The heart is a hollow, cone-shaped, muscular organ. It is a biological pump. Its size is not more than a human’s clenched fist and is made up of specialised cardiac muscles. The heart lies at the centre of the chest cavity, between the lungs and behind the sternum. It is slightly tilted towards the left. The heart has four chambers, which are separated by septum, a wall of muscles. Two upper chambers are called atria and two lower chambers are called ventricles. The valves in the heart are specialised to ensure unidirectional flow of blood.