Anemia is a blood disorder characterized by abnormally low levels of healthy red blood cells (RBCs) or reduced hemoglobin (Hgb), the iron-bearing protein in red blood cells that delivers oxygen to tissues throughout the body. Reduced blood cell volume (hematocrit) is also considered anemia. The reduction of any or all of the three blood parameters reduces the oxygen-carrying capability of the blood, causing reduced oxygenation of body tissues, a condition called hypoxia.
All tissues in the human body need a regular supply of oxygen to stay healthy and perform their functions. RBCs contain Hgb, a protein pigment that allows the cells to carry oxygen (oxygenate) tissues throughout the body. RBCs live about 120 days and are normally replaced in an orderly way by the bone marrow, spleen, and liver. As RBCs break down, they release Hgb into the blood stream, which is normally filtered out by the kidneys and excreted. The iron released from the RBCs is returned to the bone marrow to help create new cells. Anemia develops when either blood loss, a slow-down in the production of new RBCs (erythropoiesis), or an increase in red cell destruction (hemolysis) causes significant reductions in RBCs, Hgb, iron levels, and the essential delivery of oxygen to body tissues.
Anemia can be mild, moderate, or severe enough to lead to life-threatening complications. More than 400 different types of anemia have been identified. Many of them are rare. Most are caused by ongoing or sudden blood loss. Other causes include vitamin and mineral deficiencies, inherited conditions, and certain diseases that affect red cell production or destruction.
Anemia in newborn infants is noted when hemoglobin levels are lower than expected for the birth weight and postnatal age. Premature or low birth-weight infants may have lower hemoglobin levels. The normal newborn Hgb is 16.8 dL, which may be 1 to 2 dL lower if birth weight is abnormally low. Anemia may be the first sign of certain disorders in the newborn, such as blood loss that has occurred from transplacental hemorrhage, a condition in which the infant's blood bleeds back into the mother's circulation; bleeding from ruptures in the liver, spleen, adrenals, or kidneys; or hemorrhage within the brain (intracranial hemorrhage). Anemia can also be caused by the destruction of red blood cells or reduced red blood cell production. Newborns may also have low red blood cell volume (hematocrit or Hct) if they were born by cesarean section. It must be noted, however, that hemoglobin decreases naturally (physiologic decrease) in infants by eight to 12 weeks of age, leveling at a normal value of 11 g/dL or better.
L. Lee Culvert, Maureen Haggerty, Thomson Gale, Gale, Detroit,