Heart beats increase for no reason or skips beats sometimes because of anxiety or stress. When the body is put under too much stress, it needs to act out and normally it is through your heart palpitations.
Your answer is: Stress and anxiety
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Stimulants increase heart rate, increased blood pressure, constrict blood vessels, increased blood glucose
O M G i just did that one on Plato
well anyway i think the answer was the the skin because any thing that invades the body is a threat to the immune system.