The structure and function of the heart, cardiac cycle, cardiac output and its regulation, the vascular shunt mechanism, venous return, and the cardiovascular responses and adaptations to exercise and training.

What this dot point is asking

AQA wants you to explain how the heart works through the cardiac cycle, define and calculate cardiac output, explain how heart rate is regulated by the conduction system and neural and hormonal control, describe the vascular shunt and venous return, and link cardiovascular responses and long-term adaptations to exercise.

The cardiac cycle and conduction system

The heart is a double pump with four chambers. The cardiac cycle has two phases: diastole (relaxation and filling) and systole (contraction and ejection). In atrial systole the atria contract to top up the ventricles; in ventricular systole the ventricles contract, the atrioventricular valves shut (the first heart sound) and blood is ejected into the aorta and pulmonary artery; in diastole all chambers relax, the semilunar valves close (the second heart sound) and the chambers refill. The opening and closing of valves is driven entirely by pressure gradients, blood flows from high to low pressure and valves prevent backflow.

The myogenic heartbeat is initiated by the heart itself: the sinoatrial node (SAN), the pacemaker in the right atrium, fires an impulse that spreads across the atria causing them to contract, then reaches the atrioventricular node (AVN). After a short delay (which lets the atria empty before the ventricles contract), the impulse passes down the bundle of His, into the Purkinje fibres, and makes the ventricles contract from the apex upwards. This sequence is what an ECG records as the P wave, QRS complex and T wave.

Cardiac output

A resting value might be $Q = 70 \times 70 = 4900$ mL per minute (about 5 L). During maximal exercise an elite endurance athlete can reach over 30 L per minute. The rise comes from increases in both HR and SV, though SV plateaus at around 40 to 60 percent of maximal effort while HR continues to climb to maximum.

Heart rate is regulated by the cardiac control centre (CCC) in the medulla oblongata. Neural control acts through the sympathetic nervous system (the accelerator nerve raises HR) and the parasympathetic nervous system (the vagus nerve lowers HR). The CCC responds to chemoreceptors (detecting rising carbon dioxide and falling pH), baroreceptors (detecting blood pressure) and proprioceptors (detecting movement). Hormonal control by adrenaline from the adrenal medulla increases both the rate and force of contraction, and produces the anticipatory rise in HR before exercise even begins. Intrinsic control by temperature and venous return fine-tunes the response.

The vascular shunt and venous return

During exercise the vascular shunt mechanism redistributes blood: vasodilation of arterioles supplying working muscles and vasoconstriction of those supplying the gut and other inactive areas, controlled by the vasomotor centre and pre-capillary sphincters. This raises the share of cardiac output going to muscle.

Responses and adaptations to exercise

Short-term responses to exercise include rising heart rate (anticipatory rise then a further increase), greater stroke volume, increased cardiac output, and the vascular shunt. Long-term aerobic training produces cardiac hypertrophy (a larger, stronger left ventricle), increased resting and maximal stroke volume, bradycardia (a lower resting heart rate), greater capillarisation and increased blood and haemoglobin volume.