Hemoglobin is the protein molecule in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues to the lungs. Hemoglobin is made up of four protein molecules (globulin chains) that are connected together. The normal adult hemoglobin (Hbg) molecule contains 2 alpha-globulin chains and 2 beta-globulin chains. In fetuses and infants, there are only a few beta chains and the hemoglobin molecule is made up of 2 alpha chains and 2 gamma chains. As the infant grows, the gamma chains are gradually replaced by beta chains.

Each globulin chain contains an important central structure called the heme molecule. Embedded within the heme molecule is iron that transports the oxygen and carbon dioxide in our blood. The iron contained in hemoglobin is also responsible for the red color of blood.

Hemoglobin also plays an important role in maintaining the shape of the red blood cells. Abnormal hemoglobin structure can, therefore, disrupt the shape of red blood cells and impede its function and its flow through blood vessels.

Hemoglobin is usually measured as a part of the complete blood count (CBC) from a blood sample. Several methods exist for measuring hemoglobin, most of which are done currently by automated machines designed to perform several different tests on blood. Within the machine, the red blood cells are broken down to get the hemoglobin into a solution. The free hemoglobin is exposed to a chemical containing cyanide which binds tightly with the hemoglobin molecule to form cyanmethemoglobin. By shining a light through the solution and measuring how much light is absorbed (specifically at a wavelength of 540 nanometers), the amount of hemoglobin can be determined.

The hemoglobin level is expressed as the amount of hemoglobin in grams (gm) per deciliter (dl) of whole blood, a deciliter being 100 milliliters.

The normal ranges for hemoglobin depend on the age and, beginning in adolescence, the gender of the person. The normal ranges are:

• Newborns: 17-22 gm/dl
  
  
• One (1) week of age: 15-20 gm/dl
  
  
• One (1) month of age: 11-15gm/dl
  
  
• Children: 11-13 gm/dl
  
  
• Adult males: 14-18 gm/dl
  
  
• Adult women: 12-16 gm/dl
  
  
• Men after middle age: 12.4-14.9 gm/dl
  
  
• Women after middle age: 11.7-13.8 gm/dl

All of these values may vary slightly between laboratories. Some laboratories do not differentiate between adult and "after middle age" hemoglobin values.

A low hemoglobin is referred to as anemia. There are many reasons for anemia.

Some of the more common causes are:

• loss of blood (traumatic injury, surgery, bleeding colon cancer or stomach ulcer),
  
  
• nutritional deficiency (iron, vitamin B12, folate),
  
  
• bone marrow problems (replacement of bone marrow by cancer,
  
  
• suppression by chemotherapy drugs,
  
  
• kidney failure), and
  
  
• abnormal hemoglobin (sickle cell anemia).

Higher than normal hemoglobin levels can be seen in people living at high altitudes and in people who smoker. Dehydration produces a falsely high hemoglobin which disappears when proper fluid balance is restored.

Some other infrequent causes are:

• advanced lung disease (for example, emphysema),
  
  
• certain tumors,
  
  
• a disorder of the bone marrow known as polycythemia rubra vera, and
  
  
• abuse of the drug erythropoietin (Epogen) by athletes for blood doping purposes.

Sickle cell disease is a genetic condition in which the quality of hemoglobin is defective. This condition can cause abnormal hemoglobin which, in turn, can result in abnormally shaped (sickled) red blood cells. These abnormal red blood cells cannot easily pass through small blood vessels and, therefore, could deprive the body organs of adequate oxygen.

Sickle cells also have a shorter life span than normal red blood cells (10-20 days compared to 120 days). This rapid turn over may result in inadequate time to replace the red blood cells and may result in anemia.

When red blood cells die, the hemoglobin within them is released and broken up: the iron in hemoglobin is salvaged, transported to the bone marrow by a protein called transferrin and used again in the production of new red blood cells; the remainder of the hemoglobin becomes a chemical called bilirubin that is excreted into the bile which is secreted into the intestine, where it gives the feces their characteristic yellow-brown color.

Although the changes that produce abnormal hemoglobins are rare, several hundred abnormal (or more precisely, "variant") hemoglobins exist. These have accumulated over the millions of years of human existence. Most variant hemoglobins function normally, and are only found through specialized research techniques. Some hemoglobins, however, do not function normally and can produce clinical disorders, such as sickle cell disease.

Genes can suffer damage to an extent that they no longer produce normal amounts of hemoglobin. Usually, only one of the sets of hemoglobin genes is affected, that is the alpha gene set or the beta gene set. For example, one of the two beta globin genes may fail to produce a normal quantity of beta chain protein. The alpha globin gene set will continue to produce a normal quantity of alpha chain protein. An imbalance develops in the amount of alpha chain and beta chain protein in the cell. There is too much alpha chain for the amount of beta chain that is present. This imbalance is called "thalassemia ". In this example, it would be beta thalassemia, because it is the beta chain gene that has failed. An analogy would be cars coming out of the factory. Engines and bodies must be made in equal numbers to have functional automobiles. If the engine plant goes on strike (thalassemia), the bodies produced by the body plant are useless.