hemoglobin
Meaning of hemoglobin: Four protein chains around four iron atoms, arranged so that picking up one oxygen molecule makes the next three easier to catch.
Definition of hemoglobin
Hemoglobin is the protein inside red blood cells that binds oxygen in the lungs and releases it in the tissues. Each molecule is built from four protein chains, and each chain holds a heme group containing an iron atom that oxygen attaches to. The iron is also what gives blood its red color.
How does it pick up and let go of oxygen?
Binding is cooperative. When the first oxygen molecule attaches, the whole protein shifts shape so the remaining sites bind more readily, and the reverse happens on release. That is what allows hemoglobin to load almost fully in the lungs and unload substantially in the tissues, rather than releasing a little everywhere. The switch is tuned by local conditions: warmth, acidity and carbon dioxide all loosen the grip on oxygen, so tissues working hardest and producing the most of these receive the most. Hemoglobin also carries a share of the carbon dioxide back toward the lungs, though most of it travels dissolved in plasma as bicarbonate rather than on the protein itself.
What does a hemoglobin measurement show?
The concentration of hemoglobin in blood is part of a routine blood count and is the usual measure of anemia, which means having too little of it to meet the body's oxygen needs. A low result says that anemia is present but not why. Blood loss, iron or vitamin deficiency, marrow disease, chronic illness and increased destruction of red cells all produce it, and the cell size and other indices reported alongside the number are what point toward the cause. A raised result can follow dehydration, which concentrates the blood, or genuine overproduction, and living at altitude raises it as a normal adaptation.
What can go wrong with the molecule itself?
| Problem | What happens |
|---|---|
| Sickle cell disease | A single altered building block makes hemoglobin stack into rods when oxygen is low, distorting the red cell |
| Thalassemia | One type of globin chain is made in reduced quantity, so the chains are unbalanced |
| Carbon monoxide poisoning | Carbon monoxide binds far more tightly than oxygen and blocks the site |
| Methemoglobinemia | The iron is chemically altered and can no longer bind oxygen |
Fetal hemoglobin is a further variant, built from different chains and holding oxygen more tightly, which is how a fetus draws oxygen across the placenta from the parent's blood.
Also known as
Used in a sentence
The complete blood count showed a hemoglobin level below the expected range.
Good to know
A low hemoglobin level establishes that anemia is present but not its cause, since blood loss, deficiency, reduced production and increased destruction of red cells all produce the same result and are separated by further testing.
Word origin
Where “hemoglobin” comes from
Formed from hemo-, from Greek haima, meaning blood, and globin, from Latin globus, meaning a ball, describing the rounded shape of the protein.