The human heart consists of 4 chambers which rhythmically pump blood between the lungs and the extremities to gather and distribute oxygen. The heart itself is a muscle made of cardiac fibers and specialized fibers that conduct electrical impulses for the involuntary contraction and relaxation of the muscle. The fibers make up the myocardium, or the muscle walls, which are lined with a smooth skin called endocardium which prevents blood clotting within the heart.
The cone shaped tip of the heart is the apex, it sets just above the diaphragm at the base of the left lung. The heart muscle is enclosed in a membrane called the pericardium. There is a fibrous (outer) pericardium, a loose fitting sac made of connective tissue which holds the heart in place in the thoracic cavity. As the heart beats a serous fluid prevents friction between the epicardium, inner serous pericardial membrane, and the outer pericardium.
The upper chambers of the heart receive blood and are called the atria, made up of the right atrium and left atrium. The lower chambers, the left and right ventricle, expel blood by contracting. Between the atria is the interatrial septum; between the ventricles is the interventricular septum. Blood only flows between the chambers through valves which prevent the flow from reversing. Veins bring blood from the body into the right atrium through the tricuspid valve. The right ventricle relaxes and blood flows in from the atrium, when it contracts the blood is forced into the pulmonary artery which carries blood to the lungs to gather oxygen. Oxygenated blood enters the left atrium from the lungs through the mitral valve and is expelled into the aorta (an elastic artery) by the left ventricle, carrying oxygen to the body. This also provides oxygen to the outer layers of the heart via the coronary arteries.
The heart is capable of contracting spontaneously without nerve impulses. In the wall of the right atrium is the sinoatrial (SA) node where fast depolarization causes rapid contraction, initiating a heart beat. Contractile impulses then travel to the atrioventricular (AV) node in the lower interatrial septum. From there the impulses travel to the ventricles via Purkinje fibers bringing about the finish to systole. The counterpart to systole is diastole, or relaxation. A healthy human adult has a resting heart rate, pulse, of 60-80 beats per minute, the rate of SA node depolarization.
Reference: Scanlon and Sanders, Essentials of Anatomy and Physiology, 4th edition, 2003.