Blood Color: Oxygenation, Dyshemoglobinemias, and Clinical Clues

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Blood Color: Oxygenation, Dyshemoglobinemias, and Clinical Clues

The color of blood is a direct reflection of hemoglobin's oxygenation state and can be altered by dyshemoglobinemias—abnormal hemoglobin derivatives that change light absorption.

Normal Physiologic Colors

Bright red: Oxygenated arterial/capillary blood (oxyhemoglobin).
Dark red: Deoxygenated venous blood (deoxyhemoglobin).

Conditions Altering Blood Color

Condition	Color	Mechanism
Carbon monoxide poisoning	Cherry-red (arterial & venous)	Carboxyhemoglobin (COHb) has high affinity, bright red color.
Cyanide poisoning	Bright red venous blood	Cytochrome oxidase inhibition prevents O₂ extraction; venous blood remains oxygenated.
Methemoglobinemia	Chocolate-brown	Oxidized Fe³⁺ heme (metHb) cannot bind O₂.
Sulfhemoglobinemia	Greenish-black	Irreversible sulfur incorporation into heme.
Polycythemia vera	Darker red	High hemoglobin concentration.
Severe anemia	Pale, thin	Low hemoglobin concentration.

Clinical Pearl

Color alone is unreliable for diagnosis. Co-oximetry is required to quantify dyshemoglobin fractions. "Bright red blood" in trauma suggests arterial bleeding or CO/CN poisoning; chocolate-brown blood signals methemoglobinemia.