Oxygen Cascade

Gas exchange between cells and the environment occurs by a series of physiologic transport steps across different structures, as shown in Fig. 1. (This figure uses a standard set of symbols developed for quantitative descriptions of respiratory physiology, which are defined in Table 1 and in the section titled "Physical and Chemical Principles in Respiratory Physiology.'') Breathing movements bring fresh air into the lungs, and the heart pumps O2-poor blood to the lungs. O2 diffuses from the lung gas into the blood, and this O2-rich blood is returned to the heart via the pulmonary circulation. Arterialized blood is pumped to the various organs and tissues of the body via the systemic circulation. Finally, O2 diffuses out of the systemic capillaries to metabolizing tissues and ultimately to the mitochondria inside cells. CO2 moves out of the cells to the environment through these same steps in the opposite direction from O2. Figure 1 also shows normal values for the levels of O2 and CO2, expressed as partial pressures, Po2, and Pco2 (see later discussion). Note that the O2 level decreases at each step in this series of transport processes, so this transport chain is frequently called the O2 cascade.

The physiologic principles governing O2 supply-and-demand relationships can be studied by measuring maximal O2 consumption while varying the supply. Figure 2 shows how decreasing oxygen levels in the

Tissues Pto2 < 40 Ptco2 > 46

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