Also found in: Dictionary, Thesaurus, Acronyms, Encyclopedia, Wikipedia.


the main functional constituent of the red blood cell, serving as the oxygen-carrying protein; it is a type of hemoprotein in which each molecule is a tetramer composed of four monomers held together by weak bonds. It consists of two pairs of polypeptide chains, the globins, each having an attached heme molecule composed of iron plus a protoporphyrin molecule. Symbol Hb.
Chemistry and Physiology.
The iron atom has a free valence and can bind one molecule of oxygen. Thus, each hemoglobin molecule can bind one molecule of oxygen. The binding of oxygen by one monomer increases the affinity for oxygen of the others in the tetramer. This makes hemoglobin a more efficient transport protein than a monomeric protein such as myoglobin. 

Oxygenated hemoglobin (oxyhemoglobin) is bright red in color; hemoglobin unbound to oxygen (deoxyhemoglobin) is darker. This accounts for the bright red color of arterial blood, in which the hemoglobin is about 97 per cent saturated with oxygen. Venous blood is darker because it is only about 20 to 70 per cent saturated, depending on how much oxygen is being used by the tissues. The affinity of hemoglobin for carbon monoxide is 210 times as strong as its affinity for oxygen. The complex formed (carboxyhemoglobin) cannot transport oxygen. Thus, carbon monoxide poisoning results in hypoxia and asphyxiation.

Another form of hemoglobin that cannot transport oxygen is methemoglobin, in which the iron atom is oxidized to the +3 oxidation state. During the 120-day life span of a red blood cell, hemoglobin is slowly oxidized to methemoglobin. At least four different enzyme systems can convert methemoglobin back to hemoglobin. When these are defective or overloaded, methemoglobinemia can result, with high methemoglobin levels causing dyspnea and cyanosis.

A secondary function of hemoglobin is as part of the blood buffer system. The histidine residues in the globin chains act as weak bases to minimize the change in blood pH that occurs as oxygen is absorbed and carbon dioxide released in the lungs and as oxygen is delivered and carbon dioxide taken up from the tissues.

As erythrocytes wear out or are damaged, they are ingested by macrophages of the reticuloendothelial system. The porphyrin ring of heme is converted to the bile pigment bilirubin, which is excreted by the liver. The iron is transported to the bone marrow to be incorporated in the hemoglobin of newly formed erythrocytes.

The hemoglobin concentration of blood varies with the hematocrit. The normal values for the blood hemoglobin concentration are 13.5 to 18.0 g/100 ml in males and 12.0 to 16.0 g/100 ml in females. The normal mean corpuscular hemoglobin concentration, which is the concentration within the red blood cells, is 32 to 36 g/100 ml.
Variant and Abnormal Hemoglobins. There are six different types of globin chains, designated by the Greek letters α, β, γ, δ, ε, and ζ. The composition of a hemoglobin is specified by a formula such as α2β2, which indicates a tetramer containing two α chains and two β chains. The chains are coded by different genes, which are turned on and off during development in order to produce hemoglobins with the oxygen-carrying properties required at each developmental stage. In the first three months of embryonic development, when blood cells are produced in the yolk sac, embryonic hemoglobins such as Hb Gower (α2Aε2) or Hb Portland (ζ2γ2) are produced. As erythropoiesis shifts to the liver and spleen, the fetal hemoglobin Hb F (α2γ2) appears. When erythropoiesis shifts to the bone marrow during the first year of life, the adult hemoglobins Hb A (α2β2) and Hb A22δ2) begin to be produced.

Many abnormal hemoglobins arising from mutations have been discovered. Some have altered oxygen affinity, some are unstable, and in some the iron atom is oxidized, resulting in congenital methemoglobinemia. Some mutations result in a reduced rate of hemoglobin synthesis. All such conditions are known as hemoglobinopathies.

The most common hemoglobinopathy is sickle cell disease, caused by a mutation replacing the sixth amino acid in the β chain, normally glutamic acid, by valine. The variant hemoglobin α2βS2 is known as Hb S. Mutations resulting in reduced synthesis of one of the chains are called thalassemias. They can result from deletion of the gene for a chain or from a mutation in the regulatory gene that controls the synthesis of the chain.
The life cycle of red blood cells and the breakdown of hemoglobin. From Polaski and Tatro, 1996.
hemoglobin A1c hemoglobin A with a glucose group attached to the amino terminal of the beta chain; it is made at a slow constant rate during the 120-day life span of the erythrocyte. It accounts for 3 to 6 per cent of the total hemoglobin in a normal person and up to 12 per cent in persons with diabetes mellitus. Increased levels correlate with glucose intolerance in diabetics; with good diabetic control its level returns to normal range, so that periodic assays can be helpful in evaluating effective control of diabetes.
glycated hemoglobin (glycosylated hemoglobin) any of various hemoglobins with glucose attached nonenzymatically; the most common one is hemoglobin A1c. The percentage of hemoglobin that is glycosylated can be assessed over a long period of time as a gauge of blood sugar control; the normal range for a nondiabetic person is between 4 and 6 per cent.
mean corpuscular hemoglobin (MCH) the average hemoglobin content of an erythrocyte, conventionally expressed in picograms per red cell, obtained by multiplying the blood hemoglobin concentration (in g/dl) by 10 and dividing by the red cell count (in millions per ml): MCH = Hb/RBC.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


Abbreviation for hemoglobin.
Farlex Partner Medical Dictionary © Farlex 2012


 Hemoglobin, see there.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


Abbreviation for hemoglobin.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
References in periodicals archive ?
Hgb was determined using HemoCue Hb 301[R] instantly after sample collection.
The CREATE clinical trial found no difference between the two groups in regards to the initial primary endpoints; the treatment group (target Hgb 13 to 15 g/dL) and the sub-normal Hgb group (target Hgb 10.5 to 11.5 g/dL).
Table 4 reveals that resource mobilization of the HGB and the PGB have shown an increasing trend.
Some clinical laboratories may use HGB reference intervals stratified by age, including a separate category for elderly patients.
(c) decrease dosage by one increment and check Hgb two weeks after the last dose
For the baking characteristics of mixograph absorption and mix time, the HGA and HGB groups did not significantly differ from the HWC group (Table 3).
The geometric mean Hgb level for children who also had excess lead exposure was 12.8 g/dL (range 8.9-15.2 g/dL) and was not significantly different from the geometric mean Hgb level of children who had BLLs below 10 [micro] g/dL (12.9 g/dL, 10.1-16.1 g/dL).
Using a version of stochastic dominance given in Levy and Sarnat (1985), Table 6 provides results for the January returns over non-January returns for HYB, HGB, LGB, and the spread.
Height 0.276 0.039 0.009 Percentile Height 0.183 0.044 0.048 Weight 0.804 0.311 0.111 Percentile Weight 0.617 0.363 0.029 Mid-Arm Circumference 0.926 0.092 0.064 Percentile Mid-Arm Circumference 0.765 0.166 0.083 Triceps Skin Fold 0.690 0.130 0.050 Percentile Triceps Skin Fold 0.530 0.201 0.020 Mid-Arm Muscle Circumference 0.957 0.063 0.066 Percentile Mid-Arm Muscle Circumference 0.855 0.010 0.149 Mid-Arm Muscle Area 0.934 0.065 0.066 Percentile Mid-Arm Muscle Area 0.845 0.059 0.079 Protein 0.150 0.916 0.023 Vitamin A 0.115 0.417 0.092 Ascorbic Acid 0.431 0.174 0.096 Riboflavin 0.006 0.859 0.090 Calcium 0.059 0.814 0.012 Iron 0.308 0.795 0.019 Calories 0.192 0.781 0.009 Albumin 0.068 0.003 0.026 HGB 0.215 0.073 0.921 HCT 0.056 0.031 0.948 TLC 0.108 0.230 0.463
It has been noted that patients undergoing hip surgery and THA or TKA are commonly relatively older, and failure to monitor postoperative Hgb changes in these patients may lead to serious consequences.
The outcome variable in this research was glycemic control as measured by Hgb A1C.
With no [+ or -] allowable error distribution for the acceptable range limit points, 10 g/dL and 12 g/dL, the clinical goal and the reimbursement reduction risk resides within the Hgb measurement error and the ability to resolve an Hgb result of 10 g/dL and one of 9.9 g/dL.