Pulmonary Physiology, Gas Diffusion & Transport, Dr. D. Penney

Figure 7.06. Hemoglobin-oxygen association-dissociation curve at pH 7.4 and 37o C.

Hemoglobin displays a sigmoidal (S-shaped) oxygen association-dissociation curve. Oxygen is taken up more slowly at first - the "foot" region of the curve; more rapidly in the "steep" region; and less rapidly again in the "shoulder" region.

Each g of Hb at 100% saturation can carry 1.34 ml O2, whereas each 100 ml of blood can dissolve only 0.003 ml O2 per mmHg partial pressure of oxygen (i.e. 0.3 ml O2 per 100 ml blood for each 100 mmHg PO2). Thus, most of the oxygen transported by the blood to the tissues is attached to Hb - very little is dissolved in the plasma.

The relative affinity of Hb for oxygen is often expressed in terms of the P50 value. The P50 is the PO2 in mmHg at which the Hb is 50% saturated. Thus, P50 increases with decreasing affinity, and vice versa.

At the normal arterial PO2 of about 100 mmHg there are about 20 ml O2 per 100 ml blood, plus another 0.3 ml O2 in physical solution. This gives a Total O2 Concentration of 20 vols % (20 ml per dL). For each increase in arterial PO2 of 100 mmHg, dissolved O2 increases another 0.3 vols %. The O2 on Hb however will hardly rise at all since at the normal PO2 of 100 mmHg Hb is already nearly fully saturated with oxygen. Thus raising arterial PO2 above 100 mmHg will NOT ordinarily raise oxygen concentration much until very high PO2values are reached. Besides, there are species of Hb in normal blood which cannot carry oxygen (COHb, MetHb, etc.).

If 100% O2 is breathed and arterial PO2 approaches 700 mmHg, there will be 700/100 x 0.3 = 2.1 vols % O2 in physical solution, plus the normal 20 vols % on the Hb, for a total of 22.1 ml. Unfortunately the body cannot tolerate high O2 partial pressures for very long before toxic oxygen effects develop.

Last Changed 04/28/00

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