What gives red blood cells their red appearance?

The red appearance of red blood cells primarily comes from hemoglobin, which is a protein found within these cells. Hemoglobin contains iron, which binds to oxygen. When hemoglobin is saturated with oxygen, it forms oxyhemoglobin, and this specific compound enhances the red color due to its interaction with light.

Oxyhemoglobin is responsible for delivering oxygen throughout the body, and its bright red color is what you see when blood is oxygen-rich, such as in arterial blood. In contrast, venous blood that is lower in oxygen and higher in carbon dioxide has a darker red appearance, primarily due to deoxyhemoglobin rather than oxyhemoglobin.

While myoglobin is another oxygen-carrying protein found in muscles, it does not contribute to the color of red blood cells. Carboxyhemoglobin is formed when carbon monoxide binds to hemoglobin and can lead to a different color presentation, but it is not what typically gives red blood cells their characteristic red color under normal physiological conditions. Thus, oxyhemoglobin specifically highlights the oxygenated state of hemoglobin and its visual impact on blood coloration.