Brain blood flow autoregulation is abolished when CO2 levels persist abnormally high. Which statement best describes this?

• A. True
• B. False
• C. It is enhanced by low CO2
• D. It is unaffected by CO2

Answer: (A)

The main idea being tested is how CO2 levels influence cerebral autoregulation. CO2 is a powerful regulator of cerebral vessels: when CO2 is high, cerebral arterioles dilate, increasing blood flow. If CO2 remains abnormally elevated for a while, those vessels stay dilated and lose their ability to constrict in response to changes in blood pressure, so the brain’s normal autoregulatory mechanism is blunted or effectively abolished. That’s why the statement is true: persistent hypercapnia disrupts the normal, pressure-driven control of cerebral blood flow.

High CO2 shifts the baseline state toward vasodilation, which overwhelms the usual autoregulatory response. Conversely, very low CO2 causes vasoconstriction and lowers baseline flow, but it doesn’t make autoregulation work better beyond its normal range. So choices implying autoregulation is unaffected or that it’s enhanced by low CO2 aren’t correct.