heart

the hollow, muscular organ that acts as a pump and is responsible for the CIRCULATION OF THE BLOOD . The heart is cone-shaped, with the point downwards, and is situated between the LUNGS , slightly to the left of the midline. The heart projects forwards and lies beneath the fifth rib. The wall consists mainly of CARDIAC MUSCLE , lined on the inside by a membrane known as the ENDOCARDIUM . An external membrane, known as the PERICARDIUM , surrounds the heart. a SEPTUM divides the heart into right and left halves, each of which is further divided into an upper chamber, known as an ATRIUM , and a lower one, called a VENTRICLE . Four valves control the direction of blood flow at each outlet, comprising the aortic, pulmonary, tricuspid and mitral (bicuspid). These valves prevent back flow once the blood has been forced from one chamber into the next.