  The fundamentals of near infrared spectroscopy (NIRS) are reviewed. Among the
major factors controlling the cerebral blood flow (CBF), the effect of PaCO2 is
peculiar in that it violates autoregulatory CBF mechanisms and allows to
explore the full range of the CBF. A simple physical model, with a four
parameter formula, relating the CBF to PaCO2 is presented. It can be used to
transform the fits of one animal to the fits of another one. It enable the use
of rats data as monkeys data simply by rescaling the PaCO2 values and the CBF
data. Controlled breathing can change the PaCO2. Experiments on human subjects
relating the PaCO2 to rSO2, measured with brain oximeters, are presented. A
simple model relating the mean blood pressure to CBF is worked out.
