Respirometry is a general term that encompasses a number of techniques for obtaining estimates of the rates of metabolism of vertebrates, invertebrates, plants, tissues, cells, or microorganisms via an indirect measure of heat production (calorimetry).
SMR is measured while the animal is at rest (but not asleep) under specific laboratory (temperature, hydration) and subject-specific conditions (e.g., size or allometry[1]), age, reproduction status, post-absorptive to avoid thermic effect of food).
The air sealed in the container initially contains the same composition and proportions of gases that were present in the room: 20.95% O2, 0.04% CO2, water vapor (the exact amount depends on air temperature, see dew point), 78% (approximately) N2, 0.93% argon and a variety of trace gases making up the rest (see Earth's atmosphere).
As time passes, the mouse in the chamber produces CO2 and water vapor, but extracts O2 from the air in proportion to its metabolic demands.
For an open-system, design constraints include washout characteristics of the animal chamber and sensitivity of the gas analyzers.
The primary distinction between an open and closed system is that the open system flows air through the chamber (i.e., air is pushed or pulled by pump) at a rate that constantly replenishes the O2 depleted by the animal while removing the CO2 and water vapor produced by the animal.
For an open system with upstream flow meter, water (e.g., anhydrous calcium sulfate) and CO2 removed prior to the oxygen analyzer, a suitable equation is For an open system with downstream flow meter, water and CO2 removed prior to the oxygen analyzer, a suitable equation is where For example, values for BMR of a 20 g mouse (Mus musculus) might be FR = 200 mL/min, and readings of fractional concentration of O2 from an oxygen analyzer are FinO2 = 0.2095, FexO2 = 0.2072.
Assuming an enthalpy of combustion for O2 of 20.1 joules per milliliter, we would then calculate the heat production (and therefore metabolism) for the mouse as 703.5 J/h.
By the permeabilization of the cellular membrane, the cell stops to exist as a living, defined organism, leaving only the mitochondria as still functional structures.
The sample suspended in a suitable medium is placed in a hermetically closed metabolic chamber.
When plastic materials are unavoidable (e.g. for o-rings, coatings of stirrers, or stoppers) polymers with a very low oxygen permeability (like PVDF as opposed to e.g. PTFE) may be used.
The companies providing equipment for whole-animal rspirometry mentioned above are usually not involved in mitochondrial respiromety.