GAS TRANSFER



C02 and 02 are transported between the envi­ronment and the tissues by convection and dif­fusion (Fig. 17-5). In the blood, 02 combines with hemoglobin, and the resulting 02 saturation is de­termined by the oxyhemoglobin dissociation curve (Fig. 17-6A). More than 98 per cent of the 02 in the blood travels combined with hemoglo­bin; the remainder is dissolved in the plasma. Above a Pao2 of 150 mm Hg, hemoglobin is totally saturated and carries 1.34 ml 02/gm hemoglobin; further rises in Pao2 increase only the amount of 02 dissolved in the plasma at the rate of 0.003 ml O2/100 ml blood/mm Hg Po2. C02 is carried in the blood in three forms: bicarbonate (90 per cent), dissolved in plasma, or combined with protein, predominantly hemoglobin. The relationship be­tween the Pco2 and the C02 content is represented by the carboxyhemoglobin dissociation curve (Fig. 17-6B), which is steeper and more linear than the oxyhemoglobin dissociation curve.

A number of factors influence the relationship between Po2 and Pco2 and their contents, which can be described as changes in the shape or po­sition of the respective dissociation curves. In­creased Pco2 and temperature and decreased pH shift the oxyhemoglobin dissociation curve to the right, decreasing affinity of hemoglobin for 02 and expediting its release to the tissues. Converse changes in the above factors have the opposite ef­fect. Increased levels of 2,3-DPG, produced during chronic hypoxemia or anemia, also shift the curve to the right, while carbon monoxide shifts it to the left. The most important influence on the carboxy-hemoglobin dissociation curve is Po2; increased Po2 shifts the curve to the right, thus reducing the C02 content for any given Pco2 and assisting in the unloading of C02 in the lungs.