Different substrates get oxidized during respiration.

Respiratory Quotient (RQ) indicate which type of substrate, i.e., carbohydrate, fat or protein is getting oxidized by measuring the volume of carbon dioxide, i.e.,

CO2 released or evolved by the volume of oxygen O2 consumed.

A= Volume of CO2 released

B =Volume of Oxygen consumed

RQ equal to 1 = carbohydrate like glucose.

RQ higher than 1 = substrate can be organic acid like malic acid.