Venting, closure piercing & why glass persists
Five characteristics of gravity infusion from a rigid container.
An infusion bottle hangs inverted while its contents run out under gravity. Because glass does not collapse, the volume leaving must be replaced by air entering, either through a vent in the administration set or an air inlet through the closure. That single fact separates rigid infusion containers from collapsible bags in almost every respect.
Flow requires a vent, and the vent is a contamination path
Liquid leaving a rigid bottle has to be replaced by air, which is admitted through a filtered vent in the set or a second spike through the closure. That air path is filtered because it is a direct route into a sterile solution. Bags avoid the issue by collapsing, which is much of why they displaced glass for routine fluids.
The stopper is pierced once, hard, by a spike
Unlike a vial septum meeting a fine needle, an infusion closure is penetrated by a blunt plastic spike pushed with force. The stopper has to accept that without coring, without fragmenting and without leaking around the spike for hours of infusion. It is a different mechanical duty and a different stopper design from injection closures.
Glass earns its place on compatibility, not on cost
Some drugs sorb into PVC and polyolefin, and some are sensitive to oxygen ingress through a plastic wall. Nitroglycerin, insulin and several cytotoxics are classic examples where a rigid, non-sorbing, gas-tight container is chosen deliberately. Where those issues do not arise, bags win on weight, breakage and disposal without much argument.
The graduations are read while it hangs
A nurse checks the remaining volume against marks on the wall of an inverted bottle, often at a distance and in poor light. That makes graduation contrast, spacing and whether they read correctly upside down into functional requirements, and it is a place where fired enamel earns its cost over printed marking that fades in a washer.
Weight and breakage are the working objections
A full glass bottle hanging from a stand at head height is a real hazard if it is knocked, and a case of them is heavy to move. Programs that keep glass do it for compatibility reasons and then manage the handling: protective sleeves, careful hanging practice and clear labeling of which fluids are supplied in glass.
The hanging system is part of the container specification
A bottle is suspended for its whole delivery, so the neck ring or hanger has to carry the full mass without deforming the finish that the closure seals against. That load path is verified on filled units rather than assumed, and it is a reason not to lighten this format the way an ordinary bottle would be lightened.
