Medical devices, namely, cell oxygen monitors for determining, monitoring, and indicating a level of blood oxygenation, a level of hemoglobin saturation, a level of cell oxygenation, or a level of myoglobin oxygen saturation, in blood, tissue, namely, muscle tissue, or cells, namely, red blood cells and muscle cells; sensors for cell oxygen monitors, namely, optical sensors that attach to a part of the body, namely, the hand, and that are replaceable, namely, single use; pulse oximeters