Why do doctors remove air from syringes?

The answer is this is good practice because if you're putting injections into say, a vein and you were to inject a bit of air you'd get what is known as an air embolus. The air bubble would float along the vein back to the heart; it would go through the right side of your heart and into your lungs and it would lodge in a blood vessel, which is a bit smaller than it is. The result is that blood would then be stuck behind this bubble of air. You'd have to wait for the bubble to slowly be absorbed back into the blood stream. Probably pretty trivial if you just have one-off injections but if you have a lot of injections this could actually compromise the circulation through the blood. The other point is if you're having an injection into a muscle it's pretty painful and the more volume you inject the more painful it is. If you inject a volume of air as well as injection material you're just going to make the experience even more unpleasant so we try and avoid that where possible.