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.

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?