Hematocrit is used to diagnose and monitor a number of medical conditions, including rheumatoid arthritis, anemia, and a variety of cancers. This video will define hematocrit; demonstrate how to measure hematocrit; define anemia and polycythemia; describe the role that erythropoietin (or EPO) plays in the oxygen-carrying ability of the blood; explain the gender differences for the hematocrit ranges; and explain why persons at higher altitudes develop secondary polycythemia. Red blood cells, also known as erythrocytes, provide for the oxygen-carrying capacity of the blood through the iron-containing molecule called hemoglobin. The percentage of red blood cells that occupy a volume of whole blood is the hematocrit. The hematocrit is determined by centrifuging a blood sample. The heavier red blood cells (or RBCs) will move to the bottom of the tube and the plasma will stay on the top. The buffy coat is a layer sandwiched in between the plasma and the red blood cells. The buffy coat contains leukocytes and platelets. The hematocrit is performed by filling a heparinized capillary tube with blood until it is about three-quarters of the way filled. The end of the tube where the blood is located is sealed with capillary tube sealer. The tube is then placed in a centrifuge that has been equipped with a special rotor for small tubes. After centrifuging, the height of the packed red blood cells is measured in millimeters. The height of the total blood sample is measured in millimeters, and the percentage that the packed red blood cells occupy is calculated. Here, the red blood cells are 22 millimeters tall, and the total blood sample is 49 millimeters tall. To find the hematocrit, we divide 22 by 49 and convert it into a percentage. The normal ranges for hematocrit are males 42 to 52 percent, and females 37 to 47 percent. Let's check your understanding. Given the ranges males 47 plus or minus 5 percent and females 42 plus or minus 5 percent, would a hematocrit of 41 percent be in the normal range? No, for all patients; yes, for all patients; yes, if the patient is female; or yes, if the patient is male. >> Forty-one percent would only be below range for a male, since the normal range for males is 42 to 52 percent. It would be just below the middle of the range for females, who have a range of 37 to 47 percent and are therefore still in range. Anemia is a condition in which the oxygen-carrying capacity of the blood is too low to support normal metabolism. There are many reasons for anemia, including a low hematocrit. Conversely, too high a hematocrit is also abnormal. The condition of too many red blood cells is called polycythemia. The level of red blood cells is hormonally controlled. Erythropoietin (or EPO) stimulates the synthesis of erythrocytes. Normally, a small amount of erythropoietin is present, but when the oxygen delivery is too low, cells in the kidney are stimulated to release erythropoietin. Erythropoietin, in turn, stimulates the red bone marrow to increase erythropoiesis, increasing the red blood cell count and increasing the oxygen-carrying capacity of the blood. Let's check your understanding. The direct stimulus for EPO release is: a lower than normal hematocrit; number of red blood cells; or level of oxygen delivery to the tissues? >> The stimulus is hypoxia. The reason for the hypoxia could be a low hematocrit or a lower than normal number of red blood cells. The gender differences in hematocrit values are at least in part due to the effects of the male sex hormone testosterone. Testosterone enhances the kidney's production of EPO. With more EPO to stimulate the bone marrow, higher red blood cell counts result for males. Living at a high altitude can result in secondary polycythemia because of the lower oxygen content of the air. Let's check your understanding. Individuals who live at a high altitude would have a lower than normal, higher than normal, or normal hematocrit? >> The lower oxygen content in the air would stimulate EPO production that would result in an elevated hematocrit. Many athletes use blood doping to improve athletic performance by increasing the oxygen carrying capacity of their blood. As you prepare for the hematocrit lab, consider how the practice of blood doping would affect a patient's hematocrit.