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The principles of respiratory physiology underlie the understanding of pulmonary function testing. Assumptions about the respiratory system being measured are implicit in all measurements of lung function but are often poorly understood by the clinicians charged with interpreting the tests and using the results for patient care. This chapter will review simple principles of respiratory physiology with an emphasis on providing a basic understanding of how the respiratory system works and what physiologic principles and assumptions underlie common measurements of lung function used in children. This chapter will not provide a comprehensive explanation of the intricacies of respiratory physiology, nor will it provide a “how to” of lung function testing. The interested reader is directed to the references listed under Suggested Readings.


An understanding of the terminology for the different volumes and capacities of the respiratory system associated with different phases of respiration is fundamental to understanding pulmonary function. Volumes are measured and capacities are inferred from volumes. Figure 496-1 shows a schematic representation of the basic subdivision of lung volume. Tidal volume is the volume breathed in and out with each breath. By definition, the amount of air left in the lungs at the end of a tidal breath is the functional residual capacity (FRC). When an adult or older child is breathing quietly at rest, FRC may equal the elastic equilibrium volume (EEV) of the respiratory system (see the following section on static mechanical properties). However, in infants or children breathing actively, FRC may be above or below EEV. The basic principle is that the amount of air left in the lung at the end of a tidal breath is FRC, and that this volume may vary from moment to moment and from breath to breath in infants and young children, introducing instability and variability into measurements of lung function. Total lung capacity is the total amount of air that the lung can contain at the end of a maximal inspiration, and vital capacity is the total amount that can be breathed out, either slowly or with a maximal forced expiration; in that latter case, the term forced vital capacity (FVC) is used. The volume of air left over is the residual volume (RV) that cannot be expelled from the lungs.

Figure 496-1

Subdivisions of lung volumes. ERC, expiratory reserve capacity; FRC, functional residual capacity; IRC, inspiratory reserve capacity; RV, residual volume; TLC, total lung capacity; VC, vital capacity; Vt, tidal volume.


The respiratory system consists of a number of components including, from the outside moving inward, the chest wall consisting of skin and subcutaneous tissue; the rib cage, diaphragm, and intercostal muscles; the pleural membranes and virtual pleural space; the pulmonary parenchyma, with pulmonary capillaries and interstitial ...

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