The heart is a specialized organ, and the only one in the body made of cardiac muscle. Heart cells are called cardiomyocytes and make up muscle fibers that conduct electrical impulses. The function of the heart, which is to keep blood flowing throughout the body, is controlled by involuntary areas of the brain. Blood is necessary for the survival of the tissues because red blood cells carry oxygen, which is necessary for cellular processes, and the plasma carries nutrients to the tissues and waste away from the tissues.
The human heart has four chambers. The upper chambers are called the atria, and the lower chambers are the ventricles. The left and right side of the heart are separated by a thick interventricular (between the ventricles) and interatrial (between the atria) septum. The thick tissue is called the myocardium. The chambers are lined with a thin membrane called the endocardium, which prevents the blood from clotting as it passes through the heart. The heart is protected by a membrane called the pericardium. The outer pericardium resembles a fibrous sac and is made of connective tissue, holding the heart in place in the chest.
Valves separate each atrium and ventricle, as well as the exit points to the lungs and body, to prevent the backflow of blood. The tricuspid valve (also known as the right atrioventricular valve) separates the right atrium and ventricle. The mitral valve, also known as the bicuspid valve, separates the left atrium and ventricle.
The heart beat
Electrical impulses along the nerve fibers in the cardiac tissue cause the heart to contract, squeezing blood out from the heart muscle into the aorta, the first and largest blood vessel. When the fibers relax, blood flows into the heart from the inferior and superior vena cava, the endpoints of the peripheral venous system carrying deoxygenated blood from the body. A heart beat is usually thought of as "thump-thump". This is systole-diastole. Systole is atria-ventricle contraction, and diastole is atria-ventricle relaxation in overlapping events.
Systole is started by the heart's natural pacemaker in the right atrium - the sinoatrial (SA) node, a specialized group of cardiac cells. The SA node has the fastest rate of depolarization in the heart tissue. The electrical impulse started by the SA node travels to the atrioventricular (AV) node in the interatrial septum to complete systole. In the interventricular septum is the bundle of His, known as the AV bundle. The bundle of His receives the signal from the AV node and transmits it to the left and right bundle branches in the apex of the heart. The impulses travel along specialized nerve fibers, called Purkinje fibers, to other parts of the heart to produce ventricular systole while the atria have begun diastole. The atrium contracts (systole) immediately before the ventricle contracts.
During diastole blood is received by the right atrium. It is forced into the ventricle by systole, and then when the right ventricle contracts it pushes blood through the pulmonary semilunar valve and into the pulmonary artery, which carries deoxygenated blood to the lungs for oxygenation. The blood then flows into the left atrium of the heart through the pulmonary vein, and then to the left ventricle. The left ventricle has the thickest wall of any other chamber in the heart and contracts more forcibly, forcing blood through the aortic semilunar valve and out to the body (See a video of the heart functioning here).
Structure of the fetal heart
The structure of the fetal human heart is slightly different from an adult heart. An opening in the interatrial septum called the foramen ovale allows blood to bypass the pulmonary circulation. Fetal blood is oxygenated by the maternal circulation instead of the lungs. This opening is closed with a flap by the force of the blood flow upon the baby taking its first breath outside the womb. For more about the differences in heart structure and function, see the Texas Heart Institute.