Determination of gas metabolism

Animals need external energy to keep their homeostasis and numerous gradients across cellular membranes as well as for biosynthesis and muscular activities. Such biochemical processes are coupled with a significant heat dissipation that can be monitored via oxygen consumption, carbon dioxide production and nitrogen excretion. 

The classical Warburg method of manometric indirect calorimetry (see e.g. [41 ]) determined the oxygen consumption rate by a reduction of volume and the carbon dioxide production by a volume increase in the gaseous environment of an organism under research. The ratio of carbon dioxide production and oxygen consumption, the so-called respiratory quotient RQ, rendered information about the substrate(s) used in this specific metabolism. Table 2 presents the RQ values for various pure or mixed substrates and the expected heat production or gas exchanges. 

Respiratory quotient RQ, expected enthalpy change and gas exchanges for pure and mixed substrates in animal calorimetry. Adapted from [3] 

This simple approach for only two kinds of gases was later refined to include methane production and excretion of nitrogen with urine (see [3] for review). Brouwer [42] proposed the following equation for mammals 

It becomes obvious from the above equations that the contribution of F02 to the energy dissipation is the most important one. 

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