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Chapter: Medical Physiology: Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids

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Respiratory Exchange Ratio

The discerning student will have noted that normal transport of oxygen from the lungs to the tissues by each 100 milliliters of blood is about 5 milliliters, whereas normal transport of carbon dioxide from the tissues to the lungs is about 4 milliliters.

Respiratory Exchange Ratio

The discerning student will have noted that normal transport of oxygen from the lungs to the tissues by each 100 milliliters of blood is about 5 milliliters, whereas normal transport of carbon dioxide from the tissues to the lungs is about 4 milliliters. Thus, under normal resting conditions, only about 82 per cent as much carbon dioxide is expired from the lungs as oxygen is taken up by the lungs. The ratio of carbon dioxide output to oxygen uptake is called the respiratoryexchange ratio (R). That is,

R= Rate of carbon dioxide output/ Rate of oxygen uptake

The value for R changes under different metabolic conditions. When a person is using exclusively carbohy-drates for body metabolism, R rises to 1.00. Conversely, when a person is using exclusively fats for metabolic energy, the R level falls to as low as 0.7. The reason for this difference is that when oxygen is metabolized with carbohydrates, one molecule of carbon dioxide is formed for each molecule of oxygen consumed; when oxygen reacts with fats, a large share of the oxygen combines with hydrogen atoms from the fats to form water instead of carbon dioxide. In other words, when fats are metabolized, the respiratory quotient of thechemical reactions in the tissues is about 0.70 instead of1.00. (The tissue respiratory quotient.) For a person on a normal diet consuming average amounts of carbohydrates, fats, and proteins, the average value for R is considered to be 0.825.


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