Respiratory Quotient and the Measurement of Metabolism

Experiments on oxygen consumption and expiration of carbon(iv)oxide during exercises helps us ascertain how oxygen consumption and carbon(iv)oxide production which we sum up as the (metabolic rate), vary in different conditions of muscular activity, temperature and diet. Respiratory quotient also helps us work out the ratio of carbon(iv)oxide produced and oxygen consumed which gives greater insights into the nature of respiratory processes.

The Respiratory Quotient is the amount of carbon(iv)oxide produced, divided by the amount of oxygen used

RQ= Carbon(iv)oxide produced/ Oxygen used

Respiratory Quotient can also tell us what kind of food group is being oxidized(substrate being used in respiration). In theory, RQs for the complete oxidation of food groups such as fats, proteins and carbohydrates can be easily worked out using appropriate chemical equations. Proteins give an RQ of 0.9, fats 0.7 while carbohydrates give an RQ of 1.0. We might expect an organism to give one of these three RQs or at least a close approximation to it depending on the food being respired, but there are several factors that pose challenges and influence the values obtained by experiment. For instance, a respiratory substrate is rarely oxidized fully and very often a mixture of substrates is used. Most animals have an RQ of between 0.8 and 0.9 at resting conditions. Man’s RQ is generally around 0.85 and as protein is normally not used to a large extent, an RQ of less than 1.0 can be assumed to mean that carbohydrates as well as fat is being respired.

Respiratory quotient also tells us the type of metabolism that is going on and the type of food being used in respiration. For instance, a high RQ exceeding 1.0 is often obtained from organisms or from tissues which are short of oxygen. Under these conditions, they resort to anaerobic respiration with the resultant effect that the amount of carbon(iv)oxide produced exceeds the amount of oxygen used. High respiratory quotients can also occur as a result of the conversion of carbohydrates to fats. This is true because carbon(iv)oxide is being liberated in the process. This can be readily observed in organisms that are piling up extensive food reserves such as mammals preparing to hibernate and in fattening livestock.

Low respiratory Quotient on the other hand, may be assumed to mean that some or all of the carbon(iv)oxide released during respiration is being put to some kind of use by the organism. In plants, this carbon(iv)oxide produced may be used in the building of carbohydrates through a photosynthetic process,  and in animals, it could be used in the building of calcareous shells.