Hematocrit and Blood Plasma

Blood is composed of near 55% blood plasma and near 45% blood cells.  Blood plasma is composed mostly of water but contains solutes like sodium, potassium and proteins.  The majority of blood cells are red blood cells which are important in the transportation of oxygen from the lungs to other parts of the body. The term hematocrit refers to the ratio of the blood cells and other solid particles to the total amount of blood.  For example if I had 100ml of blood and 47ml of that was composed of blood cells, my hematocrit would be 47%.  The remaining 53% would be blood plasma. 

Through centrifugation it is possible to separate the red cells from the plasma and perform this calculation.  We use hematocrit to monitor the health of a person's blood, their oxygen carrying capacity, and to monitor changes in hydration state.  The figure above shows how hematocrit increases with altitude exposure in cross-country skiers.  Most people make the mistake of thinking that red blood cell production causes this change in hematocrit when it is really a loss of blood plasma commonly seen with altitude exposure.