The graph shows the pressure changes in the left atrium, left ventricle, and aorta of a healthy mammal during a single cardiac cycle.
Which statement correctly describes the events occurring at or between the labelled points?
At point W, the semi-lunar (aortic) valve opens, forcing blood rapidly from the left ventricle into the aorta.
Between points X and Y, the atrioventricular valve remains open to allow blood to flow into the ventricle under high pressure.
At point Y, the semi-lunar (aortic) valve closes because the pressure in the aorta becomes greater than the pressure in the left ventricle.
Between points Y and Z, both the atrioventricular valve and the semi-lunar valve are open as the ventricle fills rapidly with blood.
