The answer to the question – what is anemia – is threefold. Anemia is a decrease in the red blood cell count; quantity of hemoglobin (HGB) per 100 ml of blood; or volume of red blood cells per 100 ml of blood (hematocrit). The red blood cell count is in the number of red blood cells (RBCs) per cubic millimeter (mm3) of blood. It alone is not a marker of anemia though. The reason is with some forms of anemia such as thalassemia, in which the size and mass of the red blood cells in vessels are below normal, the number of RBCs is increased.
There are ranges of normal for both hematocrit and hemoglobin. Normal hemoglobin values are between 42% to 54% for adult males and 38% to 46% for adult women. The unit of expression for hemoglobin is grams per deciliter (g/dL). It reflects how much the concentration of HGB measured in the test tube would weigh in a 10th of a liter of blood. The normal values for males are from 13.8 g/dL – 17.2 g/dL. For females they are from 12.2 g/dL – 15.1 g/dL.
What Causes Anemia?
In terms of pathophysiology there are only three direct causes of anemia. All of the many different types are the result of one or more of these processes. They are:
- acute blood loss
- decreased production of red blood cells
- accelerated destruction of red blood cells
Acute blood loss is that which occurs suddenly over a short period of time. The anemia due to it might not be obvious from hematocrit and hemoglobin tests early on. The reason is the proportionate loss of plasma and RBC’s. Because the loss of fluid matches the loss of red blood cells, the concentration of red blood cells in the bloodstream does not change much until restoration of some of or all of the lost volume. Much of the volume restoration is from the administration of fluids through a vein. As fluid replacement dilutes RBCs the low red blood cell count then becomes more apparent.
Chronic blood loss alone is not a direct cause of anemia. The reason is an intact hematopoietic system under normal circumstances compensates for the slow loss of red blood cells by increasing its production to match the loss. In order to do so though, it needs some important substances obtained from the diet. They are iron, folic acid and vitamin B12.
The most common of the three deficiencies resulting in reduced RBC production is of iron. Iron deficiency is rarely solely due to insufficient iron in the diet. Rather, it is most often the result of chronic blood loss. Since iron is a part of heme and hemoglobin makes up red blood cells, loss of RBCs is also the loss of iron. Even though the body stores iron in the liver, depletion of it eventually occurs with the chronic use of it to replace lost RBCs. The gastrointestinal tract is the most common site of chronic blood loss overall. The uterus is also a common site in females.
Malabsorption and pregnancy are other fairly common causes of decreased production of RBCs due to iron deficiency. The reason for the deficiency which occurs with malabsorption is obvious. Not enough iron enters the body. In the case of pregnancy, the mother uses more stored iron to increase her volume of blood in support of the fetus as it develops. Also the fetus uses the mother’s stored iron to produce its own RBCs.
Lack of iron in the diet alone is the least common cause of decreased RBC production due to iron deficiency. The reason is the average diet contains sufficient amounts of iron which the body needs to replace red blood cells that die from old age.
There are many other causes of decreased production of red blood cells. They include various chronic conditions such as cancer, kidney disease, liver disease, various infections, disease and failure of the bone marrow, and autoimmune disease. Impaired absorption of iron, folic acid or vitamin B12 from the digestive tract can also cause decreased production of red blood cells.
The bone marrow normally replaces aged red blood cells which undergo programmed death or destruction with new ones. The average lifespan of a red blood cell is 120 days. Certain conditions cause faster destruction of red blood cells. If the rate of destruction exceeds the rate of production the RBC count drops. Hemolysis is the medical term for this accelerated rate of red blood cell destruction. Several diseases and factors can cause it. They include:
- inherited diseases such as sickle cell anemia
- immune disorders
- an enlarged and over functioning spleen
- reaction to medication
- blood transfusion reactions due to donor/recipient incompatibility (mismatch)