3. Eli5: Why do they take blood from the veins especially? When we have random cuts...

Eli5: Why do they take blood from the veins especially? When we have random cuts, blood still flows, so why can't they take blood from somewhere not the vein?

Blood is normally drawn from arteries because that is the cleanest, "freshest" blood that's coming from the heart.

One of the reasons that random cuts can take awhile to stop bleed is because they usually open several blood vessels (mostly capillaries). This means that the blood has to clot at a large (relative to the size of a blood vessel) hole in the body, and repair numerous vessels as well.

When blood is drawn properly with a needle, you receive 1 tiny puncture through a small area of the skin and into 1 blood vessel. When the pressure is applied with the cotton ball after the needle is removed, it slows the blood flow through the damaged artery, making it easier for blood clots to collect at the wound and seal it.

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