Docs can you explain what is a collapsed vein?

Venospasm: Veins don't like to be irritated with things like needles. Usually, when someone is talking about a collapsed vein they mean that they have tried to draw blood or start an IV and the vein has gone into spasm as a result of the irritation. The result is that the needle either isn't able to go into the vein or else it goes in but there is no blood return. With time the vein will relax and reopen.

Vein Shrinks: A collapsed vein is a vein that shrinks in size. This can occur from trauma to the vein such as with a needle puncture as well as a response to cold . Dehydration will also collapse a vein. Veins normally decrease in size as they get further away from the heart so it is easier for a vein to collapse in the hand as opposed to the upper arm.