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LEARNING OBJECTIVES

Learning Objectives

  • The student will be able to distinguish between the conducting zone where ventilation occurs, and the respiratory parenchyma where diffusion occurs.

  • The student will be able to define terms for normal lung volumes, including tidal volume, dead space, total lung capacity, vital capacity, functional residual capacity, inspiratory and expiratory reserve volumes, and residual volume.

  • The student will be able to describe the basic procedures used in routine pulmonary function testing, particularly bench top spirometry.

  • The student will be able to describe two techniques to quantify residual volume and functional residual capacity, including underlying assumptions of each.

  • The student will be able to explain the origin and maintenance of negative intrapleural pressures that are caused by elastic recoil forces within normal lung tissues and the chest wall/diaphragm complex.

Respiration consists of two critical and nonoverlapping processes, ventilation and diffusion. Ventilation is the bulk movement of gases in and out of the lungs via the large and intermediate airways that collectively are termed the conducting zone. Here, inspired gas is warmed to 37°C, humidified to 100% relative humidity (RH), and cleansed of most airborne particulates, but its molecular composition is changed only by addition of water vapor. This is because conducting zone airways are too wide and their walls are too thick to allow appreciable molecular diffusion of O2 and CO2 between inspired air and the pulmonary arterial blood coursing through alveolar capillaries. On the other hand, the delicate alveolar membranes of the respiratory parenchyma are thin and have an enormous aggregate surface area. As will be formally introduced in Chap. 9, molecular diffusion requires such large, thin surfaces to maximize O2 and CO2 exchange. The remainder of this chapter will focus on ventilation, and the forces and constraints that make it more or less effective. It is worth noting now that the conducting zone volume is nearly constant during normal breathing, while respiratory parenchymal volume changes very dramatically as the depth of breathing is adjusted.

NORMAL LUNG VOLUMES AND SPIROMETRY

Despite the histological complexity of lung tissue (Chap. 2), its key elements can be represented by a simple line drawing (Fig. 4.1). The many branching airways of the conducting zone that support ventilation are shown as a single tube comprising the anatomic dead space (VD) (Fig. 4.2). VD is normally ~2 mL/kg of ideal body weight, or about 160 mL in a normal adult. In Fig. 4.1, the respiratory parenchyma where diffusion occurs is shown as one contiguous compartment, the alveolar gas volume (VA), here 3.0 L. We define tidal volume (VT) as the total volume of a typical inspiration, but clearly only a portion of each breath reaches the parenchyma while the remainder occupies VD. Thus, in this example where VD = 160 mL, ...

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