Solution-Types of heart chambers, In mammals, the role of

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Solution-Types of heart chambers, In mammals, the role of

Problem:

In mammals, the role of the heart is to pump blood around the body. The events that occur during one heart beat are known as the cardiac cycle. The timing for one cardiac cycle is shown in fig.

1857_types of heart chambers.jpg

P and Q represent the two types of heart chambers.

V, W, X, Y and Z represent the symbolic (contracted) or diastolic (relaxed) state of the relevant heart chambers.

The answer says P is atrium and Q is ventricle but why? and from V,W,X,Y,Z which one is systolic and diastole?