Nitrogen equality is not a myth

Abstract

The fundamental physiological concept that elemental nitrogen is inert in the mammalian body and that consequently the amount of nitrogen expired in the steady state equals the amount inspired has recently been questioned. If expired nitrogen exceeds the amount inspired, as has been claimed, the concentration of nitrogen in the mixed venous blood in the pulmonary artery must be higher than that in arterial blood. We therefore analyzed pulmonary arterial and arterial blood simultaneously in unanesthetized dogs. We studied two dogs who had been trained to stand quietly in a sling. They were judged to be in the steady state when their rectal temperature and heart rate remained constant over a twenty minute period. Eleven pairs of blood were analyzed when the dogs were fasting and fourteen pairs one or two hours after the dogs ate meals high in protein. Our analyses were precise to within 0.6 μl/ml and we found no evidence of a difference in N 2 content between mixed venous and arterial blood. The mean N 2 concentrations ± the standard error of the mean were 9.09 ± 0.19 for mixed venous blood and 9.21 ± 0.19 for arterial blood. We therefore conclude that the classical concept of nitrogen equality is a sound one and that pulmonary function tests based on it are valid.