What the Affinity? – Hemoglobin Oxygen Affinity

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Sabrina Kroft

Registered Respiratory Therapist for 18 years working in all areas of healthcare. Professional interest is the nuances of mechanical ventilation and just learning new/old technology.

The Pre-brief

The delivery of oxygen to cells depends upon the hemoglobin oxygen affinity. What is the hemoglobin oxygen affinity (Hb-O2 affinity)? By definition, hemoglobin oxygen affinity is the ongoing relationship of hemoglobin oxygen saturation (SaO2) and oxygen tension (PaO2) plotted by the S-shaped dissociation curve.  The p50 (50% of the hemoglobin saturated with oxygen) is the oxygen tension at approximately 26mmHg, a pH 7.4, and a temperature of 37degrees Celsius. The dissociation curve is a plotted S-shape curve atp50.  The shift in curve affects how and if oxygen is picked up and released by the hemoglobin.

Why this is important

Understanding how this works will help you better understand how oxygen is transported and released. 

The Debrief

  • pH, temperature, CO2, 2,3-BPG, and CO are factors that affect the affinity of hemoglobin to oxygen 
  • Increasing the affinity of hemoglobin to oxygen shifts the curve to the left, ultimately delivering less oxygen to the tissues. 
  • Decreasing the affinity of hemoglobin to oxygen shifts the curve to the right, ultimately delivering more oxygen to tissues.


  1. McNamara, P., & El-Khuffesh, A. (2017). Oxygen Transport and Delivery. Sciencedirect.com


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