Can any medical dopers answer this one? Will giving blood give a lower blood pressure reading? If so, how long will the effects last for? Or is giving blood more a measure of how hard your heart is working and uneffected by a change in your blood volume?
I guess it depends on how much blood you give. The average person has about 70 mL/kg of blood, thus the average 70kg person has 5L. If you give one pint of blood, you might be talking 10% of blood volume.
When people lose blood in an accident, their blood pressure may go down and they are said to be “in shock”. Stage I shock, where they lose 15% or less of their blood volume, generally has few changes in heart rate (may be slightly higher), blood pressure (may be slightly lower) or pulse pressure (the difference between the top and bottom numbers of blood pressure may be slightly closer).
So to answer your question, give small amount of blood, blood pressure drops minimally, heart compensates for small drop in blood pressure, blood pressure quickly returns to normal.
Give larger amount of blood, heart speeds up to try to maintain perfusion, blood pressure still returns to normal.
Lose 15% of blood, roughly, heart speeds up but can’t quickly compensate for blood pressure so easily; starts to slowly go down as blood loss increases.
Dr_Paprika is correct, as usual.
I’d just add one other observed phenomenon associated with blood loss:
Lose a bit of blood, blood pressure rises.
The system can recognize that its volume is depleted, and to compensate, it increases heart output and arterial tone. With a minimal depletion, the net result can, and frequently is, an increase in blood pressure.
Of course, many factors come into play, so YMMV.