A hematocrit is a test that measures the proportion of a person's blood that is made up of red blood cells (RBCs). Blood consists of RBCs, white blood cells (WBCs), and platelets suspended in a fluid portion called plasma. The hematocrit is a ratio of the volume of red blood cells to the volume of all these components together, called whole blood. The value is expressed as a percentage or fraction. For example, a hematocrit value of 40% means that there are 40 milliliters of red blood cells in 100 milliliters of blood. The hematocrit is a fairly quick and simple way of evaluating a person's red blood cells and checking for conditions such as anemia. It is often performed in conjunction with a hemoglobin level and is also one component of the complete blood count (CBC), a test that is often used in the general evaluation of a person's health.RBCs are produced in the bone marrow and are released into the bloodstream when they are, or nearly are, mature. They typically make up roughly 37% to 49% of the volume of blood. RBCs contain hemoglobin, a protein that binds to oxygen. The primary function of RBCs is to carry oxygen from the lungs to the tissues and organs of the body. They also transport a small portion of carbon dioxide, a byproduct of cell metabolism, from tissues and organs back to the lungs, where it is expelled.The typical lifespan of an RBC is 120 days and the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. A number of conditions can affect either the production of new RBCs by the bone marrow or the lifespan of those in circulation or that result in significant bleeding.The hematocrit reflects both the number of red blood cells and their volume (mean corpuscular volume or MCV). If the size of the RBCs decreases, so will the hematocrit and vice versa. In general, the hematocrit will rise when the number of red blood cells increases and the hematocrit will fall to less than normal when there is a drop in production of RBCs by the bone marrow, an increase in the destruction of RBCs, or if blood is lost due to bleeding. If the bone marrow is not able to produce new RBCs fast enough, then the overall number of RBCs and hematocrit will drop, resulting in anemia.In anemia, the body does not have the capacity to deliver enough oxygen to tissues and organs, causing fatigue and weakness. In polycythemia, too many RBCs are produced (resulting in increased hematocrit) and the blood can become thickened, causing sluggish blood flow and related problems.