Respiratory Quotient: Definition and Factors | Organisms | Biology

In this article we will discuss about:- 1. Definition of Respiratory Quotient 2. Factors Affecting Respiratory Quotient 3. Value of Determining 4. Various Organs and Tissues.

Definition of Respiratory Quotient:

It is the ratio of the volume of CO₂ produced by the volume of O₂ consumed (i.e., CO₂/O₂) during a given time. It should be noted that R.Q. is simply a ratio. It gives no idea as to the absolute quantity of gaseous interchange. Proportional increase or diminution of CO₂ produced and O₂ utilised, will keep the ratio unchanged. But any disproportionate variation will be reflected by a corresponding change in the R.Q.

Normal R.Q:

In a healthy adult it is 0.85 for a mixed diet.

Method of Determination:

This is done by measuring the volume of O₂, consumed and CO₂ produced during a given time with the help of Douglas bag (Fig. 8.14) and other similar instruments.

Factors Affecting Respiratory Quotient:

i. Role of Diet:

(a) In case of carbohydrate diet the R.Q. is unity. Because in carbohydrate diet the volume of CO₂ produced is same as the volume of oxygen consumed. This is due to the fact that, in the carbohydrate molecule, the amount of O₂ present is just sufficient to oxidise the H present in the same molecule. Hence, external oxygen is necessary only to convert the C of the molecule into CO₂. So that the volume of O₂ consumed and the volume of CO₂ produced will be same.

This is represented in the following equation:

(b) In case of fats the R.Q. will be lowest and is about 0.7; because fat is an oxygen-poor compound. The oxygen present in it cannot fully oxidise the H of the molecule. So that, oxygen consumed from outside, is used for two purposes-first, for oxidising C and producing CO₂ and secondly, for oxidising H giving H₂O. Consequently, the volume of CO₂ produced will be less than the volume of O₂ utilised. Hence, R.Q. will fall and will be about 0.7.

This is shown in the following equations:

(c) In case of proteins the R.Q. is about 0.8.

In any condition where fats are burnt chiefly (starvation, advanced diabetes, etc.), the R.Q. will be about 0.7.Whereas with a predominant carbohydrate combustion the R.Q. will approach 1.

ii. Effect of Inter-Conversion in the Body:

When carbohydrates are converted into fats in the body, R.Q. will rise. Because in the process an oxygen-rich substance is converted into an oxygen-poor compound. So that some amount of O₂, liberated from carbohydrate, will be utilized for purposes of oxidation. Consequently, less oxygen will be needed from outside. Hence, the amount of CO₂ produced will be more than the amount of O₂ consumed. So that R.Q. will rise. When fat is converted into carbohydrate just the opposite effects will be produced and R.Q. will fall.

It is therefore evident that R.Q. value will indicate the following:

(a) The type of foodstuff burning in the body, or

(b) The nature of conversion of one foodstuff into another in the body.

iii. Acidosis:

During acidosis CO₂ output is greater than O₂ consumption so the R.Q. rises. The R.Q. falls due to the reverse condition of acidosis, i.e., CO₂ output is lesser than O₂ consumption.

iv. Alkalosis:

Here the R.Q. will fall, because respiration is depressed and CO₂ will be retained in the body (i.e., less CO₂ is produced).

v. Rise of Body Temperature:

It may increase R.Q. as in acidosis. [Rise of body temperature, such as in fever, will cause increased breathing and thereby will wash out more CO₂].

vi. Diabetes Mellitus:

In advanced cases of diabetes, when little carbohydrate is burning, energy is supplied mainly by oxidation of fats. Hence, R.Q. will fall. In such cases, if insulin is injected, carbohydrates will start burning and R.Q. will rise.

vii. Starvation:

Here the subject has to live on its own body tissues. In the first stages (1-2 days) energy is derived mainly from the stored glycogen, so that the R.Q., although it falls below normal (0.85), is proportionately high – 0.78. But later on, when energy is derived chiefly from the combustion of fats, R.Q. will fall still further and will be about 0.7.

viii. Voluntary Hyperpnoea:

By this process excess CO₂ is washed out without a corresponding increase of O₂ utilized, so that R.Q. will be above unity.

ix. Muscular Exercise:

a. With moderate exercise (with a normal mixed diet) the R.Q. remains almost unaltered. Because in exercise the body uses different foodstuffs in the same proportion as at rest.

b. With violent exercise lactic acid enters blood and produces acidosis. Pulmonary ventilation will be raised washing out more CO₂. Consequently, R.Q. rises and may go above 2 even.

c. During recovery from violent exercise R.Q. falls, because less CO₂ is evolved. Gradually it goes back to normal.

Value of Determining Respiratory Quotient:

i. R.Q. acts as a guide as to the type of food burning or the nature of synthesis taking place in the whole body as well as in a particular organ.

ii. R.Q. is very helpful in determining metabolic rate.

iii. Non-protein R.Q. helps in finding out the proportion of the three foodstuffs that are being utilized in the body.

iv. Determination of R.Q. helps in the diagnosis of various pathological conditions, such as acidosis, alkalosis, etc.

The R.Q. of Various Organs and Tissues:

The R.Q. of the individual organs, such as liver, spleen, heart, brain, etc., can be measured by noting:

(a) The total blood flow in a given time through the organ and

(b) The oxygen consumed and CO₂ produced by the organ at same time.

This can be found out by determining the arteriovenous difference of these gases and then multi­plying the figure by the total volume of blood flow through the organs. In this way the R.Q. of various organs can be worked out. The total gaseous interchange of an organ will show its metabolic rate and R.Q. of the organ will indicate the nature of foodstuff burning in it.