Fig. 2.

Schematic representation of the respiratory cycle illustrating the closure point (CP) between the functional residual capacity (FRC) and the residual volume (RV). The CP volume can be experimentally identified using the ¹³³Xe bolus method [26] and indicates the depth of expiration when airways closure begins. Terminal bronchiole closure and subsequent reopening occurs most prominently at expiratory volumes beyond the closure point. The blue line indicates expiration to CP and the red line indicates expiration beyond CP leading to greater aerosol production (Modified from Almstrand A et al. ref. 20). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)