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How do respiratory quotients measure oxygen use and carbon dioxide production?

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Diagram showing how epinephrine binding to a beta-adrenergic receptor on a cell membrane activates a G-protein, which then stimulates adenylyl cyclase to convert ATP into cyclic AMP (cAMP), leading to physiological effects such as increased heart rate, blood vessel dilation, and glycogen breakdown. — Biochemistry In biochemistry, researchers study the chemical substances and processes that occur in living organisms and cells. In particular, various processes by which cells gain energy, such as epinephrine-stimulated cAMP synthesis, have been deduced through the study of biochemistry.

The respiratory quotient (RQ) is a ratio used to indicate the relationship between the volume of carbon dioxide produced during respiration and the volume of oxygen consumed by an organism.12

Rather than measuring these gases directly, the RQ is calculated as:

$R Q = \frac{Volume of C O_{2} produced}{Volume of O_{2} consumed}$

In biological systems, cells rely on aerobic metabolism within the mitochondria to produce energy. During this process, oxygen is consumed to oxidize substrates like glucose, resulting in the production of water and carbon dioxide as metabolic byproducts. The respiratory quotient serves as a diagnostic tool in biochemistry to help determine which substrates are being utilized for energy production, as different fuels (such as carbohydrates, fats, or proteins) yield different ratios of carbon dioxide output to oxygen intake.3