Heart rate is controlled so that the supply of blood matches the tissues' need for oxygen and glucose and for removal of carbon dioxide. The volume of blood pumped by one ventricle per minute is the cardiac output:cardiac output = heart rate × stroke volume Worked example: at rest, heart rate = 70 beats min−1 and stroke volume = 75 cm3. Cardiac output = 70 × 75 = 5250 cm3 min−1.During exercise
1. Muscles respire faster, so the concentration of carbon dioxide in the blood rises and blood pH falls. This is detected by chemoreceptors. Stretch receptors in the muscles detect movement.
2. The receptors send more impulses to the cardiovascular centre in the medulla oblongata.
3. The cardiovascular centre sends more impulses along the accelerator nerve (sympathetic) and fewer along the vagus nerve.
4. Noradrenaline is released at the SAN, which fires more frequently, so heart rate increases. The heart also contracts more strongly, increasing stroke volume.
5. The extra blood flow removes carbon dioxide faster, so blood pH returns to normal and the stimulus is reduced.When blood pressure is too high
Baroreceptors detect the rise and send more impulses to the cardiovascular centre, which sends more impulses along the vagus nerve. Acetylcholine is released at the SAN and heart rate falls. A fall in blood pressure has the opposite effect.Each of these is an example of negative feedback: a change is detected and a response is made that reverses the change. Adrenaline in the blood also increases heart rate, reinforcing the nervous response.
1. Muscles respire faster, so the concentration of carbon dioxide in the blood rises and blood pH falls. This is detected by chemoreceptors. Stretch receptors in the muscles detect movement.
2. The receptors send more impulses to the cardiovascular centre in the medulla oblongata.
3. The cardiovascular centre sends more impulses along the accelerator nerve (sympathetic) and fewer along the vagus nerve.
4. Noradrenaline is released at the SAN, which fires more frequently, so heart rate increases. The heart also contracts more strongly, increasing stroke volume.
5. The extra blood flow removes carbon dioxide faster, so blood pH returns to normal and the stimulus is reduced.When blood pressure is too high
Baroreceptors detect the rise and send more impulses to the cardiovascular centre, which sends more impulses along the vagus nerve. Acetylcholine is released at the SAN and heart rate falls. A fall in blood pressure has the opposite effect.Each of these is an example of negative feedback: a change is detected and a response is made that reverses the change. Adrenaline in the blood also increases heart rate, reinforcing the nervous response.