Breathing at higher flow rates (e.g. during exercise) will affect the ventilation distribution because of an increasing influence of airway resistance. Owing to the vertical pleural pressure gradient, dependent airways are less dilated than those in upper lung units. Accordingly, resistance to gas flow will be higher in dependent lung regions and impede ventilation to these regions more than to upper regions. Thus the vertical distribution of ventilation within the lung becomes more uniform with increasing inspiratory flow rate (Milic-Emili 19.91). Since the pulmonary perfusion is also more evenly distributed at a higher cardiac output (see below), ventilation-perfusion matching is approximately preserved compared with resting conditions. In addition, the more efficient use of the alveolar capillary bed during exercise increases the area available for diffusion of O 2 and CO2.
The expiratory resistance may also influence the distribution of ventilation. Chronic obstructive lung disease (asthma, chronic bronchitis, emphysema) causes airway narrowing that may become apparent mainly or solely during expiration because of an expiratory muscle activity that compresses the lung and the airways. Lung regions that have not emptied completely because of an expiratory resistance exert an increased elastic recoil which will impede the inspiration of gas to those regions. It is not clear to what extent this has any bearing on acute lung disease requiring intensive care.
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