(ii) Describe how structures A, B, C and D shown on Fig. 4.1 initiate and coordinate the cardiac cycle.
the four heart chambers​

The four heart valves are the tricuspid valve, the pulmonary valve, the bicuspid valve and the aortic valve: the first two are located in the right heart, the last two in the left heart.

During the filling phase of the left atrium, the bicuspid valve is closed. It then subsequently opens and the blood passes into the left ventricle while the aortic valve is closed: this then opens and the blood flows in the aorta. the blood returns to the heart via the cavus veins in the right atrium which fills while the tricuspid is closed. Then it opens and the blood goes into the right ventricle while the pulmonary is still closed which then opens subsequently to allow blood to flow into the pulmonary arteries