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 decreases? Why does your blood pressure decrease?
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Generally in younger and healthy persons there will be no much difference in the BP in the prone or supine positions due the balanced autonomic system maintained in health.
However, in old age, cardiovascular diseases, anaemias etc., this gets affected / disturbed causing 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 variations in the blood pressure.
And naturally as a compensatory reactive mechanism, when ever the BP falls the pulse rate should immediately go up naturally to maintain normal blood supply to the vital organs especially the brain for normal function.
(Guessing) Maybe partially because the heart needs to increase the blood flow to the brain to compensate for it being raised.
Another answer would be… Because adrenalin is released when a person thinks about sitting up.
The blood pressure question is a little past me, but you could google postural hypotension.
Heres from medterms.com:
Postural hypotension: A drop in blood pressure (hypotension) due to a change in body position (posture) when a person moves to a more vertical position: from sitting to standing or from lying down to sitting or standing.
The change in position causes a temporary reduction in blood flow and therefore a shortage of oxygen to the brain. This leads to lightheadedness and, sometimes, a "black out" episode, a loss of consciousness.
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.
The BP drops because with sitting up, gravity now pulls blood out of the chest, decreasing its return to the heart.