Air Vents on IV Tubing

This may be a very silly question, but other than with glass bottles, what is the purpose of using air vents on IV tubing? Should I keep them open or closed? They are on our primary and secondary tubing sets.
The only time you need to open the vent is when the object your spiking cannot collapse as the fluid drains out. This could be rigid plastic, glass or metal.Other then that you don't have to worry about it.Hope that helps!-emerjensee

What emerjensee said.

Yes that is it...you open the vent if you need an air source to displace the IV fluid. If you do need an air source and do not open the vent the IVFs will continue to drip for awhile and then STOP and then you will see the drip chamber suck in on the tubing suck in. I see this happen when nurses forget to vent Albumin and Lipids and such. If you do not need the vent keep it closed. If you need the vent and it open and you see the tubing and/or dip chamber sucking in and you flow slows or lessons then the air vent is not working very well or at all. If the set has been primed incorrectly this can happen and the vent gets wet or the IV medication is very viscous.