Hemoglobin (haemoglobin in British English), 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. In mammals, hemoglobin makes up about 96% of a red blood cell's weight excluding water, and around 35% of the total weight including water. Hemoglobin has an oxygen-binding capacity of 1.34 mL O2 per gram, which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood plasma alone. The mammalian hemoglobin molecule can bind and transport up to four oxygen molecules.Hemoglobin also transports other gases. It carries off some of...