Figure 1 shows the two forms of the O2-equilibrium curve: (1) saturation of hemoglobin with O2 (SO2) versus O2 partial pressures (Po2), and (2) O2 concentration in blood (CO2) versus PO2. Saturation quantifies the amount of O2 in blood as the percentage of the total hemoglobin sites available for binding O2 that actually bind O2 at a given PO2. Therefore, saturation equilibrium curves are independent of hemoglobin concentration in blood. In contrast, concentration curves quantify the absolute amount of O2 in a volume of blood with a given PO2 and they depend on the amount of hemoglobin available in blood for O2.
O2 capacity (O2cap) is defined as the O2 concentration in blood when hemoglobin is 100% saturated with O2. Pure hemoglobin binds 1.39 mL O2/g Hb, and Fig. 1 shows that the O2cap for normal blood with a hemoglobin concentration of 15 g/dL is 20.85 mL/dL (= 1.39 • 15). Physically dissolved O2 also contributes a small amount to O2 concentration. Therefore, total O2 concentration in blood (in mL O2/dL blood) is calculated as:
C02 = (02cap[S02/100]) + (0.003PO2), where 02cap = 02 capacity, So2 = saturation, and
0.003.= physical solubility for 02 in blood. This equation and Fig. 1 show that dissolved O2 is not a large component of O2 concentration at PO2 levels in arterial or venous blood (i.e., 100-40 mm Hg).
The shape of the O2-Hb equilibrium curves is complex, and it can be generated only experimentally or by sophisticated mathematical algorithms. However, remembering only four points on the curve allows one to solve many common problems of O2 transport:
1. Po2 = 0 mm Hg, So2 = 0% (the origin of the curve)
2. Po2 = 100 mm Hg, So2 = 98% (normal arterial blood, which is almost fully saturated)
3. Po2 = 40 mm Hg, So2 = 75% (normal mixed venous blood)
4. Po2 = 26 mm Hg, So2 = 50% (P50, defined as the Po2 at 50% saturation)
The P50 quantifies the affinity of Hb for o2. For example, a decrease in P50 indicates an increase in o2 affinity because o2 saturation or concentration is greater for a given Po2. In adult human blood, P50 = 26 mm Hg under standard conditions of Pco2 = 40 mm Hg, pH = 7.4, and 37°C.
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