In the intact brain, cerebral blood flow is tightly coupled to the metabolic requirements of tissue (metabolic regulation) but the flow rate remains essentially constant despite alterations in blood pressure (autoregulation). An important requirement for metabolic regulation is the CO2 reactivity of cerebral vessels, which can be tested by the application of carbonic anhydrase inhibitors or CO2 ventilation. Under physiological conditions, blood flow doubles when CO2 rises by about 30 mmHg and is reduced by approximately 35% when CO2 falls to 25 mmHg. The vascular response to CO2 depends mainly on changes in extracellular pH, but it is also modulated by other factors such as prostanoids, nitric oxide and neurogenic influences.
Autoregulation of cerebral blood flow is the remarkable capacity of the vascular system to adjust its resistance in such a way that blood flow is kept constant over a wide range of cerebral perfusion pressures (80-150 mmHg). The range of autoregulation is shifted to the right, i.e. to higher values, in patients with hypertension and to the left during hypercarbia. The myogenic theory of autoregulation suggests that changes in vessel diameter are caused by the direct effect of blood pressure variations on the myogenic tone of vessel walls. Other influences are mediated by metabolic and neurogenic factors but these may be secondary and are not of great significance.
Metabolic regulation: cerebral blood flow is coupled to metabolic requirements of tissue by a vascular response to CO2.
Autoregulation: cerebral blood flow is kept constant over a wide range of cerebral perfusion pressures.
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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...