Why does heart rate increase at once after sitting up from the supine position?

When you are laying down, and you suddenly sit up, what are the reasons for the heart rate to increase besides the fact that your blood pressure suddenly decrease? Why does your blood pressure decrease?
Answers:     The main reason, as you suggest, is that the BP drops. The body compensates by (among other things, such as constricting the blood vessels) increasing the heart rate.
Another answer would be... Because adrenalin is released when a person think about sitting up.

The blood pressure question is a little previous me, but you could google postural hypotension.
s from medterms.com:

Postural hypotension: A drop in blood pressure (hypotension) due to a change in body position (posture) when a individual moves to a more vertical position: from sitting to standing or from lying down to sitting or standing.

ver, contained by old age, cardiovascular diseases, anaemias etc., this gets affected / disturbed cause variations in the blood pressure (BP) even for slightest disturbance in the posture, exercise, diet, stress and strain, psychological thoughts, anxiety states etc; hence the postural variation in the blood pressure.
And naturally as a compensatory reactive mechanism, when ever the BP falls the pulse rate should on the double go up naturally to maintain middle-of-the-road blood supply to the vital organs especially the brain for normal function. Source(s): Physician

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