The cardiac cycle, the regulation of heart rate, cardiac output and the cardiovascular drift, venous return mechanisms, and the redistribution of blood flow during exercise.

What this dot point is asking

OCR wants you to describe the cardiac cycle and its conduction, explain how heart rate is regulated, calculate and interpret cardiac output, describe the mechanisms of venous return and the Starling effect, explain cardiovascular drift, and explain how blood flow is redistributed to the working muscles during exercise.

The cardiac cycle and conduction

Regulation of heart rate

Heart rate is controlled by the cardiac control centre in the medulla oblongata, which adjusts the balance of two autonomic nerves. The sympathetic nervous system (via the accelerator nerve) speeds the heart up; the parasympathetic (via the vagus nerve) slows it down. Three groups of receptors feed information in: chemoreceptors (detecting rising carbon dioxide and falling pH), baroreceptors (detecting blood pressure) and proprioceptors (detecting movement in the muscles and joints). The hormone adrenaline also raises heart rate and the force of contraction. Proprioceptors explain the very fast rise in heart rate the instant exercise begins, before any chemical change.

Cardiac output

For example, at rest with $HR = 70$ beats per minute and $SV = 70$ ml, cardiac output is $Q = 70 \times 70 = 4900$ ml per minute. In maximal exercise a trained performer might reach $HR = 190$ and $SV = 120$ ml, giving $Q = 190 \times 120 = 22800$ ml per minute (about $22.8$ litres). An endurance athlete shows cardiac hypertrophy (a larger, stronger left ventricle), so stroke volume is high and resting heart rate is low (bradycardia, below 60 beats per minute): the same resting blood need is met with fewer, more powerful beats.

Venous return and the Starling effect

This is why a cool-down matters: keeping the muscle pump working after exercise maintains venous return, sustains cardiac output and prevents blood pooling in the veins, which would otherwise cause dizziness and slow the removal of lactate.

Redistribution of blood flow and cardiovascular drift

During exercise the vascular shunt redirects blood to where it is needed. Arterioles to the working muscles vasodilate and pre-capillary sphincters open, while arterioles to the gut, kidneys and skin vasoconstrict, raising the share of cardiac output reaching the muscles from around 20 percent at rest to over 80 percent in hard exercise. This is controlled by the vasomotor centre in the medulla.