The oxyhemoglobin dissociation curve (ODC) is one of the most recognized teachings of basic physiology. It describes the relationship between the saturation of hemoglobin and the partial pressure of arterial oxygen. Intuitively, it makes sense that the more oxygen that’s available (a higher PO2), the more saturated hemoglobin will be (% saturation). But what if the hemoglobin is in a different conformational state because of acidosis or a hemoglobinopathy? And once the hemoglobin molecule is saturated with oxygen, how readily will it “give up” the oxygen to end organs and tissues that require it?
Let’s start with the basics. The majority of adult hemoglobin is hemoglobin A (HbA), an iron-based metalloprotein made of two alpha and two beta globular protein subunits (a tetramer). Each HbA molecule can hold up to four oxygen molecules, but the hemoglobin molecule itself is also involved in physiologic buffering and carbon dioxide transport (carbaminohemoglobin).
An important teaching point is cooperativity, the phenomenon where when an atom of oxygen binds to hemoglobin, the remaining unoccupied spots on that hemoglobin molecule have increased affinity for oxygen. In other words, with each molecule of oxygen, hemoglobin gets hungrier and hungrier for the next. This positive cooperativity is responsible for the ODC’s sigmoidal shape.
In healthy adults, a PO2 of ~27 mmHg corresponds to ~50% hemoglobin saturation (blue curve). This is known as the P50 of hemoglobin. There are many physiologic stressors which can shift the curve rightward or leftward and therefore change hemoglobin’s P50. It’s important to know what these are and what they mean.
This is a fairly simplified overview of the ODC, so drop me a line if you have questions in the comments section below! 🙂