hemoglobin

American  

[hee-muh-gloh-bin, hem-uh-] / ˈhi məˌgloʊ bɪn, ˈhɛm ə- /

especially British, haemoglobin

noun

Biochemistry.

1. the oxygen-carrying pigment of red blood cells that gives them their red color and serves to convey oxygen to the tissues: occurs in reduced form deoxyhemoglobin in venous blood and in combination with oxygen oxyhemoglobin in arterial blood. Hb

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hemoglobin Scientific  

/ hē′mə-glō′bĭn /

1. An iron-containing protein present in the blood of many animals that, in vertebrates, carries oxygen from the lungs to the tissues of the body and carries carbon dioxide from the tissues to the lungs. Hemoglobin is contained in the red blood cells of vertebrates and gives these cells their characteristic color. Hemoglobin is also found in many invertebrates, where it circulates freely in the blood. It consists of four peptide units, each attached to a nonprotein compound called heme that binds to oxygen.

2. See Note at red blood cell

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hemoglobin Cultural  

1. A complex organic molecule (see also organic molecules) containing iron that carries oxygen in the blood.

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Closer Look

Ninety percent of the protein in red blood cells is made up of hemoglobin, the main oxygen transport molecule in mammals. A protein with four iron-containing subunits called hemes, hemoglobin is a complex molecule with a complex function. It must bind to oxygen in the lungs, then release that oxygen in the tissues, then bind to carbon dioxide in the tissues and release it in the lungs. Hemoglobin accomplishes oxygen transport by changing its structure, and even its substructures, around the oxygen-binding heme groups, making them more or less accessible to the environment. When oxygen binds to at least one of the heme groups (as happens in the oxygen-rich lungs), all of the heme groups become exposed to the environment and bind oxygen easily. The bond between oxygen and heme is a loose one, however, so that the oxygen can break free in the tissues, where the concentration of oxygen is relatively low, and thereby become available for use in the cells. When the last of the four heme subunits loses its oxygen, the structure of hemoglobin changes again, so that the size of the opening from the environment to the heme groups decreases, making it difficult for an oxygen molecule to rebind to the hemoglobin. In this way, hemoglobin stops itself from competing with the tissues for needed oxygen. When the red blood cell carrying hemoglobin returns to the lungs, where oxygen concentration is high, the cycle of oxygen binding, transport, and release starts again. Normally, iron binds with oxygen to form rust (iron oxide), but the structure of hemoglobin prevents this from happening, since it would inactivate the heme subunits. Carbon dioxide does not bind the heme in hemoglobin, but rather the amino groups at the ends of the hemoglobin's protein subunits. Hemoglobin transport is only one of a number of bodily mechanisms by which carbon dioxide travels from the tissues to the lungs for release to the air.

Discover More

Hemoglobin gives blood its characteristic red color.

Other Word Forms

Derived Forms

• hemoglobic
  adjective
• hemoglobinous
  adjective

Etymology

Origin of hemoglobin

First recorded in 1865–70; earlier hematoglobulin; see hemo-, globulin

Compare meaning

How does hemoglobin compare to similar and commonly confused words? Explore the most common comparisons:

• myoglobin vs. hemoglobin
• heme vs. hemoglobin

Explanation

Hemoglobin is a hemoprotein in blood that helps transport oxygen, and it gives blood its red color. Hemoglobin is the short version of the medical word haemato-globulin, which means something like "blood grains" in Greek. Like many medical terms also derived from the Greek, this one was first used in the 19th century. A doctor doing blood tests might be measuring the levels of hemoglobin in your bloodstream.

Example Sentences

Hemoglobin binds gases such as oxygen, carbon dioxide, and nitric oxide, and it is an iron carrier via the heme complex.

From Science Daily • Nov. 17, 2023

Hemoglobin is a quaternary protein comprise of four iron-containing heme groups; iron has a great affinity for oxygen.

From Textbooks • Jun. 9, 2022

Hemoglobin also has an affinity for carbon monoxide, a toxic and deadly gas.

From Textbooks • Jun. 9, 2022

Hemoglobin arose out of four interlinked globin proteins, each holding a heme, and it rapidly became ubiquitous.

From Scientific American • May 6, 2019

Hemoglobin, the oxygen carrier in blood, for instance, performs one of the simplest and yet most vital reactions in physiology.

From "The Gene" by Siddhartha Mukherjee

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