(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

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explanation:

plate tectonics and volcanic euptions

over time, plate tectonic processes cause continents to move to different positions on the globe.

for example, britain was near to the equator 300 million years ago, and therefore was hotter than it is today. for further details look at our climate through time poster or online map.

the movement of the plates also causes volcanoes and mountains to form and these too can contribute to a change in climate.

mountain ranges formed by plate tectonics. large mountain chains can influence the circulation of air around the globe, and consequently influence the climate.   warm air might be deflected somewhere cooler by the mountains.

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.