Valves of the heart (fig 2.19)
To control the bloodflow and keep it constantly moving in one direction, the heart contains valves. Each half of the heart has two sets of valves:
· One set lies between the atrium and the ventricle, preventing the blood from being forced back into the atrium when the ventricle contracts, thus forcing the blood to enter the aorta or the pulmonary artery respectively.
· The second set lies at the openings of the aorta and the pulmonary artery, preventing the blood from passing back into the ventricles when the ventricles relax.
When the heart beats both atria contract sumultaneously and fill the ventricles. Immediately afterwards the ventricles contract simultaneously, forcing the blood into the pulmonary artery and the aorta. Then the ventricles relax and immediately the valves at the openings of these vessels close, so that the blood is kept inside. It is the contraction of the ventricles which is felt as the heartbeat.