In the human body, which substance gives blood its red colour?

Hemoglobin gives human blood its characteristic red colour.

Hemoglobin is an iron-containing protein inside red blood cells. Each molecule can bind oxygen reversibly, allowing red blood cells to carry oxygen from the lungs to tissues and help transport some carbon dioxide back toward the lungs.

Blood colour varies with oxygenation. Oxygen-rich arterial blood is generally bright red, while oxygen-poor venous blood is darker red. Human blood is not actually blue beneath the skin; veins can look blue because of how light is absorbed and scattered by skin and tissue.

Hemoglobin contains heme groups with iron atoms at their centers. Oxygen binding changes the molecule's structure and contributes to efficient loading in the lungs and unloading in tissues. Low hemoglobin concentration is a defining feature of anemia, which can reduce the blood's oxygen-carrying capacity.