Have you ever wondered why scarlet blood pours out of your nose when you experience a nosebleed? Or why the veins in your skin look blue? It's all down to some colorful chemistry in your red blood cells.  Red blood cells contain a molecule that gives them their distinctive color.

Red blood cells contain a molecule called hemoglobin, which binds and transports oxygen through our bodies. Hemoglobin is made up of four protein chains that each bind an additional ring-shaped chemical structure called heme.

Our red blood cells are red because of the heme groups in hemoglobin. In turn, our blood is red because of the millions of red blood cells that it contains. Color plays an important role in many aspects of biology, explain the authors of a recent review article published in the journal Archives of Pathology & Laboratory Medicine.

Here, Dr. Sergio Piña-Oviedo, of the Department of Hematopathology at the MD Anderson Cancer Center in Houston, TX, and colleagues discuss the relevance and biochemistry of color in human organs.  Color is important for "camouflage and protection, metabolism, sexual behavior, and communication," they explain.

Metals matter The protein chains in the hemoglobin molecule in red blood cells are encoded by our genes. Mutations in the globin genes can cause diseases such as thalassemia and sickle cell disease. In order to bind oxygen, each protein chain binds to one heme group, allowing a maximum of four oxygen molecules to bind per one hemoglobin molecule. At heme's center sits an iron molecule. The iron makes heme look red-brown.