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I'm NOT quite sure about the numbers but I have the general concept for this.

As I recall 60-70% of your blood is in your veins and venules. Now I think almost half of that is not USED in active circulation. So, on increased sympathetic activity it causes constriction of the veins (NOT ARTERIES) thus leading to pump of the inactive blood into the ACTIVE circulating zone, so NOW all of the 60-70% is circulating.

When a patient donates 500 ml of blood not much of a change happens to the patient due to that compensatory mechanism. Also that's why elderly people might have orthostatic hypotension (Incompetent control of the VENOUS supply due to aging).

So, pumping that inactive blood in veins into the active circulation is typically the same as connecting an IV line to a patient and pumping fluids.

Clear?
 
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