D52 Respiratory calorimetry

Topic

This respiratory (or indirect) calorimetry measures an insect's metabolic rate by quantifying the gases it exchanges with its environment; the oxygen consumed and carbon dioxide produced are proportional to the energy released during the insect's biological oxidation reactions.

In contrast, the Brouwer system combines these gas exchange measurements to estimate the insect's metabolic heat production without requiring precise knowledge of the specific substrate being oxidized.

The infrared gas analyzer measures carbon dioxide based on the Beer-Lambert law regarding electromagnetic radiation absorption—specifically, carbon dioxide absorbs strongly within a particular infrared band—whereas infrared thermography (another method in this set) captures direct thermal emission rather than gas absorption.

The open-flow system measures the difference in carbon dioxide concentration between the incoming ambient gas and the gas exiting the insect's chamber; multiplying this difference by the volumetric flow rate yields the insect's total carbon dioxide production. The fact that the concentration difference between the incoming and outgoing gas is greater means that the insect's final carbon dioxide production is also higher for a given volumetric flow rate. Conversely, the insect's final respiratory quotient reveals which specific substrate it is oxidizing: a respiratory quotient close to the maximum value corresponds to carbohydrate combustion, whereas a considerably lower quotient corresponds to lipid oxidation, and an intermediate value indicates a mixture of these two distinct substrates. If the insect oxidizes a higher proportion of carbohydrates relative to lipids, its final respiratory quotient moves closer to that maximum value.

Thus, in this model—where the insect's final metabolic rate is determined by gas exchange—the specific metabolic rate scales with the insect's mass according to Kleiber's law, resulting in a higher rate for a small insect compared to a much larger one. Consequently, the smaller the insect's body mass, the higher its specific metabolic rate becomes, in accordance with the Kleiber's law that concludes this model.

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