The CPP is defined as the difference between the mean arterial pressure (MAP), which is the weighted average of the systolic and diastolic pressure, and the intracranial pressure (ICP), which is the pressure of the cerebrospinal fluid (CSF) in the subarachnoid space. The normal range for resting MAP is 70 to 100 mmHg and for ICP it is 5 to 15 mmHg. From Eq. (1), it is apparent that changes in perfusion pressure, changes in vascular radius (i.e., vasodilation and vasoconstriction), and changes in blood viscosity all affect the CBF. Changes in perfusion pressure may occur under normal conditions, e.g., during a change in posture or exercise, or they may result from the administration of drugs or from pathological conditions such as subarachnoid hemorrhage (SAH), traumatic brain injury (TBI), and stroke. Blood viscosity is directly related to hematocrit and the concentration of hemoglobin in blood. While lower hematocrit decreases viscosity, thus increasing CBF according to Eq. (1), it also reduces the oxygen-carrying capacity of blood. The effect of the vascular radius on CBF is of particular interest because it is responsible for the modulation and regulation of CBF, which is highly sensitive to as indicated by the fourth power of in Eq. (1), and we consider it next.