8.2.5—Partial pressures and oxygen loading

Oxygen partial pressure (pO2) is the pressure contribution of oxygen within a gas mixture. The local pO2 changes haemoglobin’s loading or unloading demand: high pO2 favours binding, while lower pO2 favours dissociation.

• Lungs — high pO2: haemoglobin encounters a high oxygen partial pressure → affinity and percentage saturation are high → oxygen loads onto haemoglobin and blood leaves the lungs carrying a high oxygen load.
• Respiring tissues — lower pO2: cells use oxygen, so tissue pO2 is lower than in the lungs → haemoglobin’s affinity is lower and saturation falls → oxygen dissociates and becomes available for cellular respiration.
• Steep-region demand: when the tissue pO2 lies on the steep part of the curve, a relatively small fall in pO2 can produce a substantial fall in saturation → a useful amount of oxygen is unloaded without requiring complete deoxygenation.
• Active/respiring tissue: greater respiration tends to maintain a lower local oxygen partial pressure → the loading–unloading system is driven towards release where oxygen demand is present, within the conditions represented by the page.

Application method: identify the location, compare its pO2 with the lung/tissue context, locate the corresponding curve region, then state whether saturation rises or falls and why.

Partial pressure is not simply the percentage of oxygen in the whole atmosphere or the total oxygen amount in blood. This card applies pO2 to location; 4608 explains the curve shape/read-off and 4610 explains the Bohr shift when CO2/H+ conditions alter affinity. No image generated or bound.