In most cases, a compartment is a chemical species in a physical place. So neither "glucose" nor "cAMP" is a compartment, and neither "central venous blood" nor "the lumen of the rough endoplasmic reticulum" is a compartment in the sense generally used by kineticists.
Ca2+ in the cytosol and plasma insulin, on the other hand, are both compartments. In the first case Ca2+ is the chemical species and cytosol is the physical place. In the second case, insulin is the chemical species, and plasma is the physical place.
Almost always, each compartment in a model will have an associated differential equation which describes the processes capable of increasing or decreasing the amount of the chemical species in the physical place.