Introduction:

Anemia is an abnormal reduction in red blood cells.

This photmicrograph shows normal red blood cells (RBCs) as seen in the microscope after staining.

Anemia is not a single disease but a condition, like fever, with many possible causes and many forms. Causes of anemia include nutritional deficiencies, inherited genetic defects, medication-related side effects, and chronic disease. It can also occur because of blood loss from injury or internal bleeding, the destruction of red blood cells, or insufficient red blood cell production. The condition may be temporary or long-term, and can manifest in mild or severe forms.

This report focuses on three of the most common forms of anemia:

• Iron deficiency anemia
• Anemia of chronic disease (ACD)
• Megaloblastic anemia (caused by deficiencies in the B vitamins folate, vitamin B12, or both)

Some less common causes and types of anemia are included in a table in this report.

Blood

Blood has two major components:

• Plasma is a clear yellow liquid that contains proteins, nutrients, hormones, electrolytes, and other substances. It constitutes about 55% of blood.
• White and red blood cells and platelets make up the balance of blood. The white cells are the infection fighters for the body, and platelets are necessary for blood clotting. The important factors in anemia, however, are red blood cells.

Red blood cells (RBCs), also known as erythrocytes, carry oxygen throughout the body to nourish tissues and sustain life. Red blood cells are the most abundant cells in our bodies. Men have about 5.2 million red blood cells per cubic millimeter of blood, and women have about 4.7 million per cubic millimeter of blood.

Hemoglobin and Iron

Each red blood cell contains 280 million hemoglobin molecules. Hemoglobin is a complex molecule, and it is the most important component of red blood cells. It is composed of protein (globulin) and a molecule (heme), which binds to iron.

In the lungs, the heme component binds to oxygen in exchange for carbon dioxide. The oxygenated red blood cells are then transported to the body's tissues, where the hemoglobin releases the oxygen in exchange for carbon dioxide, and the cycle repeats. The oxygen is used in the mitochondria, the power source within all cells.

Red blood cells typically circulate for about 120 days before they are broken down in the spleen. Most of the iron used in hemoglobin can be recycled from there and reused.

Structure and Shape of Red Blood Cells

Red blood cells -- the erythrocytes -- are extremely small and look something like tiny, flexible inner tubes. This unique shape offers many advantages:

• It provides a large surface area to absorb oxygen and carbon dioxide.
• Its flexibility allows it to squeeze through capillaries, the tiny blood vessels that join the arteries and veins.

Abnormally shaped or sized erythrocytes are typically destroyed and eliminated.

Blood Cell Production (Erythropoiesis)

The actual process of making red blood cells is called erythropoiesis. (In Greek, erythro means "red," and poiesis means "the making of things.") The process of manufacturing, recycling, and regulating the number of red blood cells is complex and involves many parts of the body:

• The body carefully regulates its production of red blood cells so that enough are manufactured to carry oxygen but not so many that the blood becomes thick or sticky (viscous).
• Most of the work of erythropoiesis occurs in the bone marrow. In children younger than 5 years old, the marrow in all the bones of the body is enlisted for producing red blood cells. As a person ages, red blood cells are eventually produced only in the marrow of the spine, ribs, and pelvis.
• If the body needs more oxygen (at high altitudes, for instance), the kidney triggers the release of the hormone erythropoietin (EPO), a hormone that acts in the bone marrow to increase the production of red blood cells.
• The lifespan of a red blood cell is 90 - 120 days. The liver and the spleen remove old red blood cells are removed from the blood by the liver and spleen.
• When old red blood cells are broken down for removal, iron is returned to the bone marrow to make new cells.