What is the inside of a tube called?

I'm trying to find out what is the inside of a tube could be (like a lab tube) called? Bulb, chamber, reservoir or store? Thanks in advance!

In the body, the inside of a tubular structure is called a lumen. This would apply to blood vessels and the stomach, but not the oral cavity, cranial vault, or upper chambers of the heart. The upper chambers are more definitively called atria (lat: rooms) while the lower chambers are ventricles (diminutive of venter (“belly, stomach, womb”)). I wouldn't hesitate to use the word lumen for the inside of a straw or a test tube, but might risk not being understood. So many great words for the insides of stuff! Lumen also refers to the International System of Units designation for a parcel of luminous flux.

It's just "the tube" or "the inside of the tube" in my opinion. There isn't a specific word for the inside of other simple vessels or containers like bottles, jars, cups, beakers or even closed containers like barrels or tanks. You only need a word when the object has more than one space inside it.

For instance a sand glass like an egg timer has two connected spaces and you might refer to sand flowing from the upper chamber to the lower chamber and the two chambers being reversed to continue timing.

I would refer to "the bowl" of a stemmed wine glass because the stem and base are other parts of the glass but I wouldn't talk about "the bowl" of a tumbler or highball glass because there are no other parts from which to distinguish it. A tumbler is a tumbler. It has a rim, a bottom, sides, an outside and an inside. The same applies to a test tube or boiling tube.

This is the chamber of the test tube.

See Definition #2 of the Merriam Webster Dictionary’s entry:

A natural or artificial enclosed space or cavity