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Hemoglobin (haemoglobin,[a] Hb or Hgb) is a protein containing iron that facilitates the transport of oxygen in red blood cells.The molecule also carries the important regulatory molecule nitric oxide bound to a thiol group in the globin protein, releasing it at the same time as oxygen.[10]

Hemoglobin is also found in other cells, including in the A9 dopaminergic neurons of the substantia nigra, macrophages, alveolar cells, lungs, retinal pigment epithelium, hepatocytes, mesangial cells of the kidney, endometrial cells, cervical cells, and vaginal epithelial cells.[11] In these tissues, hemoglobin absorbs unneeded oxygen as an antioxidant, and regulates iron metabolism.[12] Excessive glucose in the blood can attach to hemoglobin and raise the level of hemoglobin A1c.[13]

Hemoglobin and hemoglobin-like molecules are also found in many invertebrates, fungi, and plants.[14] In these organisms, hemoglobins may carry oxygen, or they may transport and regulate other small molecules and ions such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide.

Hemoglobin (haemoglobin,[a] Hb or Hgb) is a protein containing iron that facilitates the transport of oxygen in red blood cells. Almost all vertebrates contain hemoglobin,[3] with the exception of the fish family Channichthyidae.[4] Hemoglobin in the 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, a chromoprotein, and globulin.

In mammals, hemoglobin makes up about 96% of a red blood cell's dry weight (excluding water), and around 35% of the total weight (including water).[5] Hemoglobin has an oxygen-binding capacity of 1.34 mL of O2 per gram,[6] which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood plasma alone.[7] The mammalian hemoglobin molecule can bind and transport up to four oxygen molecules.[8]

Hemoglobin also transports other gases. It carries off some of the body's respiratory carbon dioxide (about 20–25% of the total)[9] as carbaminohemoglobin, in which CO2 binds to the heme protein. The molecule also carries the important regulatory molecule nitric oxide bound to a thiol group in the globin protein, releasing it at the same time as oxygen.[10]

Hemoglobin is also found in other cells, including in the A9 dopaminergic neurons of the substantia nigra, macrophages, alveolar cells, lungs, retinal pigment epithelium, hepatocytes, mesangial cells of the kidney, endometrial cells, cervical cells, and vaginal epithelial cells.[11] In these tissues, hemoglobin absorbs unneeded oxygen as an antioxidant, and regulates iron metabolism.[12] Excessive glucose in the blood can attach to hemoglobin and raise the level of hemoglobin A1c.[13]

Hemoglobin and hemoglobin-like molecules are also found in many invertebrates, fungi, and plants.[14] In these organisms, hemoglobins may carry oxygen, or they may transport and regulate other small molecules and ions such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide. A variant called leghemoglobin serves to scavenge oxygen away from anaerobic systems such as the nitrogen-fixing nodules of leguminous plants, preventing oxygen poisoning.

The medical condition hemoglobinemia, a form of anemia, is caused by intravascular hemolysis, in which hemoglobin leaks from red blood cells into the blood plasma.