Hemoglobin (also spelled haemoglobin[a], abbreviated Hb or Hgb), is the iron-containing oxygen-transport protein present in red blood cells (erythrocytes) of almost all vertebrates (the exception being the fish family Channichthyidae) as well as the tissues of some invertebrate animals. Hemoglobin in blood carries oxygen from the respiratory organs (lungs or gills) to the other tissues of the body, where it releases the oxygen to enable aerobic respiration which powers the animal´s metabolism. A healthy human has 12 to 20 grams of hemoglobin in every 100 mL of blood. Hemoglobin is a metalloprotein and chromoprotein.
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