Bohr effect and oxygen unloading in active muscle blood

Steps:

• Active muscles increase CO2 and lactic acid, lowering blood pH below 7.4, but the question specifies pH 7 conditions.
• The graph shows dissociation curves: pH 7 (left, higher affinity) and pH 7.2 (right-shifted, lower affinity via Bohr effect).
• Blood leaving active muscle has low PO2 (~20-40 mmHg) due to oxygen extraction.
• At pH 7 and low PO2, haemoglobin saturation is reduced to ~50-60%, identifying the venous point on the pH 7 curve.

Why B is correct:

• B marks ~50% saturation on the pH 7 curve at venous PO2, matching deoxygenated blood from active muscle under specified pH.

Why the others are wrong:

• A: High saturation (~95%) on pH 7 curve, typical of arterial blood, not venous.
• C: On pH 7.2 curve, but question requires pH 7.
• D: Excessively low saturation, not representative of typical active muscle venous blood.

Final answer: B