My brain immediately goes to the tourniquet example where something tight is used to prevent blood flow. That may not be accurate either though because they band your arm when drawing blood…
In: Biology
A little tightness causes an increase in blood pressure, which promotes circulation in the affected area by making it harder for venous pressure to resist arterial pressure and gravity. Tourniquets are so tight that they aim to stop blood flow entirely.
In more technical terms; the goal of a tourniquet during a blood draw is to have pressure higher than venous pressure and lower than arterial pressure. This way, blood can flow through to the area, but it cannot return to the heart from the area. This causes your veins to open up as wide as they possibly can, which will reduce the chance of the needle injuring the vein beyond the intended puncture, and promotes flow of blood into the needle.
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