Respiratory Quotient

The respiratory quotient (RQ), or respiratory ratio, is the ratio of the volume of CO₂ released to the volume of O₂ consumed during respiration:

RQ = (Volume of CO₂ released) / (Volume of O₂ consumed)

Its value depends on which respiratory substrate is actually being oxidised:

• Carbohydrates. When carbohydrates are used as the respiratory substrate and completely oxidised, RQ = 1, because the volume of CO₂ released is exactly equal to the volume of O₂ consumed, as the balanced equation for glucose oxidation shows.
• Fats and proteins. When fats or proteins are the substrate, RQ is less than 1, because the volume of CO₂ released is always less than the volume of O₂ consumed - these substrates already contain proportionally less oxygen within their own molecules, so relatively more O₂ must be drawn in to complete their oxidation. For fats, RQ works out to about 0.7; for proteins, about 0.9.
• Anaerobic respiration. RQ is always infinity, because CO₂ is evolved by fermentation without any O₂ being consumed at all. …