Explain the importance of the oxygen dissociation curve at partial pressures of oxygen in the lungs and in respiring tissues

The oxygen dissociation curve is crucial in understanding how hemoglobin (Hb) picks up and releases oxygen at different partial pressures of oxygen (pO₂), particularly in the lungs and in respiring tissues. Its sigmoidal shape allows hemoglobin to respond efficiently to changes in pO₂, optimizing oxygen delivery to the body.

1. High pO₂ in the Lungs (Loading of Oxygen)

• In the lungs, where the pO₂ is high (around 100 mmHg), hemoglobin’s affinity for oxygen is also high. This high pO₂ places the oxygen dissociation curve near the top flat portion, meaning hemoglobin becomes almost fully saturated with oxygen (around 97-98%).
• This plateau ensures that hemoglobin maximally loads oxygen, allowing it to carry as much oxygen as possible from the lungs to the rest of the body. This is crucial, as it allows for the maximum oxygen transport capacity, even if pO₂ in the lungs fluctuates slightly due to altitude or other factors.
• The steep initial increase in the curve due to cooperative binding (where each oxygen molecule bound increases affinity for subsequent molecules) allows hemoglobin to load oxygen efficiently and quickly at higher pO₂ levels.

2. Lower pO₂ in Respiring Tissues (Unloading of Oxygen)

• In respiring tissues, such as muscles, the pO₂ is much lower (typically around 40 mmHg or less) due to oxygen consumption in cellular respiration. This part of the oxygen dissociation curve corresponds to the steep, middle section.
• Here, hemoglobin’s affinity for oxygen is lower, which enables it to release oxygen more readily. This drop in affinity at lower pO₂ allows hemoglobin to unload a significant amount of oxygen (around 20-25% of its load) into tissues that require it for energy production.
• The steep slope in this range of the curve ensures that even small decreases in pO₂ (indicating active metabolism) result in a significant release of oxygen. Thus, hemoglobin can adapt its oxygen unloading to the tissues’ varying demands based on metabolic activity.

3. The Importance of Curve Shifts in Respiring Tissues

• Factors such as increased carbon dioxide, decreased pH, and increased temperature in actively respiring tissues can shift the oxygen dissociation curve to the right (the Bohr effect). This rightward shift further reduces hemoglobin’s affinity for oxygen, encouraging even more oxygen to be unloaded where it is needed most.
• This adaptation is vital for ensuring that tissues with high metabolic rates, which generate CO₂ and heat, receive more oxygen, while hemoglobin retains oxygen when these factors are lower (such as in less active tissues).