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TABLE 22-5 Relation Between Oxyhemoglobin Saturation and Plasma Po2

FACTORS AFFECTING OXYHEMOGLOBIN DISSOCIATION The best known of the factors affecting the oxyhemoglobin dissociation curve are pH, temperature, and the amount of 2,3-diphosphoglycerate (2,3-DPG) in the red blood cells. Other related factors include P co2 and exercise.

pH The more acidic the blood, the more readily hemoglobin gives up its oxygen and the higher the Pa o2 (the partial pressure of oxygen dissolved in blood) is for a particular oxyhemoglobin saturation. In contrast, alkalosis makes hemoglobin hold on to its oxygen more tightly, lowering the Pa o2 present at a particular oxyhemoglobin saturation. In general, a rise or fall in pH of 0.10 causes a fall or rise (i.e., an opposite change) in the Pa o2 of about 10% (T§bleiiii2.2-§,).

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TABLE 22-6 Changes in Pao2 Related to pH

Partial Pressure of Carbon Dioxide A shift of the oxyhemoglobin dissociation curve, as a result of changes in the blood levels of carbon dioxide (Haldane effect) and hydrogen ions (Bohr effect), enhances oxygenation of the blood in the lungs and promotes release of oxygen from the blood in the tissues. As the blood passes through the lungs, carbon dioxide diffuses from the blood into the alveoli. This reduces the blood P co2 and decreases the hydrogen ion concentration because of the resulting decrease in the blood carbonic acid level. Both changes shift the oxyhemoglobin dissociation curve to the left. With a shift to the left, the quantity of oxygen binding to hemoglobin at any given Pao2 is increased, allowing greater oxygen transport to the tissues. Then, when the blood reaches the tissue capillaries, the opposite effect occurs. Carbon dioxide entering the blood from the tissues shifts the curve to the right. This displaces oxygen from the hemoglobin and delivers oxygen to the tissues at a higher Po2 than would otherwise occur.

Temperature As blood temperature increases, hemoglobin gives up oxygen more readily, raising the P o2 in the plasma. The opposite occurs during cooling. For each 1°C rise in temperature, the Pao2 rises about 5% (TabileM2i2M-7). With hypothermia, the Pco2 falls by about the same amount.

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