The left ventricular wall is approximately three times thicker than the right ventricular wall. What best explains this structural difference?
AThe left ventricle contains more cardiomyocytes to generate body heat
BThe left ventricle must generate higher pressure to drive blood through the systemic circulation
CThe left ventricle receives blood from more veins than the right ventricle
DThe left ventricle pumps a larger volume of blood per beat than the right ventricle
Wall thickness reflects the pressure a chamber must generate. The right ventricle drives blood through the pulmonary circuit at ~25 mmHg; the left ventricle must overcome systemic vascular resistance of ~120 mmHg. Greater pressure requires greater muscle mass — thicker walls. Options C and D are wrong because both ventricles receive and eject the same volume per beat (otherwise blood would pool on one side), and body heat is not a ventricular function.
Question 2 Multiple Choice
The mitral valve snaps shut during ventricular systole. What triggers this closure?
AAn electrical signal from the sinoatrial node directly closes the valve leaflets
BPapillary muscles actively pull the valve closed through chordae tendineae
CVentricular pressure exceeds atrial pressure, reversing the pressure gradient and pushing the leaflets shut
DThe valve is pulled shut by the elastic recoil of the myocardium
Valves are passive structures that respond to pressure gradients — they have no independent motor function. During atrial contraction, atrial pressure exceeds ventricular pressure, pushing the valve open. When the ventricle contracts, ventricular pressure quickly rises above atrial pressure, reversing the gradient and pushing the leaflets back to the closed position. The chordae tendineae (option B) prevent prolapse — they keep the leaflets from flipping backward into the atrium — but they do not actively close the valve.
Question 3 True / False
A ventricular septal defect (VSD) — a hole between the left and right ventricles — causes oxygenated and deoxygenated blood to mix.
TTrue
FFalse
Answer: True
The interventricular septum normally keeps the high-pressure, oxygenated left side completely separated from the lower-pressure, deoxygenated right side. A defect allows blood to shunt from left to right (because left ventricular pressure is higher), mixing oxygenated blood with the deoxygenated blood heading to the lungs. In large VSDs this reduces systemic oxygen delivery and forces the right ventricle to work harder against the extra volume load.
Question 4 True / False
The right and left ventricles pump the same volume of blood per beat, so they is expected to generate approximately equal pressures during contraction.
TTrue
FFalse
Answer: False
Cardiac output requires that both ventricles eject equal volumes (otherwise blood would accumulate in the pulmonary or systemic circulation), but equal volume does not mean equal pressure. Pressure reflects resistance: the pulmonary circulation is a low-resistance, low-pressure circuit (~25 mmHg systolic), while the systemic circulation has much higher resistance (~120 mmHg systolic). The left ventricle generates roughly five times the pressure of the right ventricle despite ejecting the same stroke volume.
Question 5 Short Answer
Why do the semilunar valves (pulmonary and aortic) close after ventricular contraction ends, rather than remaining open continuously?
Think about your answer, then reveal below.
Model answer: When ventricular contraction ends and the ventricles begin to relax, ventricular pressure falls below the pressure in the aorta and pulmonary artery. This pressure reversal drives blood backward toward the ventricles, pushing the cup-shaped semilunar leaflets closed. The valves prevent backflow because the aorta and pulmonary artery maintain residual pressure (diastolic pressure) that keeps forcing the leaflets shut throughout ventricular relaxation.
Valves are entirely pressure-driven: they open when pressure is higher upstream and close when it reverses. The semilunar valves close at the start of diastole because the arteries retain pressure from the previous contraction while the relaxing ventricles rapidly drop their pressure. This creates the diastolic blood pressure that continues driving blood through capillaries between heartbeats.