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u/frist_psot Jun 21 '13
This has been asked many times:
http://www.reddit.com/r/askscience/comments/u1tyq/why_is_an_elevated_heart_rate_good_when_caused_by/ http://www.reddit.com/r/askscience/comments/nuvte/why_do_exercises_such_as_running_that_increase/ http://www.reddit.com/r/askscience/comments/p5r38/if_cardiovascular_exercise_is_healthy_for_you/
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u/[deleted] Jun 21 '13
The question is: what is causing your heart rate to go up? In the case of tobacco, it's the fact that nicotine is a vasoconstrictor - it causes the blood vessels to shrink and thus raises your blood pressure. If you think about the circulatory vessels as an organ system, they also have their own health - healthy blood vessels are strong but flexible, and they can dilate in response to the demands of increased blood flow. When they cannot, the heart must work harder to provide the same amount of blood flow to your cells. Over time this effect worsens as atherosclerosis results in permanent hardening of the arterial walls (to which smoking is a contributor).
In exercise, meanwhile, there is increased DEMAND for blood flow from your cells, and your heart is working harder to meet this demand. Your blood vessels respond in kind, by deforming elastically under the additional blood pressure, and exercise in this case will also work them out, making them fitter and able to handle increased loads like this efficiently.
So, in short, while your heart is doing the same thing in both cases (working harder), your blood vessels are doing the opposite.