2.64 explain how the heart rate changes during exercise and under the influence of adrenaline

Exercise
When you exercise your muscles need more energy, therefore, you respire more. In order to get more oxygen to your muscles quicker (and the carbon dioxide away faster) your heart rate increases. This increases the rate of blood flow which will increase the amount of oxygen/carbon dioxide going in/out of your muscle cells. It's done like this...

- Exercise will inevitably increase the amount of carbon dioxide in the blood (as oxygen is being converted into carbon dioxide quicker)
- This high level of carbon dioxide is detected by receptors in the aorta artery (and also in the carotid artery, in your neck)
- The receptors in these arteries send signals to the brain
- The brain then sends signals to the heart, causing more forceful and frequent contractions (basically, making it pump more)

Adrenaline
Adrenaline is released by the adrenaline glands when an organism is threatened.

This adrenaline will bind to a specific receptors in the heart, causing the heart (cardiac) muscles to contract more frequently and more forcefully. This will increase the amount of oxygen supplied to the tissues.