This is called a venous cannula or venflon.
we hold it like this
When the needle is inserted, the doctor would fiddle with the bottom end of the plastic (which is connected to the needle) by withdrawing it a little to look for any blood (flashback) which will appear in the tube.
If there is blood, hurray to both the doc and the patient coz the doc has hit the vein!
If there is no blood, the patient has to suffer while the doc probe or wriggle the needle in the person's arm.
Did you notice the thin plastic tube surrounding the needle? So when the needle is withdrawn a bit, blood will enter the plastic tube so the doc knows whether he has found the vein or not.
When the doc noticed flashback in the plastic tube, he gently but swiftly withdraws the plastic end which has the needle.
It is by no means magic or the needle dissolves in the body which makes the needle disappear! lolz.
So the plastic bit will sit in the vein of the patient with the screw back on again while the needle bit will be thrown away in a special bin called the sharps bin.
Happy New Year everyone!