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The Respiratory System
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Overview of the Respiratory System
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The primary function of the respiratory (pulmonary) system is to maintain systemic arterial blood gas levels within normal range. To achieve this, the rates of O2 uptake and CO2 excretion at the lungs must match the respective rates of O2 use and CO2 production by cellular respiration. The main components of the respiratory system are the lungs, the chest wall, and the pulmonary blood vessels. Muscles of the chest wall power the movement of air into the lungs during inspiration. Distribution of the pulmonary blood flow, to match ventilation, ensures the proper gas exchange. The levels of systemic O2 and CO2 are monitored by chemoreceptors, allowing the pulmonary system to respond to changes in cellular respiration. Treatment of respiratory disorders requires an understanding of factors that govern ventilation (gas flow), diffusion of gases, and perfusion (blood flow) in the lungs.
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The lung is specialized for gas diffusion and has an internal surface area of 50–100 m2. The large surface area is produced by repeated branching of the airways, which begins at the trachea and terminates in over 300 million closed air sacs called alveoli. Ventilation is the process whereby air enters the lungs and comes into contact with alveoli, which are the sites of gas exchange. Each alveolus is surrounded by a dense network of pulmonary capillaries. The blood gas interface is less than 1-μm thick and consists of the following four elements in series (Figure 5-1):
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Thin layer of surface liquid.
Alveolar lining cells (type 1 pneumocytes), plus associated basement membrane.
Thin layer of interstitial fluid.
Pulmonary capillary endothelial cells, plus associated basement membrane.
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The volume, pressure, and temperature of gases are all closely related by physical laws (Table 5-1). These laws can be applied to help explain the mechanics of air movement into and out of alveoli as well as the diffusion of gases across the blood-gas interface.
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