The ratio of the volume of carbon dioxide eliminated from the lungs per minute to the volume of oxygen taken into the lungs during the same time. RER = VCO₂/VO₂. During rest, the respiratory exchange ratio usually equals the respiratory quotient (RQ), but during exercise an unstable state arises due to expired air containing carbon dioxide and oxygen derived from stores within the body, and the R value is not the same as the RQ value.

The Oxford Dictionary of Sports Science & Medicine. Copyright © Michael Kent 1998, 2006, 2007. All rights reserved.