Q.Mark the incorrect statement in context to O2 binding to Hb

Concept and Intuition

The oxygen-haemoglobin dissociation curve illustrates the relationship between the partial pressure of oxygen ($P_{O_2}$) and the percentage saturation of haemoglobin with oxygen. It's typically S-shaped, reflecting how readily haemoglobin binds to oxygen at different oxygen levels.

• High affinity: When haemoglobin has a high affinity for oxygen, it binds oxygen tightly and releases it less readily. This is desirable in the lungs, where oxygen needs to be picked up efficiently.
• Low affinity: When haemoglobin has a low affinity for oxygen, it releases oxygen more easily. This is crucial in active tissues, which have a high metabolic demand and require more oxygen.

A rightward shift of the curve signifies that for a given $P_{O_2}$, haemoglobin is less saturated with oxygen. In simpler terms, haemoglobin's affinity for oxygen has decreased, meaning it releases oxygen more readily to the tissues. This is physiologically important because it ensures that oxygen is delivered precisely where it is most needed, such as in metabolically active tissues (e.g., exercising muscles).

Conversely, a leftward shift indicates an increased affinity of haemoglobin for oxygen, meaning it binds oxygen more tightly and releases it less readily. This occurs in conditions where oxygen needs to be picked up efficiently, like in the lungs, or in situations like foetal haemoglobin, which has a higher affinity for oxygen than adult haemoglobin to extract oxygen from the mother's blood.

Step-by-Step Explanation

Several factors influence the affinity of haemoglobin for oxygen, causing the dissociation curve to shift. These factors are typically associated with increased metabolic activity in tissues, signalling a greater need for oxygen.

1. Effect of pH (Bohr Effect):

   • When tissues are metabolically active, they produce more carbon dioxide ($CO_2$).
   • $CO_2$ reacts with water to form carbonic acid ($H_2CO_3$), which then dissociates into hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$).
   • $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$
   • An increase in $H^+$ concentration leads to a decrease in pH (more acidic conditions).
   • These $H^+$ ions bind to haemoglobin, altering its structure and reducing its ability to bind oxygen. This effectively lowers haemoglobin's affinity for oxygen.
   • Therefore, low pH (high $H^+$ concentration) causes a rightward shift of the oxygen-haemoglobin dissociation curve, promoting oxygen release to the tissues.
   Watch out

   A common mistake is to confuse low pH with high pH. High pH (alkaline conditions, low $H^+$) would increase haemoglobin's affinity for oxygen, causing a leftward shift.

2. Effect of Carbon Dioxide ($P_{CO_2}$) (Bohr Effect):

   • High concentrations of $CO_2$ in the blood also contribute to a rightward shift.
   • As explained above, $CO_2$ increases $H^+$ concentration, lowering pH.
   • Additionally, $CO_2$ can directly bind to the amino groups of haemoglobin to form carbaminohemoglobin. This binding also reduces haemoglobin's affinity for oxygen.
   • Thus, high $P_{CO_2}$ causes a rightward shift, facilitating oxygen unloading. …