Hemoglobin and Cell Color: Understanding the Red Hue

Red blood cells (erythrocytes) owe their characteristic red color to hemoglobin, an iron-containing protein that constitutes ~95% of the cell's dry weight.

Mechanism

• Each hemoglobin molecule has four heme groups, each containing an iron (Fe²⁺) ion.
• Iron binds oxygen (O₂) in the lungs, forming oxyhemoglobin (bright red).
• In tissues, oxygen is released, forming deoxyhemoglobin (darker, bluish-red).
• The iron-oxygen interaction alters light absorption, giving blood its color.

Key Points

• No nucleus or organelles—maximizes hemoglobin packing.
• Hemoglobin also transports CO₂ (as carbamate) and buffers H⁺.
• Anemia (low hemoglobin) causes pallor; carbon monoxide poisoning gives cherry-red color (carboxyhemoglobin).

Clinical Relevance

Hemoglobin concentration (g/dL) is a primary measure of oxygen-carrying capacity. Pulse oximetry exploits color change to estimate oxygen saturation.