Transport of respiratory gases is the primary function of the respiratory system. Pulmonary gas exchange describes the process of oxygen (o2) uptake and carbon dioxide (Co2) elimination by the lungs. This chapter focuses on pulmonary gas exchange and on how the lungs load adequate o2 in the blood to meet tissue 02 demand. It also covers the physiology of 02 and C02 transport by the cardiovascular system and tissue gas exchange. The relation between all of these elements is illustrated by the o2 cascade, which shows how Po2 decreases along the o2 transport chain between the environment and the mitochondria (Fig. 1).
Figure 1 shows normal Po2 values that could be measured in gas, blood, and tissue samples from a resting individual at sea level. The Po2 drop between dry room air and tracheal gas is due to the humidification of inspired gas. Ventilation is the primary determinant of the much larger drop in Po2 between inspired and
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