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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