A laboratory reports the P50 of a patient's blood as 18 mmHg (normal 26 to 27 mmHg). This value indicates that the patient's hemoglobin:

• A Has reduced oxygen affinity and will release oxygen more readily in tissues
• B Has an increased oxygen-carrying capacity per gram of hemoglobin
• C Has lost cooperative binding entirely
• D Has increased oxygen affinity and will unload less oxygen at tissue PO2 ✓

Explanation

P50 is the PO2 at which hemoglobin is 50 percent saturated. A low P50 means the dissociation curve is shifted left, so hemoglobin holds oxygen more avidly and unloads less at ordinary tissue PO2, causing tissue hypoxia and compensatory erythrocytosis. Carrying capacity per gram depends on hemoglobin concentration and intact heme groups, not on affinity. Loss of cooperativity would flatten the curve shape but is a separate property from P50.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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