Respiratory Quotient

Introduction

Respiration is the process of getting oxygen into the body from the outer environment and relieving carbon dioxide from the body to the outer environment. These gases play an important role in physiological processes. The ratio of carbon dioxide released to the oxygen absorbed is called the Respiratory Quotient, which can be abbreviated as RQ. The respiratory quotient can be called as respiratory ratio or respiratory coefficient also.

What is a Respiratory Quotient?

A respiratory quotient is a logarithmic number used to calculate basal metabolic rate by the ratio of carbon dioxide liberated to oxygen absorbed.

$$\mathrm{RQ=CO_{2}\:liberated/O_{2}\:absorbed}$$

With the help of an instrument called a respirometer, we can detect the respiratory quotients of different substrates. RQ can be calculated for a specific substrate such as protein, sugar, or lipids.

RQ for Different Respiratory Substrates

The respiratory quotient may be different for different substrates.

• The respiratory quotient for carbohydrates − When carbohydrates are used as the substrate, the amount of carbon dioxide evolved is the same as that of the amount of oxygen absorbed. So RQ for carbohydrates is 1.

$$\mathrm{C_{6}H_{12}O_{6}+6O_{2}\rightarrow 6CO_{2}+H_{2}O}$$

$$\mathrm{So\:RQ\:of\:carbohydrates\:is\:6CO_{2}/6O_{2}=1}$$

• The respiratory quotient for proteins − In the case of protein substrates the oxygen absorbed is more than the carbon dioxide released. RQ for proteins may be in the range of 0.8 to 0.9. Let us take albumin protein as an example.

$$\mathrm{C_{72}H_{112}N_{18}O_{22}S+77O_{2}\rightarrow 63CO_{2}+38H_{2}O+SO_{3}+9CO(NH_{2})_{2}}$$

$$\mathrm{So\:RQ\:for\:albumin\:protein\:is\:63CO_{2}/77O_{2}=0.8}$$

• The respiratory quotient for fats − In the case of fats the oxygen absorbed is more than the carbon dioxide released. So RQ for fats is always lower than 1. Let us take palmitic acid as an example.

$$\mathrm{C_{16}H_{32}O_{2}+23O_{2}\rightarrow 16CO_{2}+16H_{2}O}$$

$$\mathrm{So\:RQ\:for\:albumin\:palmitic\:acid\:is\:16CO_{2}/23O_{2}=0.696}$$

• The respiratory quotient for organic acids − In the case of organic acids substrate oxygen absorbed is less than the carbon dioxide released. So RQ of organic acids is always higher than 1.

  Examples of organic acids include, RQ for malic acid is 1.33, and RQ for citric acid is 1.14.

• The RQ for a mixture of substrates is 0.8.

Characteristics of Respiratory Quotient

• RQ is a non-integer number that can be calculated by the ratio of carbon dioxide released to the oxygen absorbed.

• The RQ of the body is almost 1 which confirms the volume of oxygen absorbed is the same as that of the carbon dioxide released. If it is more than 1, it indicates the amount of oxygen is less than carbon dioxide, and if RQ is less than 1 it indicates the amount of oxygen is more than the amount of carbon dioxide released.

• The molecules with low oxygen content have more RQ and the molecules with high oxygen content have less respiratory quotient.

Terms Related to Respiratory Quotient

• Aerobic Respiration − The process of respiration in the presence of oxygen that releases ATP is called aerobic respiration.

• Anaerobic Respiration − Anaerobic respiration occurs when sugar is broken down to release ATP in the absence of oxygen.

• Cellular Respiration − Number of metabolic reactions that occur inside a living cell during the process of respiration.

• Basal Metabolic Rate − The energy an individual needs to maintain at a resting state is termed basal metabolic rate.

Application of Respiratory Quotient

• The RQ can help to detect the lacking of potting soil in culture media.

• The RQ of chronic obstructive pulmonary disease (COPD) patients can be reduced by increasing the fat content of their food. This will reduce the burden of removing carbon dioxide from their bodies.

• The RQ of less than 0.85 shows underfeeding results in the usage of fat and above 1 shows overfeeding which results in lipogenesis.

• The RQ can be used to diagnose many hepatic conditions.

• A decrease in RQ which is caused due to damaged carbohydrate metabolism indicates diabetes.

• An RQ helps in determining conditions like acidosis and alkalosis.

• An RQ is used to measure the metabolic rate.

Definition of Respiratory Quotient

The respiratory quotient is defined as the amount of carbon dioxide liberated over the amount of oxygen absorbed during the process of respiration. The substrate is oxidized by aerobic respiration resulting in the evolution of carbon dioxide.

Utilization of Respiratory Quotient

• RQ can be used to detect the utilization of lipids and carbs.

• RQ is used to measure the severity of diabetes.

• RQ is used to diagnose hepatic conditions like cirrhosis.

• People can use RQ to monitor their physical fitness levels and guide them in obtaining adequate nutrition.

Clinical Significance

• Chronic Obstructive Pulmonary Disease (COPD) − The loss of elasticity of the alveoli prevents the COPD patient from fully exhaling, resulting in carbon dioxide remaining in the alveoli. When patients with an RQ ratio greater than 0.75 are given fat-rich meals, they produce less CO2, resulting in a reduction in alveolar ventilation.

• Weight Gain During Type-2 Diabetes − RQ can be used to analyze the gain of weight in type-2 diabetes patients. Studies have shown that patients who are on oral hypoglycemic agents and who have a higher BMI have a higher RQ ratio than people with normal BMI.

Conclusion

This tutorial helps to give a brief idea about what is respiratory quotient. Characteristics, utilization, and clinical significance of RQ have been described in this tutorial. This tutorial also helps to understand terms related to RQ, and respiratory quotients for different substrates.

FAQs

Q1. What is resting metabolic rate? Which equation is used to calculate BMR?

Ans. An individual's resting metabolic rate measures how much energy the body requires to function at rest. Harris-Benedict equation is used to calculate BMR.

Q2. What is the function of a respirometer?

Ans. The respirometer measures the oxygen and carbon dioxide exchange rate of a living organism during respiration.

Q3. What is lipogenesis? How it is related to RQ?

Ans. The process of converting glucose into fatty acids and triglycerides is called lipogenesis. Overfeeding results in lipogenesis which increases the RQ by more than 1.

Q4. How do you measure the severity of diabetes with the help of RQ?

Ans. In diabetics without treatment, the glucose levels were higher, and RQ values were lower than in diabetics who had received treatment.

Q5. What is the role of RQ in detecting liver cirrhosis?

Ans. A lower respiratory quotient (RQ) can be caused by an increase in gluconeogenesis and fatty acid oxidation in patients with cirrhosis.