Both alveolar volume and airway dimensions are dependent on the transpulmonary pressure. Airway caliber decreases during expiration. Airways close during a deep expiration, when pleural and extramural airway pressures become positive and higher than luminal pressure ( Milic-Emili 1991). Closure of airways during expiration will begin in the most dependent lung regions, owing to the pleural pressure gradient, and spread up the lung with deeper expiration. Thus airway closure will impede ventilation mostly in dependent lung regions.
Airway closure is smallest in subjects aged about 20 years, where it may be difficult to demonstrate even after expiration to residual volume. Above the age of about 65 years airway closure becomes more prominent and can be detected even above functional residual capacity (FRC) in the upright position. Closing capacity, which is the lung volume at which airways begin to close during expiration, appears to be independent of body position. Since the change in position from upright to supine reduces FRC by approximately 0.7 to 0.8 1iters, airway closure will be seen more regularly above FRC in the supine position and can be expected at ages of about 50 years and older. Thus ventilation of dependent lung regions will decrease with increasing age. Since blood flows predominantly to dependent lung regions (see below), the phenomenon of airway closure seems to be an explanation of the well-known decrease in PaO2 with age.
The increase in airway closure with increasing age is probably caused by a loss of elastic tissue in the lung. Thus the elastic recoil of the lung decreases, making the pleural pressure less negative. This also explains why FRC slowly increases with age (corrected for changes in body size) ( Fig 2). Further causes of an increase in closing capacity include a loss of elastic recoil due to lung disease (chronic bronchitis, emphysema) or an increase in airway tone (asthma). Decrease in FRC can also be seen during anesthesia which may promote closure of airways and impede ventilation in dependent lung regions. Long-standing closure of airways will promote resorption atelectasis. While collapse of a lung unit will take several hours if it is filled with air, gas resorption and subsequent atelectasis may occur within 10 min on oxygen breathing. Breathing oxygen at very low lung volume, close to or at residual volume, further promotes atelectasis formation. Atelectasis is also a common finding in anesthetized subjects, and it can be hypothesized that any acute lung failure accompanying a small lung volume is sensitive to the composition of the respiratory gases. It is possible that the administration of oxygen and nitrous oxide contributes to the pathology of the lung as seen on CT scans.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.