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INTRODUCTION AND CONCEPTUAL OVERVIEW OF OXYGEN TRANSPORT

Learning Objectives

  • The student will be able to describe the general process of oxygen transport and identify steps in this cascade that depend upon the lung to be achieved.

  • The student will be able to translate key words and phrases of pulmonary medicine into their internationally recognized abbreviations and acronyms.

  • The student will be able to calculate the partial pressures of constituent gases in normal dry air using Dalton's law and explain the effects of altitude upon them.

  • The student will be able to use Boyle's law and Charles's law to convert gas volumes between standard conditions [Standard Temperature and Pressure, Dry (STPD)] and ambient conditions [Body Temperature and Pressure, Saturated (BTPS)].

  • The student will be able to compute the content of gases equilibrated with aqueous media using Henry's law and their established solubility coefficients.

There is little argument that the urge to breathe is the most compelling drive that humans face, one they must satisfy every minute of life. Our dependence on aerobic metabolism to perform life's complex functions requires coordinated interaction among multiple organ systems to deliver sufficient O2 for those demands (Fig. 1.1). Although only the upper elements of this O2 transport cascade are the formal purview of this book, physicians need to appreciate each aspect since failure at any step can become an O2 transport bottleneck with catastrophic consequences for their patients.

FIGURE 1.1

The oxygen transport cascade. See text for details.

From this perspective, respiration really consists of two sequential processes within the lung, ventilation and diffusion. Each will require several chapters to describe their constituent parts and boundary conditions. As an obvious example, ventilation is the algebraic product of respiratory frequency, f, multiplied by the amount of air moved per breath, termed the tidal volume (VT). Less obvious is the fact that only a portion of each VT enters the alveolar volume (VA) where diffusion can occur, while the remaining portion of VT is confined to anatomical regions unsuitable for diffusion, collectively called the dead space volume (VD). Thus, one goal of this text is to provide the intellectual tools both to determine the fraction of VA in each VT, and to identify possible treatments for patients whose VA/VT is not optimal. As will be seen, the work of breathing and the calories required are substantial to inhale the 8,000-12,000 L of air per day just to support basal oxygen consumption. Clinicians must be able to distinguish if a patient's work of breathing is excessive, and if so, whether due to altered recoil properties of the lungs and chest wall, or to reduced airway diameters which increase dynamic resistance. Such functional distinctions, and the structural ...

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