The diagram below depicts the human heart in one of its phases. Answer the questions that follow.
Give a suitable reason to justify your answer in (a).
Topic: Evidence for ventricular systole
Answer
Because the ventricles are shown CONTRACTING to force blood out — the atrioventricular valves are CLOSED, preventing backflow into the atria, while the SEMILUNAR valves are OPEN, allowing blood to be pumped into the aorta and the pulmonary artery.
The reason must describe what is VISIBLE in the diagram — the positions of the two sets of valves and the direction of the arrows — rather than what you remember about the cardiac cycle generally.
The valve positions are the proof, because they are OPPOSITE in the two phases: VENTRICULAR SYSTOLE — AV valves CLOSED, semilunar valves OPEN VENTRICULAR DIASTOLE — AV valves OPEN, semilunar valves CLOSED
This also explains the heart sounds: the AV valves closing make "LUBB" at the start of systole, and the semilunar valves closing make "DUP" at the start of diastole — the point tested in Question 1(iii) of this same paper.