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The heart and the cardiac cycle: pressure decides everything questions
Why large active animals need a mass transport system, how double circulation differs from single, the structure of the mammalian heart including the thicker left ventricle wall, the three stages of the cardiac cycle driven by pressure changes, reading a pressure-time graph, cardiac output as stroke volume times heart rate, and myogenic control through the SAN, AVN, bundle of His and Purkyne fibres.
6 original questions · 20 marks · the heart and the cardiac cycle: pressure decides everything notes · Animal transport and cardiovascular biology
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Describe the pressure changes responsible for every valve movement in one cardiac cycle, starting at the beginning of ventricular systole.
Mark scheme
- B1 ventricular pressure rises above atrial pressure, so the atrioventricular valves are pushed shut, which is the first heart sound
- B1 for a short period both sets of valves are shut, so ventricular pressure climbs with no blood moving and no change in volume
- B1 ventricular pressure rises above aortic pressure, so the semilunar valves are forced open and blood is ejected
- B1 as the ventricles relax their pressure falls below aortic pressure, so blood in the aorta pushes the semilunar valves shut, which is the second heart sound
- A1 ventricular pressure keeps falling until it is below atrial pressure, at which point the atrioventricular valves open and the ventricles begin to fill again
At rest an adult has a stroke volume of 70 cm³ and a cardiac cycle lasting 0.80 s. Calculate her heart rate in beats per minute and her cardiac output in dm³ min⁻¹.
Mark scheme
- M1 heart rate = 60 ÷ 0.80, converting the time for one cycle into beats per minute
- A1 75 beats per minute
- M1 cardiac output = stroke volume × heart rate = 70 × 75 = 5250 cm³ min⁻¹
- A1 5.25 dm³ min⁻¹ after dividing by 1000, with the unit given
A heart removed from the body goes on beating for a while. Explain why it does so, and explain what the delay at the atrioventricular node achieves.
Mark scheme
- B1 cardiac muscle is myogenic, so the beat originates within the muscle itself and needs no nerve impulse to start it
- B1 the sinoatrial node in the wall of the right atrium depolarises spontaneously about 75 times a minute and acts as the pacemaker, while nerves and hormones only alter the rate
- B1 a band of connective tissue between atria and ventricles will not conduct, so the atrioventricular node is the only route through and it holds the impulse for about 0.13 s
- A1 that pause lets the atria finish emptying before the ventricles contract, so the ventricles are properly filled; without it both would contract together and the ventricles would fill badly
Compare the single circulation of a fish with the double circulation of a mammal.
A baby is born with a hole in the septum between the two ventricles. Suggest why the baby becomes breathless and tires easily.
State what causes a valve in the heart to open or to shut.
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