The heart consists of four separate chambers. There are two receiving
chambers and two pumping chambers. One receiving chamber receives 
blood from the body (right atrium) and the other from the lungs (left
atrium): one pumping chamber pumps blood via an artery called the
pulmonary artery to the lungs (right ventricle) and the other to the body via
connected to the pumping chambers by one-way valves, the right atrium and
an artery called the aorta (left ventricle). The two receiving chambers are
right ventricle by the tricuspid valve, the left atrium and left ventricle by the
mitral valve. The two pumping chambers are also connected to their arteries
by valves; the right ventricle to the pulmonary artery by the pulmonary
valve, the left ventricle to the aorta by the aortic valve. The role of all these
valves is quite simply to make sure that the blood continually moves in a
forward direction and does not move backwards so that, for example, when
the left ventricle contracts, blood moves forwards through the aortic valve
and not backwards through the mitral valve.