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Course: MCAT > Unit 2

Lesson 3: Foundation 3: Organ Systems

Cardiovascular system: Oxygen affinity of hemoglobin and myoglobin


Adult hemoglobin (HbA) differs significantly from fetal hemoglobin (HbF), the primary hemoglobin component found in fetal blood, and from myoglobin (Mb), the oxygen carrier present in muscle cells. These differences are essential for facilitating the appropriate transport of oxygen between the mother and baby during fetal development, as well as between the blood and muscle tissues throughout the body.
Figure 1 shows the oxygen saturation curves of normal HbA, HbF, and Mb. The oxygen saturation curve, also known as the oxygen dissociation curve, measures the relationship between the partial pressure of oxygen (PO2) in the surrounding environment and the percentage of hemoglobin molecules that are bound to oxygen in solution (% saturation, or SO2).
Figure 1 Oxygen saturation/dissociation curves for HbA, HbF, and Mb at physiological pH
The p50 value on the oxygen dissociation curve refers to the PO2 at which the hemoglobin molecule is 50% saturated with oxygen. It is an important parameter on the oxygen dissociation curve because it indicates the affinity of hemoglobin for oxygen.
Oxygen affinity, and therefore the p50 value, can be influenced by a variety of factors, including:
  1. pH levels: Acidic conditions lead to a decrease in hemoglobin’s affinity for oxygen, a phenomenon known as the Bohr effect. This effect is more pronounced in HbA compared with HbF, meaning that in acidic conditions, HbA releases oxygen more readily than HbF, enhancing oxygen unloading in tissues with higher metabolic activity. Changes in pH may also affect oxygen binding by myoglobin.
  2. Temperature: Higher temperatures reduce the oxygen affinity of both HbF and HbA. In warmer tissues, hemoglobin tends to release oxygen more efficiently, ensuring oxygen supply to cells with higher energy demands. Unlike hemoglobin, myoglobin does not show a significant temperature dependence on oxygen affinity.
  3. 2,3-Bisphosphoglycerate (BPG) concentrations: BPG is a molecule in red blood cells that binds to hemoglobin and decreases its oxygen affinity. Higher levels of BPG are generally found in situations with low oxygen availability (e.g., at high altitudes or in chronic lung diseases). HbF naturally has a higher affinity for BPG compared with HbA, which contributes to its ability to unload oxygen in fetal tissues more effectively. Myoglobin does not bind to BPG like hemoglobin does. Instead, myoglobin has a higher affinity for oxygen and acts as an oxygen reservoir in muscle tissues, allowing efficient oxygen transfer from hemoglobin in the blood to the muscles during periods of high demand.
Why do fetuses use HbF instead of HbA?
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