system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: After having my blood taken from a vein in my arm, I am told to put pressure on the withdrawal site to prevent 'bruising' of that area. How does it 'bruise', and how does putting pressure on it prevent that?
gpt
When they take the needle out, sometimes blood will leak out of the vein and pool up under the skin leaving you with a "bruise" in the area.  Not all bruises are from being hit, a bruise is just when blood gets in places it shouldn't normally be and if it's close enough to the surface you can see the discoloration through the skin.