Human Heart

The human heart is a hollow pumping organ. It is somewhat conical in shape. It is about the size of man's fist. The heart is enclosed in a thin tough transparent sac, the pericardium. There is a fluid between the heart and the pericardium called pericardial fluid. It is lubricating fluid, which reduces friction between the pericardium and heart. The major portion of the heart is called myocardium. It consists largely of cardiac muscle tissue.

Dissections of a human heart, as seen from the front, with the ventral part of both atria and both ventricles removed.

Internally, the heart is divided by a vertical partition into two halves, the right and left. The vertical partition is called septum. Each half is again divided into an upper thin-walled atrium and a lower thick-walled ventricle. Thus, the heart consists of four chambers, the right and left atria and the right and left ventricles. 

The atria receive blood and the ventricles distribute it. Blood from the head,neck and arms is returned to the right atrium by superior vena cava. Blood from lower parts of the body is brought back inferior vena cava to the right atrium. Thus, the right atrium receives deoxygenated blood from the two-vena cava. When the right atrium contracts, the blood flows into the right ventricle through the tricuspid valve.

       
                   Circulation of blood


When the right ventricle contracts the blood pressure closes tricuspid valve. This prevents back flow of blood into the atrium. The blood leaves the right ventricle by pulmonary trunk. The pulmonary trunk divides into two pulmonary arteries one to each lung. Return of blood into the ventricle is prevented by semilunar valves in the pulmonary trunk. Oxygenated blood from the lungs is brought back to the heart by way of the pulmonary veins, which open into the left atrium. When the left atrium contracts the blood enters the left ventricle through bicuspid valve.
   
        When the left ventricle contracts, blood leaves by a large artery, the aorta. From the aorta blood is distributed to all parts of the body except lungs. Aorta also has semilunar valves to prevent back flow into the left ventricle. 

Heart beat

The alternating contraction and relaxation of the atria and ventricles is called cardiac cycle. The two atria contract simultaneously emptying blood into ventricles. A fraction of a second later, the two ventricles contract simultaneously, forcing blood into arteries leaving the heart. Both the ventricles then relax for less than a second before the cycle is repeated. The period of contraction is called systole and the period of relaxation is called diastole. A heartbeat includes one systole and one diastole.


Pulse Rate

The beating of the heart is also felt in the arteries as pulse. Pulse is the vibration felt in arterial walls due to expansion of the aorta following ventricular contraction. If you place fingers on an artery on the wrist you will feel the pulse. A normal adult pulse rate can vary from 60-100 times per minute. Normally it is 68-72. The rate of heartbeat is indicated by the pulse rate.