Abstract: The invention comprises methods for noninvasively monitoring physiological characteristics of a patient's blood. Determinations of blood constituent concentrations may be made by comparing absorbance of radiation at varying parameters, such as path length and blood pressure. Preferably, changes in pressure are effected by changing the height of the probes relative to the patient's heart. Determinations of blood pH may be made by comparing absorbance of the blood at different wavelengths. The temperature of the blood, and thus of the patient's core, may also be accurately determined. Further, cardiac output characteristics and blood pressures may be noninvasively determined using the methods of the invention.