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2015-05-27 19:34:47
2015-05-27 19:34:47

Hemoglobin is a protein-based component of red blood cells which primary role is transferring oxygen from the lungs to the rest of the body. Although not many people know about this, hemoglobin is actually the reason red blood cells appear red, although oxygen-rich blood is noticeably brighter than the depleted blood returning to the heart and lungs. Fresh hemoglobin is produced in the bone marrow as needed.

Structure of hemoglobin

The hemoglobin molecule is a group of four globular protein subunits and each of these subunits is composed of a protein chain tightly associated with a non-protein heme group. It is proven that each individual protein chain arranges in a set of alpha-helix structural segments connected together in a specific arrangement called myoglobin fold. This folding pattern contains a pocket which is perfectly suitable to strongly bind the heme group.

What exactly is this heme group?

A heme group consists of an iron atom (Fe) held in a heterocyclic ring, known as a porphyrin. This iron atom is the exact site of oxygen binding. The fact is that an iron atom is bonded four nitrogens in the center of the ring and two additional bonds perpendicular to the plane on each side can be formed with the iron to form the fifth and sixth positions, one connected strongly to the protein, the other available for binding of oxygen.

Types of hemoglobins in humans

In the embryo:

Gower 1 (ξ2ε2)

Gower 2 (α2ε2) (PDB 1A9W)

Hemoglobin Portland (ξ2γ2)

In the fetus:

Hemoglobin F (α2γ2) (PDB 1FDH) - Hemoglobin F is the predominant hemoglobin during fetal development

In adults:

Hemoglobin A (α2β2) (PDB 1BZ0) - This is the designation for the normal hemoglobin that exists after birth.

Hemoglobin A2 (α2δ2) - This is a minor component of the hemoglobin found in red cells after birth

Hemoglobin F (α2γ2)

The creation of hemoglobin

Like all proteins, the exact copy or blueprint for hemoglobin exists in DNA and normally, every individual has four genes that code for the alpha protein, or alpha chain.Two other genes code for the beta chain. The alpha chain and the beta chain are made in precisely equal amounts, despite the differing number of genes. The protein chains join in developing red blood cells, and remain together for the life of the red cell. The fact is that essentially, hemoglobin develops a hunger for oxygen molecules.

Heme synthesis

Heme is synthesized in a complex series of steps involving enzymes in the mitochondrion and in the cytosol- two organelles of the cell. First one molecule called 5-aminolevulic acid (ALA) is being transported to the cytosol where a series of reactions produce a ring structure called coproporphyrinogen III which then returns to the mitochondrion where an addition reaction produces protoporhyrin IX. The enzyme ferrochelatase inserts iron into the ring structure of protoporphyrin IX to produce heme.

Globin synthesis

The fact is that two globin chains combine to form hemoglobin. One of the chains is designated alpha and the second chain is called "non-alpha". This is because there is a number of variables influence the nature of the non-alpha chain in the hemoglobin molecule. For example:

The fetus has a distinct non-alpha chain called gamma.

After birth, a different non-alpha globin chain, called beta, pairs with the alpha chain.

The combination of two alpha chains and two non-alpha chains produces a complete hemoglobin molecule

Physiology of blood and hemoglobin circulation

Whenever the blood is carried into the lungs, these hemoglobin proteins attract whatever oxygen is available and this oxygenated blood then travels throughout the entire bloodstream, releasing oxygen into the various muscles and organs all around the body. When these red blood cells are being spent, they are being transferred to the gastrointestinal system for disposal and new red blood cells with hemoglobin take their place in the bloodstream.

How is hemoglobin measured?

There are several methods exist for measuring hemoglobin and most of them are done currently by machines designed to perform several different tests on blood. In this machine, the red blood cells are broken down to get the hemoglobin into a solution. The free hemoglobin is exposed to some specific chemicals that are 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, the amount of hemoglobin can be determined.

Normal values of hemoglobin

The hemoglobin level is expressed as the amount of hemoglobin in grams per deciliter of blood and the normal ranges for hemoglobin depend on the age and, beginning in adolescence, the sex 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

How the test is performed?

Hemoglobin levels are amongst the most commonly performed blood tests, usually as part of a full blood count or complete blood count. The test is rather simple. Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand and the puncture site is cleaned with antiseptic. A small needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation and once the blood has been collected, the needle is removed.

Test results

Lower-than-normal hemoglobin may indicate:

anemia (various types)

erythropoietin deficiency (from kidney disease)

red blood cell destruction associated with transfusion reaction

bleeding

lead poisoning

malnutrition

nutritional deficiencies of iron, folate, vitamin B-12, vitamin B-6

over hydration

Higher-than-normal hemoglobin may indicate:

congenital Heart disease

cor pulmonale

pulmonary fibrosis

polycythemia vera

increased RBC formation associated with excess erythropoietin

What does a low hemoglobin level mean?

Low hemoglobin is referred to the condition called anemia. There are many reasons for anemia. Some of the more common reasons are:

loss of blood (traumatic injury, surgery, bleeding colon cancer),

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)

Possible causes of elevated hemoglobin

A high hemoglobin count usually suggests that you have too many red blood cells. This condition is also called erythrocytosis. It can lead to a several complications because it can impair circulation and lead to abnormal clotting.

An elevated hemoglobin concentration is usually the result of 2 mechanisms:

increased red blood cell production as a compensatory mechanism when blood oxygen carrying capacity is compromised to meet the demand of tissue

contracted plasma volume resulting in an appearance of greater red cell volume

Causes of an elevated hemoglobin count may include:

Living at high altitudes- Because the higher the altitude is, the lower oxygen level is in the air- body is producing more hemoglobin to bind as much oxygen as it is possible!

Smoking- Several studies done in the past with smokers volunteers have proven that great percentage of the smokers have the elevated hemoglobin level, although the mechanism is still unknown. Some experts believe that it could be because of low level of "pure" oxygen in the smoker's lungs! So, this could be an adoptive mechanism on low oxygen levels!

Dehydration - Dehydration produces falsely high hemoglobin which disappears when proper fluid balance is restored.

The symptoms and possible complications

Although much of the clinical focus has been on anemia as a condition where there is lack of hemoglobin, there are also problems at the high end.

The most notable consequence is increased blood viscosity.

Several studies have proven that the relation between hemoglobin and blood viscosity is linear when the hematocrit is <0.50 (equivalent to a hemoglobin value of 160 g/L). Above this hemoglobin concentration, the relation becomes exponential, which means that a small increase in hemoglobin or hematocrit results in a large increase in viscosity. Once hemoglobin concentrations reach 180 g/L, the blood viscosity reaches a level that impairs microcirculation and an inadequate amount of oxygen is transported to tissues, similar to the situation with severe anemia.

Symptoms

Peripheral cyanosis and impaired mental function resulting from compromised cerebral blood circulation are the two most common symptoms of elevated hemoglobin! Additionally, because of the poor blood flow, the risk of thrombo-embolism increases significantly.

Treatment

Because the high hemoglobin couldn't be considered as a special disorder it could be only treated as a symptom of some other underlying disease. That's why- treating the underlying disease will probably lower the hemoglobin level!

Article sources

http://www.ajcn.org

http://www.nlm.nih.gov

http://www.sickle.bwh.harvard.ed

http://www.wikipedia.com

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