How does an intravenous pack work?

On most IV bags there are two ports at the bottom (type "IV bag" into Google Images), which are initially sealed. One can be used to add medication with a needle; it's rubber and self-seals after the needle is removed. When the bag is hung, the other port is opened and "spiked" -- the end of the IV tubing has a spike that will break the seal and allow liquid to flow into the tubing from the bag. Gravity drives the drip. Atmospheric pressure is not involved.