which of the following has a greater affinity for oxygen in 1A and 2A
Answers
Answer:
Explanation:
Hemoglobin oxygen affinity is the continuous relationship between hemoglobin oxygen saturation and oxygen tension. It is customarily plotted as the sigmoidal oxygen equilibrium curve, and it can be summarily expressed as P50—that is, the oxygen tension at which 50% of hemoglobin is saturated with oxygen at standard temperature and pH (Figure 71-2). The sigmoidal shape of the oxygen-hemoglobin equilibrium curve relates to the fact that the heme groups react with oxygen in a fixed sequence, and oxygenation and deoxygenation of one heme group profoundly affect the oxygenation and deoxygenation of the other heme groups. This latter phenomenon has been termed heme-heme interaction. As each heme group accepts oxygen, it becomes progressively easier for the next heme group of the molecule to pick up oxygen. This concept is implicit in the Hill equation for percent saturation:
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Figure 71-2. Oxyhemoglobin (HbO2) saturation curve under standard conditions for normal blood of pregnant and nonpregnant women.
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where y is percent saturation with oxygen, k is an equilibrium constant, Po2 is oxygen partial pressure, and the exponent n is the average number of iron atoms per hemoglobin molecule. For normal hemoglobin, n is approximately 2.9.
As blood circulates through the normal lung, Po2 increases from approximately 40 mm Hg to 110 mm Hg, a pressure sufficient to ensure at least 95% saturation of hemoglobin with oxygen. The oxygen-hemoglobin equilibrium relationship is such that any further increase of oxygen tension in the lung results in only a small increase in saturation. In the normal adult, when oxygen tension has fallen to approximately 27 mm Hg, at a pH of 7.40 and a temperature of 37° C, 50% of hemoglobin is saturated with oxygen (i.e., P50 for whole blood is 27 mm Hg).
The steep and flat parts of the curve reflect definitive processes in oxygen unloading. As oxygen diffuses from capillary to tissue, at first a rapid fall in Po2 (represented by the flat part of the curve) occurs until the steep part is reached, after which capillary Po2 decreases little even though large amounts of oxygen are released. Because oxygen tension at the mitochondrial surface, the point of oxygen utilization, is always approximately 0.5 to 1.0 mm Hg,5 the driving pressure for, and consequently the rate of, oxygen delivery is determined solely by the mean Po2 in capillary blood. This is in turn set by the position of the dissociation curve on the Po2 axis and by its steepness, such that relatively little change in driving pressure occurs as the red blood cell moves through the capillary. As the partial pressure of oxygen decreases, tissue oxygenation may become impaired. The term critical Po2 was introduced to indicate the oxygen tension of blood below which diffusion is impaired and organ function is disturbed.24 A critical Po2 cannot be a well-defined value, as cellular needs will likely vary between organs and are influenced by metabolic activity. For example, in the brain, an organ in which an adequate oxygen supply is essential for maintaining energy metabolism, the critical Po2 appears to be approximately 20 mm Hg.
When hemoglobin oxygen affinity is increased (i.e., with lower P50), the curve is shifted to the left, and oxygen (which is bound more tightly to hemoglobin) is released only at lower partial pressures. For example, whereas a Po2 of 40 mm Hg results in an oxygen saturation of 75% at 37° C and pH 7.40, a leftward shift of the curve results in a higher saturation at the same Po2. The resulting change in oxygen unloading ultimately could result in impaired diffusion. When affinity is decreased (i.e., with higher P50), the curve is shifted to the right. Consequently, oxygen is bound less tightly to hemoglobin and is released at higher partial pressures, thereby enhancing oxygen unloading at the tissue level. Therefore the release of oxygen from the blood at the tissue level depends on the position of the oxygen equilibrium curve, which in turn is modified by intraerythrocytic pH, Pco2, temperature, and other factors, including electrolyte concentration, organic phosphate levels, and hemoglobin type.