Respiratory system: Residual volume.

The idea

Residual volume is the volume of air that remains in the lungs after a maximal expiration. It cannot be exhaled, ensuring that the alveoli and airways remain partly inflated to prevent the lungs from collapsing. Because this air stays in the lungs, residual volume cannot be measured by simple spirometry. Residual volume is not part of vital capacity, but it is added to vital capacity to calculate total lung capacity. During exercise, residual volume remains relatively unchanged. However, because a proportion of each breath mixes with this residual stale air, effective gas exchange relies on increasing tidal volume and breathing frequency to ensure sufficient oxygen reaches the alveoli.

What earns the marks

• Residual volume is the volume of air remaining in the lungs after a maximal expiration.
• Residual volume cannot be measured by simple spirometry because the air does not leave the lungs.
• Residual volume is excluded from vital capacity but is added to vital capacity to calculate total lung capacity.
• During exercise, residual volume remains relatively unchanged while tidal volume increases.

Common mistakes and how to put them right

• Misunderstanding: students believe residual volume can be exhaled with enough effort.

  Put it right: It remains in the lungs after the most forceful expiration and cannot be breathed out.

• Misunderstanding: students include residual volume within vital capacity.

  Put it right: Vital capacity excludes residual volume, while total lung capacity includes it.

• Misunderstanding: students think a spirometer alone can measure residual volume.

  Put it right: Simple spirometry cannot record it because the air is not exhaled.

Exam tips

• Anchor the definition to the phrase 'after a maximal expiration' so you do not confuse residual volume with expiratory reserve volume.
• Remember the equation: total lung capacity equals vital capacity plus residual volume.
• If a question asks about spirometry traces, remember that residual volume will not be directly plotted as an exhaled volume.