Blood Smear Showing Reduced Red Blood Cell Count : Anemia is caused when there are too few RBCs circulating in the bloodstream. That can happen when they are lost through bleeding, destroyed too quickly or produced too slowly. When a person is anemic, the blood is less effective at transporting inhaled oxygen to cells and retrieving carbon dioxide from the tissues and carrying it back to the lungs to be exhaled.

The hematocrit (Ht or HCT, from the Greek "αιματοκρίτης" which means blood judge'), also known as packed cell volume (PCV) or erythrocyte volume fraction (EVF), is the volume percentage (%) of red blood cells in blood. It is normally 45% for men and 40% for women. It is considered an integral part of a person's complete blood count results, along with hemoglobin concentration, white blood cell count, and platelet count. Because the purpose of red blood cells is to transfer oxygen from the lungs to body tissues, a blood sample's hematocrit—the red blood cell volume percentage—can become a point of reference of its capability of delivering oxygen. Additionally, the measure of a subject's blood sample's hematocrit levels may expose possible diseases in the subject. Anemia refers to an abnormally low hematocrit, as opposed to polycythemia, which refers to an abnormally high hematocrit. For a condition such as anemia that goes unnoticed, one way it can be diagnosed is by measuring the hematocrit levels in the blood. Both are potentially life-threatening disorders.

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