Modulation of Blood Oxygen Equilibrium Curves

The o2-equilibrium curve in blood can be physiologically modulated in three ways: (1) The vertical height of the concentration curve (but not the saturation curve) can change, indicating a change in 02cap; (2) the horizontal position of saturation and concentration curves can change, indicating a change in Hb-o2 affinity; and (3) the shape of saturation and concentration curves can change, indicating a change in the chemical reaction between o2 and hemoglobin. The maximum height of the saturation curve cannot change by definition; the maximum is always 100% when o2 is bound to all available hemoglobin sites. However, changes in hemoglobin concentration [Hb] will change the maximum height of the concentration curve, according to the relationship between 02cap and [Hb] described earlier. Mean corpuscular Hb concentration (MCHC) quantifies [Hb] in red blood cells, and hematocrit (Hcrit) quantifies the percentage of blood volume that is red blood cells. Therefore [Hb], in g/dL of blood, depends on both of these factors:

Typical human values of MCHC = 0.33 and Hcrit = 45%, and [Hb] = 15 g/dL are used for Fig. 1, which shows normal o2 concentration in arterial blood (Cao2) = 20 mL/dL and 15 mL/dL in mixed venous blood (CVo2). If [Hb] decreases, for example with decreased hematocrit in anemia, then 02cap and concentration decreases at any given Po2. The o2cap increases when [Hb] increases, for example, by the stimulation of red blood cell production in bone marrow by the hormone erythropoietin. Erythropoietin transcription is a regulated hypoxia inducible factor (HIF-1a) and is released from cells in the kidneys in response to decreases in arterial o2 levels. Polycythemia, or increased hemato-crit, occurs with chronic hypoxemia in healthy people (e.g., during acclimatization to altitude) and with disease.

The horizontal position of Hb-o2 equilibrium curves reflects the affinity of Hb for oxygen, and changes in horizontal position are quantified as changes in P50. A decrease in P50 is referred to as a left shift of the equilibrium curve, and indicates increased Hb-o2 affinity; o2 saturation or concentration is increased for a given Po2. Similarly, increased P50 or a right shift reflects decreased Hb-o2 affinity. Figure 2 shows the three most important physiologic variables that can modulate P50: pH, PCo2, and temperature.

The Bohr effect describes changes in P50 with changes in blood PCo2 and pH. Decreased PCo2 causes Hb-o2

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