Human heart is somewhat triangular and has the size of our heart. It lies between the two lungs, a little to the left. It is surrounded by a double-walled membrane called pericardium. A fluid called pericardial fluid fills the space between the heart and the pericardium. This fluid helps to protect the heart from shock. The heart is divided into four chambers. The upper chambers are called auricles or atria and the lower chambers are called ventricles. There are valve between the auricles and ventricles. The valve between the right auricle and right ventricle is called the tricuspid valve and the valve between left auricles and left ventricles is called the biscuspid valve. The right and left half of the heart are separated by a septum.
Impure bloos from all over the body is carried to right atrium by superior vena cava and inferior vena cava. Similarly, pure blood from the lungs is collected in the left atrium by pulmonary veins. When the auricles contract, this blood goes to the ventricles through the valves. These valves prevent the blood from flowing back. When the ventricles contract, the impure blood flows to lungs through the pulmonary artery and the pure blood is pumped to different parts of the body through aorta and its branches. Both these arteries have valves at their bases called semilunar valves.