Nervous control of the heart rate

The SAN sets the heart's basic rhythm. The brain and autonomic nervous system can adjust the heart rate based on the body's demands.

The medulla oblongata in the brain stem acts as the control centre. It receives information from receptors in the carotid arteries and aorta.

These receptors are:

1. Baroreceptors - Blood pressure receptors.
2. Chemoreceptors - Chemical receptors.

The medulla oblongata then regulates the SAN's firing rate to adjust the heart rate.

How the autonomic nervous system responds during exercise

This automatic system allows the heart rate to adapt swiftly to the body's changing needs.

For example, during exercise:

1. Blood has a higher concentration of carbon dioxide, and so a lower pH.
2. Chemoreceptors in the carotid arteries and aorta detect this and increase the frequency of nervous impulses sent to the medulla oblongata.
3. The medulla oblongata increases the rate of impulses to the sinoatrial node (SAN) via the sympathetic nervous system.
4. Heart rate increases.
5. This provides the extra oxygen required for increased respiration in active muscles.
6. The increased blood flow also helps remove excess carbon dioxide via the lungs.
7. Carbon dioxide concentration reduces.

Hormones, such as adrenaline and noradrenaline, can also influence heart rate. In times of stress, these hormones are released and cause the SAN in the heart to increase the heart rate as part of the 'fight or flight' response.