Anatomical dead space is measured using the single-breath nitrogen technique. The volume of gas exhaled before the nitrogen concentration begins to rise sharply from zero represents:

• A Alveolar dead space
• B Anatomical dead space ✓
• C Closing volume
• D Functional residual capacity

Explanation

The subject inhales a single breath of pure oxygen and exhales while nitrogen concentration is recorded. The initial exhaled gas comes from conducting airways that did not participate in gas exchange, so it contains no nitrogen. The sharp rise marks arrival of alveolar gas, and the volume up to that transition approximates anatomical dead space, about 150 mL. This is Fowler's method; the later sloping phase III reflects uneven emptying.

Reference: West's Respiratory Physiology: The Essentials, 11th ed.

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