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FIGURE 1 The oxygen cascade in a healthy subject breathing room air at sea level shows the pattern of Po2 decrease between the different steps of o2 transport as described in the text. The difference between alveolar and arterial Po2 occurs because of pulmonary gas exchange limitations.

alveolar gas. Diffusion across the blood-gas barrier, and other factors such as shunts and the matching of ventilation to pulmonary blood flow, explains the relatively small decrease in Po2 between alveolar gas and arterial blood. The circulation and o2 diffusion from capillaries to tissues cause the large decreases in Po2 between arterial and venous blood, and between blood and mitochondria in the tissues.

Changes in o2 supply or demand can have different effects on the 02 cascade, depending on the nature of the change. Because 02 transport occurs in a series of steps, changes at one level will affect Po2 at other levels. Therefore, understanding the physiologic factors that determine the Po2 at any step in the o2 cascade is useful for diagnosing abnormalities in gas exchange.

Some General Principles of Gas Exchange

Before covering each step of 02 transport in detail, it is helpful to introduce some general principles that apply to all steps. Chapter 18 introduced the principles of mass balance that will be used here to predict Po2 values for each step in the o2 cascade. The reader should refer to Chapter 18 for derivations of the simple models used here to describe 02 transport by (1) convection, that is, ''bulk flow'' transport by ventilation and blood flow; and (2) diffusion across blood-gas or blood-tissue barriers. It is important to note that o2 is not actively transported or secreted across membranes in the human body.

Quantitative descriptions of gas exchange are also referred to as ''models'' of gas exchange. Gas exchange models in this chapter assume a steady state, defined as an equal and constant rate of gas transport at each step in the o2 cascade. Steady-state conditions do not necessarily imply resting conditions. o2 transport can be elevated but still equal at every step in the o2 cascade, for example, during steady-state exercise. However, non-steady-state conditions occur frequently, for example, at the onset of exercise.

Gas exchange models are useful for quantifying respiratory function. For example, in an ''ideal'' model of alveolar gas exchange, arterial blood equilibrates with

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