OxyHemoglobin Dissociation Curve

Oxygen Delivery to Tissues

The oxygen dissociation curve (ODC) is a graph depicting the relationship between the partial pressure of oxygen (pO2) and the percentage of hemoglobin saturation. This curve quantifies the affinity of hemoglobin for oxygen. It shows the percentage of hemoglobin that is saturated at a given pO2. We can evaluate the O2 affinity of hemoglobin in a blood sample by identifying the pO2 at which 50% of the hemoglobin is saturated (p50). The normal p50 value for adult humans is 24–28 mmHg, or 26.7 mmHg at pH 7.4 and 37°C.

Certain conditions can influence hemoglobin's affinity for O2, causing the curve to be shifted. A left shift means that a p50 will be reached at a lower pO2, indicating increased hemoglobin-oxygen affinity. An alkalotic pH can induce a left shift. Conversely, a decrease in pH (acidosis) can cause the curve to be shifted to the right, denoting a decreased affinity of hemoglobin for oxygen.

• The horizontal axis is Pa0₂, it represents the partial pressure of oxygen in arterial blood
• The vertical axis is SaO₂, reflects the percentage of hemoglobin binding sites occupied by oxygen.
• Once the PaO₂reaches 60 mm Hg the curve flattens, indicating very little change in saturation that can occur if above PaO₂ is increased above this point.
  • So, PaO₂ of 60 or more is usually considered adequate.
  • But, at less than 60 mm Hg the curve is very steep, and small changes in the PaO₂ greatly reduce the SaO₂.

• The term "affinity" is used to describe hemoglobin's ability to attract and hold O₂ at the heme binding sites.
  • Affinity changes with:
    • variation in pH,
    • temperature,
    • CO₂ and,
    • 2,3,-DPG, a metabolic by-product which competes with O₂ for binding sites.
  • Traditionally the curve starts with:
    • pH at 7.4,
    • temperature at 37 Centigrade, and
    • PaCO₂ at 40.
    • Changes from these values are called "shifts".
• A left shift will increase oxygen's affinity for hemoglobin.
  • In a left shift condition (alkalosis, hypothermia, etc.) oxygen will have a higher affinity for hemoglobin.
  • SaO₂ will increase at a given PaO₂, but more of it will stay on the hemoglobin and ride back through the lungs without being used. This can result in tissue hypoxia even when there is sufficient oxygen in the blood.
• A right shift decreases oxygen's affinity for hemoglobin.
  • In a right shift (acidosis, fever, etc.) oxygen has a lower affinity for hemoglobin. Blood will release oxygen more readily.
  • This means more O₂ will be released to the cells, but it also means less oxygen will be carried from the lungs in the first place.

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