Cerebral Autoregulation is the tendency of cerebral blood flow to remain constant over a broad range of arterial blood pressures. The concept was introduced by Lassen in 1959. Cerebral auto regulation was thought to be an exclusive property of the brain but later other organs (e.g. the kidney) were shown to display similar behavior in response to fluctuations in blood pressure.
Both the brain as well as the kidney are organs quite capable of influencing systemic blood pressure by humoral and for the brain neural pathways.
Where cerebral blood flow remains constant over a broad range of arterial blood pressures, it is highly sensitive to changes in pCO2 (or pH). Cerebral blood flow steeply decreases during hyperventilation and increases during CO2-retention. This is called the metabolic response and is present loco-regionally within brain tissue. For instance, this response makes it possible to generate functional images by BOLD-MRI: brain regions that are active during a certain task will show vasodilatation causing local hyperemia whereas inactive brain regions decrease their blood supply. Again, metabolic coupling is not exclusive to the brain but most likely a property more widespread in body tissues.
The cardiovascular model combines metabolic coupling and cerebral auto regulation to a single concept: at low blood pressures the resulting hypoperfusion causes an increase in regional CO2 leading tot arteriolar vasodilatation and at high blood pressures hyper perfusion leads to a decrease in regional CO2 an thus vasoconstriction. The above graph was obtained by varying the blood pressure through variation in venous capacitance.