Exercise:
During exercise, muscles require more energy which is released by respiration. This means that more oxygen need to be brought to cells and more carbon dioxide taken away.
This means the heart needs to increase its speed so more blood is sent to muscles.
Adrenaline:
-adrenaline is a hormone produced when an organism is threatened.
-the adrenaline will bind to specific receptors in the heart, causing cardiac muscles to contract more frequently and forcefully.
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