Respirometry

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).

The metabolism of an animal is estimated by determining rates of carbon dioxide production (VCO₂) and oxygen consumption (VO₂) of individual animals, either in a closed or an open-circuit respirometry system. Two measures are typically obtained: standard (SMR) or basal metabolic rate (BMR) and maximal rate (VO2max). 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), age, reproduction status, post-absorptive to avoid thermic effect of food). VO₂max is typically determined during aerobic exercise at or near physiological limits. In contrast, field metabolic rate (FMR) refers to the metabolic rate of an unrestrained, active animal in nature. Whole-animal metabolic rates refer to these measures without correction for body mass. If SMR or BMR values are divided by the body mass value for the animal, then the rate is termed mass-specific. It is this mass-specific value that one typically hears in comparisons among species.

Respirometry depends on a "what goes in must come out" principle. Consider a closed system first. Imagine that we place a mouse into an air-tight container. 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. Therefore, as long as we know the volume of the system, the difference between the concentrations of O₂ and CO₂ at the start when we sealed the mouse into the chamber (the baseline or reference conditions) compared to the amounts present after the mouse has breathed the air at a later time must be the amounts of CO₂/O₂ produced/consumed by the mouse. Nitrogen and argon are inert gasses and therefore their fractional amounts are unchanged by the respiration of the mouse. In a closed system, the environment will eventually become hypoxic.

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