Blood doping is the act of artificially increasing the number of red blood cells in a person’s blood for the purpose of improving athletic performance. This works based on the fact that red blood cells are responsible for carrying oxygen to muscles, and thus more red blood cells can provide more endurance against muscle fatigue. This was originally done by transfusions of red blood cells, either from another person or by collecting and storing a person’s own red blood cells to be used later. In the past couple of decades, a new type of blood doping has come about. This is based on the hormone erythropoietin, which stimulates the body to produce red blood cells. Erythropoietin is produced artificially in mass quantities and is commonly used to treat anemia, but is also sometimes used by athletes for the purposes of blood doping.