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Valves respond to pressure differences. Explain the cardiac cycle by following contraction, pressure, valve position and blood movement in that order.
What you'll be able to do
The venae cavae return systemic blood to the right atrium; blood passes to the right ventricle and then through the pulmonary artery to the lungs. Pulmonary veins return blood to the left atrium; the left ventricle supplies the aorta and systemic circulation. The renal arteries supply kidneys and renal veins return blood; coronary arteries supply the heart muscle itself.
The left ventricle has a thicker muscular wall because it generates the pressure needed for the systemic circuit. Both ventricles normally pump the same average volume over time in a steady circulation. Artery and vein describe direction relative to the heart, not an absolute rule about oxygen content.
During ventricular filling, atrial pressure exceeds ventricular pressure and atrioventricular valves are open. Atrial contraction can complete filling. As ventricles contract, their pressure rises above atrial pressure, closing the atrioventricular valves and preventing backflow.
When ventricular pressure exceeds that in the pulmonary artery or aorta, semilunar valves open and ejection occurs. As ventricular muscle relaxes and pressure falls below arterial pressure, semilunar valves close. There is a short interval with all valves closed before ventricular pressure falls sufficiently for filling to resume. Valves are moved by pressure differences, not by their own rhythmic contraction.
End-diastolic ventricular volume is the volume before contraction; end-systolic volume remains after ejection. Their difference is stroke volume. Cardiac output is stroke volume multiplied by heart rate, with consistent time and volume units.
On a pressure diagram, infer valve changes from pressure crossings and identify phases where ventricular volume is constant. A pressure rise alone need not mean blood is already being ejected. Link any proposed adaptation to pressure, resistance or output rather than simply saying the heart “works harder”. Data on cardiovascular risk also require evaluation of association, confounding and evidence quality.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
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Test yourself
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