Respiratory Quotient (RQ)

The RQ is the ratio of CO₂ : O₂

RQ = volume of CO₂ evolved / volume of O₂ absorbed 

or

moles or molecules of CO₂ evolved / moles or molecules of O₂ absorbed

The equation shows that a given volume of carbon dioxide is produced from respiration.

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This volume equals that of oxygen consumed.

These values relate to the respiration of carbohydrates.

RQ =

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Different respiratory substrates result in different RQ values.

• Physical measurements of oxygen intake against carbon dioxide output can give information about the nature of the substrate being used for respiration.
• Such information can supply an idea of the metabolic rate or activity of an organism.
• The metabolic rate can be calculated by measuring the rate of oxygen consumption.

Tripalmitin is a typical fat. The RQ for this fat is 0.70 because to use this fat, the value of CO₂ is 51 (51CO₂) whilst the value of O2 is 72.5 (72.5O₂). 51 / 72.5 = 0.70.

Thus a considerably greater volume of oxygen is required to metabolise a fat than a carbohydrate but, proportionally less carbon dioxide is evolved. HEAT is released in greater kJ with oxidation of fats than of carbohydrates.