What binds to hemoglobin to give it a bright red color?

Hemoglobin binds to oxygen in the lungs, forming oxyhemoglobin, which gives the blood its bright red color. The iron ions in the heme groups of hemoglobin interact with oxygen, resulting in a structural change that enhances the red hue. When hemoglobin releases oxygen to the tissues, it becomes deoxyhemoglobin, which has a darker, purplish color. This reversible binding is essential for efficient oxygen transport throughout the body.