Raising a cut on your arm above your heart I often read that when you have a deep cut on your arm, you should raise your arm to the point where the cut is above your heart. This should reduce the amount of blood loss because less blood reaches the cut.
However, I'm not clear on the physics of this. Which of the following is correct?


*

*The amount of blood reaching the cut changes suddenly at the point where the cut is level with the heart. Therefore, the cut needs to be raised only just above the heart.

*The amount of blood reaching the cut changes gradually as the cut is raised. Therefore, the cut needs to be raised as high as possible.
 A: Blood pressure is measured in mm-Hg.  If we take a typical blood-pressure 120-80, that can be adjusted to height above or below the heart provided we make a very bad assumption that the human body is a bag of water.   In reality, the human body is nowhere near that simple.
120-80 mm-Hg where Hg is a density of 13.59 g/cc and blood is about 1.06, then every 12.8 mm a wound is raised, the blood pressure can drop by as much as 1 mm-Hg.  So, raising 1/2 meter above the heart corresponds to 500/12.8 which gives a change in -39 from the blood pressure for 1/2 meter above, similarly +39 for 1/2 meter below the heart.   However, the human body has structure, musculature and containment and a whole series of valves which all reduce the simple calculation of drop in pressure of a liquid based on relative height.  The actual change should be quite a bit less than the above estimate.   But there should be some change.  It's not hard to google and find that blood pressure does change, arm raised, arm lowered, etc.
The body can also regulate itself, for example, the effort of holding an arm high might undo any benefit from the lower pressure in the raised body part.
You can run experiments on this just for fun.  Buy a blood pressure monitor and check your blood pressure with your arm in different positions.  I'm not sure this will work, but take the blood pressure of your leg too, standing, sitting and prone and leg raised with physical effort vs raised on pillows.    
But check with your doctor before taking a physicists medical advice.  ;-)
