Blood is red because of the presence of a protein called hemoglobin. Hemoglobin is found in red blood cells, which are responsible for carrying oxygen from the lungs to the body’s tissues and carrying carbon dioxide from the body’s tissues to the lungs. Hemoglobin contains iron, which gives it a red color. When oxygen binds to the iron in hemoglobin, the iron changes from a reduced (Fe2+) state to an oxidized (Fe3+) state. This change in the iron’s valence state causes the hemoglobin to adopt a red color.

Hemoglobin gives blood its red color, but the actual color can vary depending on the oxygenation of the blood. Blood that has just been oxygenated in the lungs is bright red. Blood that has given up its oxygen to the body’s tissues is dark red. Blood that is low in oxygen may appear purple or blue, which is why veins look blue under the skin. Blood that is low in oxygen is sometimes referred to as “deoxygenated blood.”

Apart from red blood cells, Blood also have white blood cells and platelets, which are colorless and help in forming clots. But the majority of the blood volume(about 45%) is red blood cells which gives blood its red color.