Hypoxia will be induced by on each subject, as levels of oxyhemoglobin saturation (between 70% and 100%) are achieved by breathing mixtures of nitrogen, room air and carbon dioxide. Inspired O2 concentration will be adjusted breath-by-breath using a computed saturation, based on end-tidal PO2 and PCO2, as sampled by a mass spectrometer. Predicted levels of oxyhemoglobin saturations will be attained and held stable. At a minimum of sixty seconds into each plateau, a 0.5cc waste blood draw through the arterial line, followed by the first 1.0cc sample blood draw. After approximately thirty additional seconds, the second 1.0cc sample blood draw will be taken. The samples will immediately be analyzed by a Radiometer OSM-3 multi-wavelength co-oximeter and recorded. The SpO2 data from the oximeters will be collected via a laptop computer. Concurrent with the end of each blood draw, a marker will be generated on the laptop computers to identify the event.