Respiratory Quotient (RQ)

Respiratory quotient is denoted by R.Q. It is the ratio of volume of CO2 expired to the volume of O2 inspired, this ration can be represent as below:

For complete oxidation of carbohydrate R.Q is 1 because in the oxidation of glucose which is represented by equation, C6H12O6 + 6O2 ----------------> 6CO2 + 6H2O, the CO2 produced is equal to O2 utilized. Therefore R.Q of glucose combustion  1.0.

For fats, e.g. triolen, the oxidation is represented by the following equation

   C57H104O6 + 80 O2 -------------------->57 CO2 + 52 H2O

R.Q for this fat is there fore 57/80 = 0.71. Other fats also have R.Q near 0.70
Alcohol sometimes is responsible for providing enough energy to man. Its R.Q is 0.67 as is clear from the following equation

   C2H5OH + 3O2 --------------------------> 2CO2 + 3H2O

The determination of R.Q of protein is more difficult because protein contain N and S in addition to C, H, and O. However using specialized methods the R.Q of protein has been found to be 0.80.

Under normal circumstance an organism does not utilize only one type of foodstuff as its source of energy but obtains its energy from combustion of all three food stuffs, i.e. carbohydrates, fats, and proteins. Thus the R.Q of an intact animal will be the average of the R.Q of all these 3 types of foodstuffs. It has been found that on an ordinary mixed diet, the R.Q is 0.85. In the post-absorptive state, i.e. when the person has not ingested any food article in the last 14 hours, the R.Q is usually 0.82. An excess of carbohydrates in the diet tends to raise the R.Q while greater fat oxidation (more fat intake, starvation) leads to a fall in the R.Q

   Methods of Determining R.Q in Human Being

There are two methods of determining R.Q in man which are open circuit method and closed circuit method. In both of these methods the amount of O2 consumed and CO2 evolved by a person over a given length of time are to be found out first.

Open Circuit Method

The subject is made to breathe in a Douglas bag for few minutes. The volume of air breathed is measured in a gas meter and a sample is analyzed for the O2 and CO2 concentrations. For this purpose, Haldane gas analysis apparatus is used. In this apparatus a known volume of the gas sample is first treated with KOH solution. CO2 is taken up by KOH resulting in a corresponding decrease in the original volume of the gas sample being analyzed. From this the concentration of CO2 in the expired air is found out. Later the remaining gas is made to react with alkaline pyrogallate (pyrogallic acid in KOH) which absorbs O2; from this the concentration of O2 in the expired air is found out. Because the subject inhales atmospheric air whose composition is known (practically constant) it is quite easy way to find out the amount of O2 used and CO2 given off in the experiment. From these values R.Q is calculated by the formula, R.Q = Volume of CO2 exhaled/Volume of O2 utilized.

Closed Circuit Method

A spirometer is filled with O2 for inhalation by the subject. The subject inhales from and then exhales into the same apparatus. The expired gases are made to pass over a concentrated solution of NaOH which absorbs all CO2 present in these gases. As the subject continues breathing from and into the Spirometer, the amount of O2 in the spirometer falls resulting in a fall in the spirometer volume. The decrease in the volume of gas in the spirometer is automatically recorded on a calibrated paper which is wound on a drum rotating at a prefixed speed. The fall in the volume of the spirometer gives the volume of O2 consumed. For the determination of CO2, it is liberated from the absorbent by adding to it H2SO4. The reaction which take place are given below

   2NaOH + CO2 -------------------------> Na2CO3 + H2O (Absorption of CO2)
   Na2CO3 + H2SO4 --------------------> Na2SO4 + H2O + CO2 (Release of CO2)

The CO2 released from the absorbent is made to enter the spirometer which shows an increase in its volume. This increase in the volume of the spirometer gives the volume of CO2 exhaled by the subject. From the values of O2 consumed and CO2 liberated, and putting these values in the formula of R.Q  (R.Q = Volume of CO2  exhaled/Volume of O2 utilized), R.Q can be determined.