Questions on the heart and cardiac cycle

Question 1

Describe the pressure changes responsible for every valve movement in one cardiac cycle, starting at the beginning of ventricular systole.

Accepted Answer

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

Question 2

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.

Accepted Answer

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 5.3 dm³ min⁻¹ (5.25 unrounded) after dividing by 1000, with the unit given

Question 3

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.

Accepted Answer

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

Question 4

An electrocardiogram is recorded from electrodes placed on a patient's skin. Describe what the trace measures, and state which event in the heart produces each of the P wave, the QRS complex and the T wave.

Accepted Answer

B1 it measures the potential difference between the electrodes against time, produced by the wave of depolarisation spreading through the heart muscle, so it records electrical activity rather than movement of the walls or pressure in the chambers B1 the P wave is produced by depolarisation of the atria, spreading out from the sinoatrial node, and atrial contraction follows it B1 the QRS complex is produced by depolarisation of the ventricles, and it is the largest deflection because the ventricle walls contain far more muscle than the atria A1 the T wave is produced by repolarisation of the ventricles, returning the muscle to its resting state

Question 5

On an electrocardiogram the first R peak is at 0.32 s and the fifth R peak is at 2.32 s. Calculate the patient's heart rate in beats per minute, and state whether the trace shows tachycardia, bradycardia or neither.

Accepted Answer

M1 four intervals separate the first and the fifth peak, so the mean interval is (2.32 − 0.32) ÷ 4 A1 2.00 ÷ 4 = 0.50 s between beats M1 heart rate = 60 ÷ the interval in seconds A1 60 ÷ 0.50 = 120 beats per minute, which is above 100, so the trace shows tachycardia

Question 6

A flatworm has no heart, no blood and no vessels, and relies on diffusion to supply oxygen to every cell. Explain why a large, active mammal could not rely on diffusion alone.

Accepted Answer

B1 as an animal's size increases, its volume rises with the cube of its length while its surface area rises only with the square, so its surface area to volume ratio falls B1 a large animal therefore has far less exchange surface available per gram of tissue than a small one such as a flatworm B1 doubling the diffusion distance across a large body does not double the time diffusion takes, it roughly quadruples it, so diffusion alone is far too slow over centimetres A1 a large, active, endothermic mammal also has a high metabolic rate and so a high demand for oxygen and glucose, which makes a mass transport system, rather than diffusion, necessary to meet that demand

Question 7

At rest a man's cardiac output is 5.25 dm³ min⁻¹ with a heart rate of 75 min⁻¹. During exercise his heart rate rises to 150 min⁻¹ and his stroke volume rises to 105 cm³. Calculate his cardiac output during exercise in dm³ min⁻¹, and calculate the factor by which it has increased compared with rest.

Accepted Answer

M1 cardiac output = stroke volume × heart rate = 105 × 150 A1 15 750 cm³ min⁻¹, which is 15.75 dm³ min⁻¹ (accept 15.8) M1 factor increase = exercise cardiac output ÷ resting cardiac output = 15.75 ÷ 5.25 A1 3.0 times (three-fold) higher during exercise than at rest

Question 8

Compare the single circulation of a fish with the double circulation of a mammal.

Accepted Answer

B1 blood passes through the heart once per circuit in a fish but twice per circuit in a mammal B1 in a fish blood travels straight from the gill capillaries to the body and so reaches it at low pressure, whereas in a mammal it returns to the heart and is re-pressurised by the left ventricle A1 delivery is therefore faster in the mammal, which supports a higher metabolic rate, while its gas exchange surface sees only about a fifth of the systemic pressure rather than the full pressure of the ventricle

Question 9

A baby is born with a hole in the septum between the two ventricles. Suggest why the baby becomes breathless and tires easily.

Accepted Answer

B1 the septum normally keeps oxygenated and deoxygenated blood apart, and a hole lets the two mix B1 the left ventricle generates about 120 mmHg against the right ventricle's 24 mmHg, so blood is pushed from left to right and oxygenated blood is sent back to the lungs instead of to the body A1 less oxygenated blood therefore reaches the body, so less oxygen is delivered to respiring tissue and the heart has to work harder to compensate

Question 10

Two traces are recorded from different patients. In the first the complexes are evenly spaced and every QRS is preceded by a P wave. In the second the baseline is unsteady, no P waves can be found and the QRS complexes are unevenly spaced. Explain what the second trace shows and why the patient's ventricles fill less well than normal.

Accepted Answer

B1 the second trace shows atrial fibrillation, identified by the absence of P waves and by the irregular intervals between QRS complexes B1 the atrial muscle is depolarising chaotically rather than in one organised wave from the sinoatrial node, so there is no single deflection to record and the atria do not contract as a unit A1 atrial systole normally tops the ventricles up before they contract, so without it the ventricles are less full at the start of ventricular systole and stroke volume falls

Question 11

The atrioventricular valves are held by tendinous cords running to papillary muscles in the ventricle wall. Suggest what would happen to blood flow through the heart if these cords were torn.

Accepted Answer

B1 the tendinous cords are inelastic strands that normally stop the valve flaps being pushed too far when ventricular pressure rises B1 papillary muscles tense as the ventricle contracts, so the cords stay taut and hold the flaps in place at the moment ventricular pressure is highest A1 if the cords were torn, rising ventricular pressure would turn the flaps inside out into the atrium instead of holding them shut, so blood would leak backwards into the atrium during ventricular systole

Question 12

During exercise, a rise in blood carbon dioxide concentration leads to an increase in heart rate. Explain the sequence of events linking the two, as a negative feedback loop.

Accepted Answer

B1 more carbon dioxide dissolves in the blood and forms carbonic acid, so blood pH falls B1 chemoreceptors in the carotid bodies, the aortic body and the medulla itself detect this fall in pH, and the medulla responds by increasing sympathetic impulses to the sinoatrial node, raising heart rate A1 the faster heart rate carries carbon dioxide to the lungs and removes it faster, so blood pH returns towards normal and the frequency of impulses from the chemoreceptors falls again, completing the negative feedback loop

Question 13

Explain how pressure receptors in the carotid sinus and the aorta respond to a rise in blood pressure, and explain how their response brings the pressure back down.

Accepted Answer

B1 a rise in blood pressure increases the frequency of impulses from the pressure receptors to the medulla B1 the medulla responds by increasing parasympathetic impulses to the sinoatrial node, and reducing sympathetic impulses A1 this produces a slower, weaker heartbeat, which reduces cardiac output and brings the blood pressure back down towards normal

Question 14

State why a resting heart rate of 45 beats per minute is not a sign of disease in a trained endurance athlete, although it might be in someone else.

Accepted Answer

B1 in the athlete a large stroke volume delivers the same cardiac output in fewer, bigger beats, so the low rate is normal for them B1 in someone untrained, the same low rate would not be compensated by an unusually large stroke volume, so it might indicate reduced cardiac output and be a sign of disease

Question 15

State what causes a valve in the heart to open or to shut.

Accepted Answer

B1 a difference in pressure across the valve, with blood pushing harder on one face than on the other