Hemoglobin (or haemoglobin) is a protein in red blood cells which contains iron. It is used to transport oxygen around the human body. Hemoglobin is found in the red blood cells of all vertebrates apart from white-blooded fish. It also occurs in some invertebrates.
Hemoglobin is involved in the transport of other gases. It carries some of the body's respiratory carbon dioxide (about 20-25% of the total).
Red blood cells get their color from hemoglobin, which is red. There are millions of hemoglobin molecules in each red blood cell and millions of red blood cells in the human body. When hemoglobin has oxygen attached, it is called oxyhemoglobin.
The most common type of hemoglobin in mammals contains four such subunits. Each subunit of hemoglobin is a globular protein (globin) with a heme group inside it. Each heme group has one iron atom. This binds one oxygen molecule. So the complete hemoglobin molecule has four globin chains, four heme molecules, and four iron atoms. When hemoglobin is in the lungs, it picks up oxygen in its hemes, and carries it to the rest of the body.
Images for kids
A schematic visual model of oxygen-binding process, showing all four monomers and hemes, and protein chains only as diagramatic coils, to facilitate visualization into the molecule. Oxygen is not shown in this model, but, for each of the iron atoms, it binds to the iron (red sphere) in the flat heme. For example, in the upper-left of the four hemes shown, oxygen binds at the left of the iron atom shown in the upper-left of diagram. This causes the iron atom to move backward into the heme that holds it (the iron moves upward as it binds oxygen, in this illustration), tugging the histidine residue (modeled as a red pentagon on the right of the iron) closer, as it does. This, in turn, pulls on the protein chain holding the histidine.
A hemoglobin concentration measurement being administered before a blood donation at the American Red Cross Boston Blood Donation Center.
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