[hem- + globin]
ABBR: Hb, Hbg, Hgb The iron-containing pigment of red blood cells (RBCs) that carries oxygen from the lungs to the tissues. The amount of hemoglobin in the blood averages 12 to 16 g/100 ml in women, 14 to 18 g/100 ml in men, and somewhat less in children. Hemoglobin is a crystallizable, conjugated protein consisting of heme and globin. In the lungs, 1 g of hemoglobin combines readily with 1.36 cc of oxygen by oxygenation to form oxyhemoglobin. In the tissues where oxygen concentration is low and carbon dioxide (CO2) concentration is high (low pH), hemoglobin releases its oxygen. Hemoglobin also acts as a buffer for the hydrogen ions produced in RBCs when (CO2) is converted to bicarbonate ions for transport in the plasma.
When old RBCs are phagocytized by macrophages in the liver, spleen, and red bone marrow, the iron of hemoglobin is reused immediately to produce new RBCs or is stored in the liver until needed. The globin is converted to amino acids for the synthesis of other proteins. The heme portion is of no further use and is converted to bilirubin.
Hemoglobin combines with carbon monoxide (in carbon monoxide poisoning) to form the stable compound carboxyhemoglobin, which renders hemoglobin unable to bond with oxygen and results in hypoxia of tissues. Oxidation of the ferrous iron of hemoglobin to the ferric state produces methemoglobin.
Hundreds of different types of hemoglobin have been discovered.
A hemoglobin molecule composed of two alpha and two beta chains.
HEMOGLOBIN A MOLECULE
ABBR: Hb A1c Hemoglobin A that contains a glucose group linked to the terminal amino acid of the beta chains of the molecule.
Levels of hemoglobin A1c can be used to determine both the presence of diabetes mellitus (in previously undiagnosed patients) and the degree of glycemic control of known diabetics. The amount of glucose bound to the hemoglobin depends on the average concentration of glucose in the blood over time. In patients with diabetes mellitus, when the blood glucose level is optimally and carefully regulated over 8 to 12 weeks, the Hb A1c level is normal or slightly elevated. If the blood glucose level has not been controlled (and has been abnormally elevated) in the preceding 8 to 12 weeks, the Hb A1c blood level is increased. Hb A1c is a good indicator of long-term glycemic control. The blood test for it may be performed when the patient is not fasting.
The hemoglobin A1c test is unreliable in pregnancy and in hemolytic anemias. In conditions in which the longevity of red blood cells is decreased, diabetes mellitus should be diagnosed with measurements of fasting plasma glucose or with oral glucose tolerance tests.
SYN: SEE: glycohemoglobin; SEE: glycated hemoglobin; SEE: glycosylated hemoglobin
SEE: Barts hemoglobin
A hemoglobin molecule in which lysine is substituted for glutamic acid at the sixth position of the beta chain. This substitution decreases the solubility of the hemoglobin molecule and increases the rigidity of the red blood cell membrane.
A hemoglobin molecule in which lysine is substituted for glutamic acid at the 26th position of the beta chain. This variation is found most often in those of Southeast Asian ancestry.
The type of hemoglobin found in the erythrocytes of the normal fetus. It has better oxygen-binding capacity than adult hemoglobin and is able to extract oxygen from the placenta to meet the needs of the fetus.
The induction of fetal hemoglobin (with drugs such as hydroxyurea) in patients with sickle cell anemia often improves their clinical status because fetal hemoglobin does not deform or “sickle” in the circulation. It is capable of taking up and giving off oxygen at lower oxygen tensions than the hemoglobin in adult erythrocytes.
free plasma hemoglobin
SEE: Plasma hemoglobin.
SEE: Hemoglobin A1c.
SEE: Hemoglobin A1c.
A variant hemoglobin formed by an unequal crossover and fusion of the beta and delta genes. A single copy of the variant gene causes thalassemia minor. Homozygotes have thalassemia intermedia.
mean cell hemoglobin
The hemoglobin content of the average RBC, usually expressed in picograms per red cell and calculated by multiplying the number of grams of hemoglobin/100 ml by 10 and dividing by the red cell count.
SYN: SEE: mean corpuscular hemoglobin
mean corpuscular hemoglobin
ABBR: MCH SEE: Mean cell hemoglobin.
Hemoglobin released from red blood cells when they are destroyed (lyzed). It circulates in the blood and extravascular tissues.
SYN: SEE: free plasma hemoglobin
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