The technique on which near infrared spectroscopy relies is closely analogous to the technique of pulse oximetry. The main difference is the tissue being sampled. Pulse oximetry calculates the percentage of oxygenated hemoglobin in the arterial blood. NIRS calculates the changes in oxy- and deoxyhemoglobin (and optionally the percentage of oxygenated hemoglobin) in the tissue under investigation (capillaries), which contains both arterial and venous blood.