The respiratory exchange ratio is the ratio of carbon dioxide output to oxygen uptake, and is usually determined by simultaneous measurement of the partial pressure of carbon dioxide and the partial pressure of oxygen at the mouth.


At steady state, there is a dynamic equilibrium between the respiratory exchange ratio in the alveolus, alveolar capillary blood, and tissue. The respiratory exchange ratio at steady state is termed the "respiratory quotient" and is a reflection of cellular metabolism. It is usually designated by the variable "R".


respiratory quotient (R) =

= (carbon dioxide production) / (oxygen consumption)


This can be approximated by the following equation:


respiratory quotient =

= ((partial pressure of oxygen in the inspired gas) - (PAO2)) / (PACO2)



• PACO2 is approximated by PaCO2

• The partial pressure of oxygen in the inspired air is derived in the previous section.

• PAO2 is discussed in the next section.



• The normal respiratory quotient in resting persons is 0.8, with range 0.7-1.0.

• The respiratory exchange ratio may differ significantly from the respiratory quotient depending on the person's ventilation. If a person hyperventilates then the ratio may be increased above the normal resting level.

• With exercise the ratio tends to increase from resting levels and may exceed 1.00 as carbon dioxide production increases. However, at steady state with exercise for several minutes at a constant workload, the ratio will approximate the respiratory quotient.

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