Hemoglobin F

  • Hemoglobin F is a special isoform of hemoglobin only expressed in the fetus. The F isoform of hemoglobin displays a higher affinity for oxygen than the normal adult isoform; consequently, when it is included in a hemoglobin tetramer the Oxygen-Hemoglobin Dissociation Curve discussed in Oxygen Transport is shifted leftward. Consequently, for the same partial pressure of oxygen, blood containing Hemoglobin F will display a higher hemoglobin saturation, reflecting its higher affinity. Expression of the F isoform in fetuses is critical for facilitating oxygen transfer from the maternal to the fetal circulation. Because fetal blood displays a higher affinity for oxygen than maternal blood, oxygen will diffuse from the pregnant maternal to the fetal circulation within the placenta, allowing for oxygenation of fetal tissues.

Modulation of the Oxygen-Hemoglobin Dissociation Curve
A variety of environmental factors can shift the Oxygen-Hemoglobin Dissociation Curve. Effects which are associated with increased peripheral tissue metabolism, such as reduced pH, increased CO2, increased temperature, shift the curve to the right, reducing hemoglobin’s affinity for oxygen and thus improving oxygen unloading. Chronic hypoxia increases the blood’s concentration of 2,3-DPG which also shifts the curve to the right. The presence of HbF and carbon monoxide (CO) shift the curve to the left, increasing the oxygen affinity of hemoglobin.