What is the difference between respiratory quotient and respiratory exchange ratio?

The respiratory quotient (RQ) is a measure of the ratio of carbon dioxide produced to oxygen consumed during metabolism at the cellular level, typically reflecting the type of substrate being metabolized (e.g., carbohydrates, fats). In contrast, the respiratory exchange ratio (RER) is measured at the level of the lungs and can vary based on factors like exercise intensity and the metabolic state of the body, providing a broader view of gas exchange. RQ is usually a steady-state measurement, while RER can fluctuate with changes in activity and metabolism.