system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you get a shot in the arm, how can you be sure the contents are being released into a blood vessel?

When you donate blood the needle goes into a large, visible vein in your arm or hand but when getting a shot the needle just goes somewhere in your arm.
gpt
Many injections don't have to be directly into a blood vein/artery.   Just injecting into muscular tissue alone provides fairly rapid absorption into the bloodstream without the risks of using a vien.