Free Cardiac Cycle and Heartbeat MCQs with Answers

1. The pacemaker of the human heart is the

• A. atrioventricular node
• B. bundle of His
• C. sinoatrial node in the wall of the right atrium
• D. network of Purkinje fibres

Explanation: The sinoatrial node depolarises spontaneously about seventy times a minute and sets the rhythm, which is why the heartbeat is myogenic and a heart continues beating when removed from the body. The atrioventricular node can also fire on its own but at a slower rate, so it takes over only if the sinoatrial node fails. The bundle of His and the Purkinje fibres conduct the impulse rather than generate it.

2. The brief delay of the impulse at the atrioventricular node is important because it

• A. slows the overall heart rate
• B. prevents the impulse from reaching the ventricles at all
• C. allows the semilunar valves to open early
• D. gives the atria time to empty into the ventricles before the ventricles contract

Explanation: A delay of about a tenth of a second lets atrial systole finish and the ventricles fill completely, so the following ventricular contraction ejects a full stroke volume. Without it the two sets of chambers would contract almost together and pumping would be very inefficient. The node conducts the impulse onwards rather than blocking it, and the semilunar valves open only after ventricular pressure has risen.

3. If the stroke volume is 70 cm3 and the heart rate is 72 beats per minute, the cardiac output is about

• A. 5 dm3 per minute
• B. 0.5 dm3 per minute
• C. 14 dm3 per minute
• D. 50 dm3 per minute

Explanation: Cardiac output is stroke volume multiplied by heart rate, so 70 cm3 times 72 gives 5,040 cm3 per minute, which is about 5 dm3. That figure is close to the total blood volume of an adult, meaning the whole circulation passes through the heart roughly once every minute at rest. During heavy exercise both factors rise and the output can reach five times this value.

4. The first heart sound, lubb, is produced by

• A. blood rushing into the atria
• B. the closure of the semilunar valves
• C. contraction of the atrial walls
• D. the closure of the atrioventricular valves at the start of ventricular systole

Explanation: When the ventricles begin to contract the pressure inside them rises above atrial pressure, the tricuspid and bicuspid valves snap shut and the vibration gives the long low lubb. The shorter, sharper dupp comes later, when the semilunar valves close at the end of ventricular systole. A murmur heard between these sounds usually means a valve is leaking or narrowed.

5. The QRS complex in ECG represents

• A. Atrial depolarization
• B. Ventricular depolarization
• C. Atrial repolarization
• D. Ventricular repolarization

Explanation: The QRS complex represents ventricular depolarization (contraction); the P wave is atrial depolarization.

6. Cardiac output equals

• A. Heart rate + Stroke volume
• B. Heart rate × Stroke volume
• C. Heart rate - Stroke volume
• D. Stroke volume only

Explanation: Cardiac output (mL/min) = Heart rate (beats/min) × Stroke volume (mL/beat).