Beyond this, however, it is possible to measure the concentrations of the inert gases in the expired gas from the subject. The ratio of the mixed expired concentration to the mixed venous concentration has been termed excretion () and describes the ventilation to regions of varying . When taken together: where: denotes volume of an inert gas eliminated per minute (mL/min) denotes minute ventilation (mL/min) denotes cardiac output (mL/min) When observing a collection of alveoli in which PO2 and PCO2 are uniform, local alveolar ventilation and local blood flow define : From these equations it can be deduced that to have knowledge of either retention or excretion implies knowledge of the other. Moreover, a similar understanding exists for the relationship between the distribution of blood flow and the distribution of ventilation. Limitations The data produced by the MIGET is an approximation of the distribution of ratios across the entire lung. It has been estimated that nearly 100,000 gas exchange units exist in the human lung; this could lead to a theoretical maximum of VA/Q compartments as high as 100,000, in that case. References Pulmonology Medical tests