Hematocrit

Human blood is made up of erythrocytes or red blood cells, leukocytes or white blood cells, and platelets suspended in a fluid called plasma. Each of the component of blood performs a specific function. The packed cell volume or hematocrit is a ratio of the volume occupied by the RBCs to the total volume occupied by all the blood components or whole blood.

The RBCs transport inhaled oxygen from the lungs to all the cells of the body, and also a small amount of carbon dioxide from the cells to the lungs to be exhaled. The majority of carbon dioxide is transported in solution in plasma as bicarbonate ions. They contain a protein called hemoglobin which binds to oxygen for transport.

RBCs are produced in the erythropoietic cells of the bone marrow in response to the hormone Erythropoietin secreted by the kidneys when oxygen saturation of blood is detected to be low (hypoxia). The average lifespan of RBCs in circulation is 120 days. Hence, the bone marrows continuously produce RBCs to maintain a steady concentration in blood. The Packed Cell Volume Test depends on the count as well as the average size of the RBCs (Mean Corpuscular Volume or MCV). Higher than normal amount of RBCs produced by the bone marrow can cause the hematocrit to increase, leading to increased blood density and slow blood flow. Lower than normal hematocrit can be caused by low production of RBCs, reduced lifespan of RBC in circulation, or excessive bleeding, leading to reduced amount of oxygen reaching the cells.