Key Points

• Skeletal muscle contractions generate pressure gradients to pull fresh air into the lung; the diaphragm is the most important muscle for inspiration.

• Expiration at rest is passive, but expiratory muscles are recruited at higher levels of ventilation (e.g., exercise).

• Lung volume is determined by the pressure difference between the airways inside the lungs and intrapleural space outside the lungs. Intrapleural pressure is subatmospheric at most lung volumes.

• Compliance quantifies the pressure-volume relationship of the lungs, which changes with both physiologic and pathologic conditions.

• Pulmonary surfactant, a lipoprotein that lines the alveoli and reduces surface tension, stabilizes the alveoli, reduces the pressures required for ventilation, and helps prevent pulmonary edema.

• The functional residual capacity of the lungs is determined by a balance of lung and chest wall forces, and is a sensitive indicator of lung compliance changes with disease.

• The main site of airway resistance is in the medium-sized bronchi. Pulmonary disease usually starts in small airways, so it is difficult to detect increased airway resistance in early stages of disease.

• Maximum expiratory flow is independent of muscular effort at most lung volumes less than total lung capacity (TLC) and is very dependent on lung compliance. In contrast, maximum inspiratory flow rate is proportional to pressures generated by respiratory muscles at most lung volumes.

• Local differences in lung compliance and airway resistance are the main causes of regional differences in ventilation.

• The oxygen cost of breathing is small in healthy individuals, ranging from 1-3% of total oxygen consumption at rest and less than 15% at maximal exercise.

Essential Medical Physiology, Third Edition

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