Human heart

The heart is roughly triangular in shape. It has four compartments called chambers inside it. The upper two chambers of the heart are called atria and the lower two chambers are called ventricles. The atria receive blood from the two main veins. The left atrium is connected to the left ventricle through a valve V1 and the right atrium is connected to the right ventricle through another valve V2. These valves prevent the backflow of blood into atria when the ventricles contract to pump blood out of the heart to the rest of the body because when the ventricles contract, the valves V1 and V2 close automatically so that the blood may not go back into the atria. The job of heart is to pump blood around our body. All the atria and ventricles of the heart contract and relax at appropriate times and make the heart behave like a pump. A sheath of tissue called pericardium protects the muscular heart.