Free Circulation MCQs with Answers

1. The wall of the left ventricle is much thicker than that of the right ventricle because it

• A. holds a far larger volume of blood
• B. must pump blood against the much greater resistance of the systemic circuit
• C. contains more valves than the right ventricle
• D. receives blood at a higher pressure from the left atrium

Explanation: Both ventricles eject the same volume with each beat, but the left has to drive blood through the whole body while the right only sends it to the lungs a short distance away and at low pressure. More cardiac muscle means a higher pressure can be generated, which is why the left wall is roughly three times thicker. Blood enters both ventricles from the atria at low pressure.

2. The valve between the right atrium and the right ventricle is the

• A. tricuspid valve
• B. bicuspid valve
• C. aortic semilunar valve
• D. pulmonary semilunar valve

Explanation: The right atrioventricular valve has three flaps and is therefore called tricuspid, while the left one has two and is called bicuspid or mitral. Both are held by chordae tendineae attached to papillary muscles, which stop them turning inside out when the ventricle contracts. The semilunar valves guard the exits into the pulmonary artery and the aorta, not the openings between chambers.

3. The muscle of the heart itself is supplied with blood by the

• A. pulmonary arteries
• B. coronary arteries
• C. blood passing through the chambers
• D. superior vena cava

Explanation: The coronary arteries branch from the base of the aorta and spread over the heart wall, because the myocardium is far too thick for the blood inside the chambers to supply it by diffusion. Blockage of one of these arteries starves a part of the muscle of oxygen and causes a heart attack. The pulmonary arteries carry deoxygenated blood away to the lungs.

4. 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.

5. 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.

6. 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.

7. 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.

8. Which vessel carries oxygenated blood despite being called a vein?

• A. Superior vena cava
• B. Hepatic vein
• C. Pulmonary vein
• D. Renal vein

Explanation: The pulmonary vein returns freshly oxygenated blood from the lungs to the left atrium, so it is the exception to the rule that veins carry deoxygenated blood. The naming depends on direction of flow rather than on oxygen content: a vein carries blood towards the heart and an artery away from it. Its partner exception is the pulmonary artery, which carries deoxygenated blood.

9. Valves are present in veins but not in arteries because

• A. veins carry deoxygenated blood
• B. blood in veins is at low pressure and would otherwise flow backwards
• C. arteries have thinner walls
• D. veins lie closer to the skin

Explanation: By the time blood reaches the veins the pressure created by the heart has largely been lost in the capillaries, so flow depends on the squeezing of skeletal muscles, and the pocket valves ensure that this squeezing pushes blood only towards the heart. Arteries need no valves because the pressure from ventricular systole is high enough to keep blood moving in one direction. Failure of these valves produces varicose veins.

10. Exchange of materials between blood and tissues occurs only in capillaries because their walls are

• A. one cell thick, formed of a single layer of endothelium
• B. made of smooth muscle that contracts rhythmically
• C. lined with valves at close intervals
• D. thick and elastic to withstand pressure

Explanation: A capillary wall is a single layer of flattened endothelial cells, giving a diffusion distance of well under a micrometre, and the enormous total surface area and slow flow complete the design. Arteries and veins have thick walls of muscle and elastic tissue that make exchange impossible, so they are transport vessels only. The narrowness of a capillary also forces red cells through in single file.