5. Anxiety and fear
The heart rate, as well as the rest of the vital signs and some functions of internal organs and glands, can be altered by your mental state. There are general systems that modulate your behavior and your vitals in order to adapt to certain circumstances. Even though anxiety is currently seen as a health problem and even a mental condition, it is not in itself a pathological manifestation of our behavior.
Anxiety is meant to prepare the body for a potential or real danger, and as a result, the organism reacts, getting ready for a possible burst in physical activity. Originally, anxiety was meant to get our body ready for a fight or for escaping a threat we want to avoid. For that reason, anxiety activates our blood circulation in order to get the blood to the muscles; that is why it can increase your heart rate and give you tachycardia.