Oxygen Dissociation Curve Shifts

The oxygen dissociation curve illustrates the relationship between the partial pressure of oxygen ($P_{O_2}$) and the saturation of hemoglobin with oxygen. A shift to the right indicates that hemoglobin has a lower affinity for oxygen, meaning oxygen is more readily released to the tissues.

Factors Causing Rightward Shift

Several physiological conditions cause the oxygen dissociation curve to shift right, indicating decreased oxygen affinity:

• Increased Temperature: Higher temperatures, often associated with increased metabolic activity, reduce hemoglobin's affinity for $O_2$.
• Hypercapnia ($↑ P_{CO_2}$): An increase in the partial pressure of carbon dioxide ($P_{CO_2}$) in the blood decreases blood pH (via the bicarbonate buffer system), which lowers hemoglobin's affinity for $O_2$. This is part of the Bohr effect.
• Raised 2, 3 DPG Level: 2,3-Diphosphoglycerate (2,3-DPG) is a molecule that binds to hemoglobin and stabilizes its low-affinity state, promoting oxygen release.

Metabolic Alkalosis and Curve Shift

Metabolic alkalosis is a condition characterized by an increase in blood pH (alkalemia). This typically involves a decrease in hydrogen ion concentration ($[H^+]$). Low $[H^+]$ means hemoglobin binds oxygen more tightly, resulting in a leftward shift of the oxygen dissociation curve, not a rightward one.

Therefore, metabolic alkalosis is the condition among the options that does NOT cause a rightward shift in the $O_2$ dissociation curve.