Respiratory Quotient: Meaning, Effect, Application

Respiration in simple terms is defined as the process of breathing. The basic function of inhaling and exhaling gases is something all of us are aware of. Now, we will explore a different facet of breathing called Respiratory Quotient.

Let us take a quick look at what respiratory quotient is, and its relationship with aerobic respiration.

What is Respiratory Quotient?

The process of respiration in the presence of oxygen (aerobic) is used by most living beings in order to make efficient use of energy derived from food. This respiratory process comprises of two basic functions – inhalation of oxygen and exhalation of carbon dioxide.

The respiratory quotient is the real proportion of the quantity of oxygen inhaled and the quantity of carbon dioxide released during the process of respiration. This is also known as the respiratory ratio which is represented by the symbol RQ. 

What effect does the kind of respiratory substrate have on the respiratory quotient:

The kind of substrate used in the process of respiration determines the respiratory quotient. The respiratory quotient can also be used to calculate the BMR (Basal Metabolic Rate) when the production of carbon dioxide is taken into account. These measurements collectively are indirect calorimetry which can be gauged by using the Ganong respirometer.

Respiratory Quotient for Carbohydrates:

This can be illustrated by the chemical equation for glucose oxidation.

C6H12O6 + 6O2 → 6CO2+ 6H2O

RQ = 6 CO2 / 6 O2 

In this case of complete oxidisation of all carbohydrates, the respiratory quotient is one. This means equal amounts of oxygen and carbon dioxide are consumed and released.

Respiratory Quotient for Fats:

Since fats often play a role in respiration, the respiratory quality drops to less than one. This is also because fats contain reasonably lesser oxygen atoms in proportion to atoms of hydrogen and carbon and hence have a different chemical composition. 

For instance, 

C16H32O2 + 23 O2 → 16 CO2 + 16 H2O

RQ = 16 CO2 / 23 O2 = 0.696

The respiratory quotient is approximately less than 7 for fats. 

Respiratory Quotient for Proteins:

For proteins, the respiratory quotient can be calculated by the chemical equation for the oxidation of albumin. 

For instance,

C72H112 N18 O22S + 77 O2 → 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2

RQ = 63 CO2 / 77 O2

The respiratory quotient is roughly 0.8 for proteins.

It is important to take cognizance of the fact that the respiratory quotient in living beings is not always one. In fact, it can often be greater than one. Proteins and fats are almost never utilised as respiratory substrates.

Where is Respiratory Quotient applied?

In extreme cases, such as chronic and obstructive lung diseases, the respiratory quotient may be used. Others applications include:

• It is used to analyse liver functioning
• It is used as a gauge of under or over feeding
• It is utilised for diagnosing liver cirrhosis patients’ conditions who have non-protein respiratory quotient