Exercise - Blood Pressure & Heart Beats/Pulse Rate after a Long Run

I notice that every time after a long run, my blood pressure drops.
For example:

After the long-run last Sunday July 24, 2011, the readings were:

- Blood pressure = 112/72
- Pulse = 39 beats per min

I suppose that's good news as high blood pressure leads to all kinds of health issue. But I wonder what causes the drop.

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