Results of analyses and experiments concerning metrological problems with pulse oximetry principles that are utilized in noninvasive measurements of oxygen content in arterial blood, have been presented. Phenomena which limit processing reliability of the quantity to be sensed, i.e., the oxygen saturation SaO2%, are considered. Specific groups of affecting factors are distinguished in an approach of their contribution to the measurement uncertainty components. These factors are connected with properties of a living object to be measured, a way the optoelectronic sensor is applied, and external interferences, respectively. In comparative experiments, chosen models of pulse oximeters and the testing device especially made for the author's research, were used. The achieved results are being utilized in works on a design of the novel computer-aided system based on the pulse oximetry idea.