Questions: Cardiac Cycle and Heart Function

Question 1

During a stethoscope exam you hear two sounds: 'lub' and 'dub'. What physical events produce these sounds?

• A. Ventricular contraction followed by ventricular relaxation
• B. Atrial contraction followed by ventricular contraction
• C. Closure of the mitral/tricuspid valves followed by closure of the aortic/pulmonic valves
• D. Opening of the AV valves followed by opening of the semilunar valves

Answer: (C)

Heart sounds are caused by valve closure, not muscle contraction. S1 ('lub') is produced when the mitral and tricuspid valves close at the start of ventricular systole. S2 ('dub') is produced when the aortic and pulmonic valves close at the end of systole. This is a classic misconception: it feels intuitive that 'the heartbeat sound' comes from the heart muscle contracting, but the sounds come from the hydraulic snap of the valves.

Question 2

Increasing heart rate generally increases cardiac output.

• T. True
• F. False

Answer: False

Cardiac output = stroke volume × heart rate. At very high heart rates, diastole is shortened so severely that the ventricles do not fill adequately, causing stroke volume to fall. If stroke volume decreases faster than heart rate increases, cardiac output can actually decline. This is why extreme tachycardia can be hemodynamically dangerous.

Question 3

State Frank-Starling's law and explain why it is useful for matching cardiac output to metabolic demand.

Model answer: Frank-Starling's law states that the more the ventricle is stretched during diastolic filling (greater end-diastolic volume), the greater the force of the subsequent contraction and the larger the stroke volume.

This intrinsic mechanism means the heart automatically pumps out more blood when more returns to it — without requiring nervous system input. For example, during exercise, increased venous return stretches the myocardium, increasing stroke volume and therefore cardiac output. It also ensures the left and right ventricles remain balanced: if the right side pumps slightly more, the left receives more filling and responds with greater output.