system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why and how do blood veins "roll"?

Reading Reddit today, I saw an appeal for blood and platelets. I'd be down with giving both more often except when I do, I leave with painful dark bruises all over both arms and hands because the Red Cross says I have "veins that roll".

What does it mean for blood veins that "roll", why do they do this, and what can I do to make giving blood easier and more painless? I'm really sick of the bruises, being jabbed and poked with needles for hours to find "a good vein", and having to sigh and tell people in advance "please use the butterfly". Thanks for all serious answers.
gpt
Paramedic here, most of the time when a healthcare provider tells you that you have veins that roll it's an excuse because their IV starting skills aren't adequate. The actual meaning of this is when you are first inserting the catheter if you hit the vein on the side rather than dead on it can basically push the vein to the side causing you to either shear of the side of the vein or just flat out miss out. However this almost only happens in geriatric (elderly) patients with loose skin. The cause of the bruising is blood leaking out of the vein from it being nicked. Now it could just be that you don't have any veins close to the surface of your skin, but it is rare to find a patient that doesn't have ANY potential arm IV sites unless they are obese or a pediatric.