Figure 8. Contribution of excreted nitrogen to expired nitrogen concentration from a child lung model.

Expired nitrogen (N₂) is shown with (solid line) and without (dotted line) N₂ excretion for a two compartment lung model adjusted to represent a child with cystic fibrosis. N₂ excretion has been reduced by a factor of 0.45 (A) to reflect the lower body mass and blood volume, or increased by a factor 1.6 (B) to reflect increased tissue perfusion and relative cardiac output. Washout ends at expired N₂ = 1.975% (1/40^th of the starting concentration). Increase in expired N₂ at the true LCI point as a result of excreted nitrogen is indicated by the arrow.