Respiratory Quotient

The respiratory quotient (RQ), also called the respiratory ratio, is a simple but informative measurement that allows a physiologist to infer which organic substrate a respiring tissue is actually oxidising, without having to identify that substrate directly. It is defined as the ratio of the volume of carbon dioxide evolved by a respiring tissue to the volume of oxygen consumed by that same tissue, over the same period of time and under the same conditions: RQ = (volume of CO₂ evolved) / (volume of O₂ consumed). Because the volume of a given number of moles of any gas is the same under identical temperature and pressure conditions, this ratio of gas volumes is equivalent to, and can be calculated directly from, the ratio of moles of carbon dioxide released to moles of oxygen consumed in the underlying chemical reaction.

The value taken by the RQ depends directly on the chemical composition of the substrate being respired, because different classes of organic molecule differ in how much oxygen they already contain relative to their carbon and hydrogen content, and therefore in how much additional external oxygen is required to oxidise them completely to carbon dioxide and water.

When a carbohydrate such as glucose is the substrate being respired, the RQ works out to exactly 1. This follows directly from the balanced equation for complete glucose oxidation, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, in which six moles of oxygen are consumed and exactly six moles of carbon dioxide are released -- an equal volume of each gas, giving an RQ of 6/6 = 1. Because carbohydrates already contain hydrogen and oxygen in the same 2:1 ratio found in water, oxidising them completely requires no more external oxygen, mole for mole, than the carbon dioxide they release, so any respiratory substrate belonging to the carbohydrate class reliably gives an RQ at or very close to 1.

When fats are the substrate being respired, the RQ works out to a value distinctly less than 1, commonly cited as roughly 0.7 for a typical fat such as tripalmitin. Fats are chemically more reduced than carbohydrates -- they contain proportionally far more carbon and hydrogen and far less oxygen for the same number of carbon atoms -- so completely oxidising a fat molecule requires a substantially larger volume of external oxygen relative to the volume of carbon dioxide it eventually releases, pulling the ratio below 1.

When proteins are the substrate being respired, the RQ takes an intermediate value, commonly cited as approximately 0.9, reflecting the fact that proteins are chemically intermediate in their oxygen content between carbohydrates and fats, and also that proteins are not, in practice, fully oxidised by respiration in the first place -- the nitrogen-containing amino group of each amino acid is first removed by deamination, before the remaining carbon skeleton enters the respiratory pathway, so protein respiration does not go to fully balanced completion in quite the same simple way that carbohydrate or fat oxidation does. …