What is anemia?

Anemia is a deficiency of circulating red blood cells or low hemoglobin levels. There are number of different types of anemia and this term may encompass a number of defects in size, shape, structure and quantity of red blood cells in the blood. Irrespective of the type of anemia, the oxygen-carrying capacity of the blood is reduced and this causes a number of effects as outlined in Too Few Red Blood Cells.

Causes of Anemia

There are hundreds of types of anemia and the causes of anemia can divided according to :

  1. Blood loss
  2. Decreased red blood cell production
  3. Red blood cell destruction (hemolysis)

Anemia due to Blood Loss

Acute blood loss is often due to trauma and the hemorrhage has to be massive for it to result in anemia. Death is always a risk in acute blood loss but if attended to rapidly and if the patient survives, anemia will set in unless there is a transfusion. It can take several days for the bone marrow to replace the lost red blood cells, but this is only possible if there is sufficient iron available.

The body constantly recycles iron as outlined under Red Blood Cells Life Cycle. Since 80% of the body’s iron is in circulation compared to 20% in the stores, a massive hemorrhage may mean that a large portion of this iron is lost. Iron deficiency anemia will set it and iron supplementation may therefore be necessary.

In chronic blood loss, the hemorrhage is usually minor and while the bone marrow may be able to accommodate with replenishing the red blood cell count, the hemoglobin levels are often lower as a result of chronic iron loss. This is frequently seen in young menstruating women, especially where gynecological disturbances lead to heavy periods or frequent periods. Another cause for chronic blood loss leading to anemia is gastrointestinal bleeding – refer to Gastric Bleeding and Stomach Bleeding.

Anemia due to Decreased Red Blood Cell Production

Red blood cell production may be hampered for a number of reasons including disruptions, defects and deficiencies of :

  • bone marrow, hematopoietic stems cells and other red blood cell precursors
  • nutrients like iron, vitamin B12, folic acid
  • building blocks of red blood cells like heme or the globins, which is often related to a lack of the nutrients
  • erythropoietin which is the hormone responsible for stimulating red blood cell production
  • unknown mechanisms

Most cases of decreased red blood cell production are related to nutritional deficiencies. Here the red blood cells are formed but do not develop or mature appropriately.

  • Iron-deficiency anemia is the most common and while in developed countries it is often due to chronic blood loss, in developing countries a low dietary iron and malnutrition is one of the leading causative factors.
  • Vitamin B12 and folate deficiency may lead to megaloblastic anemia and pernicious anemia. The latter deficiencies may be seen in chronic gastritis, pregnancy, alcohol abuse, strict dieting, fasting or starvation

In certain cases, the bone marrow may not produce sufficient red blood cells for various reasons. Aplastic anemia is where the bone marrow is damaged and may be caused by :

  • Autoimmune diseases
  • Cancer and cancer treatment (chemotherapy and radiation therapy)
  • Chronic infections
  • Medication or toxins
  • Pregnancy
  • Idiopathic (unknown)

Anemia of chronic disease may be linked to aplastic anemia although the bone marrow may not be disturbed yet the utilization and maturation of  red blood cells may be disrupted. In chronic kidney disease is often associated with a disruption in the production and/or secretion of erythropoietin. This hormone is primarily produced by the kidney and if it is not available in sufficient quantities in the bloodstream, the bone marrow will not produce new red blood cells even if other stimuli, like hypoxia, is present. Anemia of chronic disease is also seen in conditions like cancer, hypothyroidism, rheumatoid arthritis or systemic lupus erythematosus (SLE).

Anemia due to Red Blood Cell Destruction

This is known as hemolytic anemias. Red blood cells are destroyed and removed out of the bloodstream prematurely and often at a rate that is faster than the bone marrow’s ability to replenish red blood cells. The trigger for this premature destruction may be intrinsic, where there are defects in red blood cells, or extrinsic, where outisde factors causes even normal red blood cells to be destroyed. Due to the rapid breakdown of red blood cells, jaundice may occur. This is known as hemolytic jaundice discussed under Types of Jaundice.

  • Thalassemia is a genetic condition where the formation of the globin chains are affected thereby impairing hemoglobin formation. Alpha-thalassemia indicates a problem with alpha-globin chain formation while beta-thalassemia is a result of problems with beta-globin chain formation. Refer to Red Blood Cells Structure for more information on hemoglobin formation.
  • Sickle cell anemia is another genetic condition where the shape of the red blood cells are distorted (sickle-shaped)  and this drastically decreases its life span for various reasons. Another condition known as hereditary spherocytosis is due to a defect in the red blood cell membrane thereby resulting in a cell that is sphere shaped. Refer to Red Blood Cells Shape.
  • Eyrthoblastosis fetalis occurs in a fetus and newborn baby where the immune system attacks and destroys red blood cells as a result of Rhesus incompatibility between mother and child. Refer to Neonatal Jaundice.

Other causes of rapid red blood cell destruction may be a result of :

  • Infections like malaria
  • Toxins
  • Poisons like venom
  • Medication
  • Cardiovascular disturbances like a thrombus (blood clot)
  • Unknown causes (idiopathic)

Related Articles

  1. Red Blood Cells
  2. Too Few Red Blood Cells
  3. Low Iron in the Blood


Article reviewed by Dr. Greg. Last updated on November 25, 2010