Hemoglobin is a substance contained within erythrocytes (red blood cells) that is responsible for their color and their remarkably high oxygen-carrying capacity. Hemoglobin – the most efficient oxygen-carrier known – has the unique property of combining reversibly with oxygen and is the medium by which oxygen is transported within the body. It takes up oxygen as blood passes through the lungs and releases it as blood passes through the tissues, where the oxygen pressure is low. Oxyhemoglobin is scarlet in color; reduced hemoglobin is of a purplish color.
Hemoglobin is a member of the same class of porphyrins to which chlorophyll also belongs. A hemoglobin molecule consists of a porphyrin ring with a central iron atom (heme), hooked to a clump of protein called globin. Hemoglobin is found throughout the animal kingdom, in almost all vertebrates and in the circulatory fluids of many invertebrates.
The normal hemoglobin content of blood is 13.5–18.0 g/100 ml in men and 11.5–16.5 g/100 ml in women. The average human contains about 4 grams (0.14 ounce) of iron, a lot of which circulates as hemoglobin. If the diet does not contain the 6 milligrams of iron needed each day, anemia will eventually develop.
Hemoglobin also combines very easily with carbon monoxide, forming carboxyhemoglobin. It also plays a part in regulating the acidity of blood and in the carriage of carbon dioxide. Muscle hemoglobin acts as a respiratory catalyst.