A subject is given 100% FiO2 and takes a single vital capacity breath followed by a full exhalation. Nitrogen concentration is measured over time and an increase in nitrogen concentration is seen after a plateau. Why does this occur?

  1. Effect of gas exchange in poorly compliant part of the lung
  2. Increased dead space
  3. Less oxygen available in apical alveoli
  4. Effect of closing capacity exceeding FRC