Hemoglobin, abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in red blood cells (erythrocytes) of almost all vertebrates as well as the tissues of some invertebrates. Hemoglobin in the blood carries oxygen from the respiratory organs to the rest of the body. There it releases oxygen to allow aerobic respiration, to provide energy to power the functions of an organism in the process called metabolism. A healthy individual human has 12 to 20 grams of hemoglobin in every 3.4 ounces of blood.

Source: Wikipedia