Hemoglobin (haemoglobin in British English), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocytes) of almost all vertebrates (the exception being the fish family  Channichthyidae ) as well as the tissues of some invertebrates.Hemoglobin in blood carries oxygen from the respiratory organs (e.g. lungs or gills) to the rest of the body (i.e. tissues). There it releases the oxygen to permit aerobic respiration to provide energy to power functions of an organism in the process called metabolism. A healthy individual human has 12 to 20 grams of hemoglobin in every 100 mL of blood.