Why do I have a high pulse every time I go in to donate plasma?

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I keep asking myself that and I can't find the answer. I try to calm myself beforehand. I try not to worry about having a high pulse. I do what the nurses suggest. Cut off caffeine the day before and try to just focus on the tree swaying in the wind outside. Put it out of your mind. They ask me if I smoke. I don't. I'm not necessarily fat, but I could be in better shape. (I'm 5'10-6'feet tall and weigh around 197.) I try to exercise, but maybe I don't get enough. What can I do to lower my pulse?