(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

doctors i believe if i'm wrong sorry

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!