Understanding Red Blood Cells and Their Functions

5. Hemoglobin

In vertebrates, red blood cells consist mostly of a metalloprotein -- hemoglobin. Within hemoglobin are heme groups with iron atoms that can temporarily bind to oxygen molecules. This is what allows the cells to collect oxygen in the lungs and transport it through the body. Hemoglobin is red, and gives red blood cells their signature color. The cells can change color depending on the level of oxygen in the cell, however. Hemoglobin with oxygen, oxyhemoglobin, is scarlet. After the red blood cell releases the oxygen, oxyhemoglobin becomes deoxyhemoglobin and develops a dark red color.