system
Given the following post, respond as if you're commenting on Reddit:
human
What happens to veins after they are injected with a needle?
gpt
Not sure what you mean... so hopefully the following helps; 

First, IDK if you can say they're injected with a needle; they're punctured/pierced by the needle - you then inject the contents of the syringe into the vein. 

Second, the needle breaks the wall of the vessel, but since it's sharp and small it does fairly little damage... and when removed the body's response quickly closes the hole. Any leakage turns into a small bruise/blood under the skin, which then gets re-absorbed over the next few days.