Respiratory Quotient RQ

the ratio of the volume of carbon dioxide gas (CO₂) given off by the body to the volume of oxygen (O₂) absorbed in the same time interval. The respiratory quotient may be expressed as

Determination of the RQ is important in research on the characteristics of gas exchange and metabolism in animals and plants.

Animals and humans. When carbohydrates are being oxidized in the organism and the requisite oxygen is available, the RQ is 1. In the oxidation of fats, the RQ is 0.7 and in the oxidation of proteins, 0.8. The RQ of a healthy human at rest is 0.85 ±0.1. Humans engaged in moderate work and animals that feed primarily on plants have an RQ that approaches 1. In humans during prolonged work or hunger, in carnivorous animals, and in hibernating animals, when dissimilation of fats is increased as a result of the limitation of carbohydrate reserves, the RQ is approximately 0.7. The RQ is greater than 1 when there is an increase in the deposition of fat formed from carbohydrates entering the body in the diet. (This occurs, for example, in humans during the restoration of normal weight after starvation or a long illness and in animals that are being fattened.) The RQ rises to 2 during strenuous work and hyperventilation of the lungs, when the body discharges supplementary, bound CO₂. The RQ reaches even higher values in anaerobic organisms, in which a large portion of the CO₂ that is given off is formed by means of anaerobic oxidation (fermentation). An RQ lower than 0.7 occurs during illnesses due to metabolic disturbances or after heavy physical labor.

Plants. In plants the RQ is a function of the chemical nature of the respiratory substrate, the CO₂ and O₂ content of the atmosphere, and other factors. Thus, it characterizes the quality and condition of respiration. When the cell is using carbohydrates for respiration (for example, in the sprouting of grains), the RQ is equal to approximately 1. When it is using fats and proteins (for example, the germinating seeds of oil-producing plants and legumes), the RQ is 0.4-0.7. If O₂ is lacking or is not readily accessible to plant cells (for example, seeds with hard coats), the RQ is 2-3 or higher. A high RQ is also characteristic of cells at growth points in plants.