side of the heart, blood enters through two veins, either the left or right pulmonary vein into the left atrium, and blood exits the left ventricle through the largest artery of the body, the aorta.
The heart contains four valves that open and close to ensure unidirectional blood flow through the heart and avoid backflow. Two of the four valves are classified as atrioventricular valves (AV valves) and are flap-like valves found between the atria and ventricles. The other two are semilunar valves that are cup-shaped valves, or "half-moons," located at the major arteries exiting the heart. The right side of the heart has an AV valve called the tricuspid valve and a semilunar valve called the pulmonary valve located at the entrance of the pulmonary trunk, which is the artery that carries blood out of the right side of the heart. The left side of the heart has an AV valve called the bicuspid (mitral) valve and a semilunar valve called the aortic valve found in the entrance of the aorta, the artery that carries blood out of the left side of the heart. Although the AV valves and semilunar valves both function to keep blood flowing in one direction, they differ structurally.
The AV valves are made up of two or three flaps (cusps) that operate much like doors. The right tricuspid valve has three flaps, and the left bicuspid valve has two. These AV valves are connected to stringy tendons called chordae tendineae, which anchor to papillary muscles. Papillary muscles are fingerlike extensions of the myocardium on the inner walls of the ventricles that resist the pull of the valves during ventricular contraction. When the ventricles relax and the atria contract, the valve cusps open into the ventricles, allowing blood to pass from the atria to the ventricles, and when the ventricles contract, they close to prevent blood from reentering the atria. The length of chordae tendineae is such that they prevent the valves from inverting into the atria.The semilunar valves are named for their cup shapes, with each valve containing three cups arranged concentrically at the entrance of the arteries of the heart. When the ventricles contract, the cups of the semilunar valves collapse, and blood passes through the valves and into the arteries. When the ventricles relax, blood fills the cups of the valves, which blocks blood from reentering the ventricles. This careful unidirectional control is essential for maintaining the blood pressure responsible for efficient delivery of oxygen and nutrients to all tissues of the body.