Which statement about brain blood flow autoregulation is true?

Which statement about brain blood flow autoregulation is true?

• A. It is abolished when abnormally high CO2 levels persist
• B. It is enhanced by high CO2 levels
• C. It is unaffected by CO2 levels
• D. It depends only on blood pressure

Answer: (A)

Brain blood flow autoregulation keeps cerebral blood flow constant across a range of arterial pressures by fine-tuning the diameter of cerebral vessels. Carbon dioxide is a powerful modulator of cerebral vessel tone: elevated CO2 (hypercapnia) causes vasodilation, partly by lowering pH inside smooth muscle cells, which reduces their constrictive response. When CO2 remains abnormally high, this vasodilation can overpower the usual myogenic and metabolic mechanisms that normally adjust resistance to keep blood flow steady. As a result, the autoregulatory ability is diminished or abolished, and cerebral blood flow becomes more directly influenced by pressure and CO2 levels rather than being kept constant by the vessels. This is why persistent hypercapnia disrupts autoregulation. The other statements don’t fit because autoregulation is not enhanced by high CO2, it is not unaffected by CO2, and it does not depend solely on blood pressure.

Brain blood flow autoregulation keeps cerebral blood flow constant across a range of arterial pressures by fine-tuning the diameter of cerebral vessels. Carbon dioxide is a powerful modulator of cerebral vessel tone: elevated CO2 (hypercapnia) causes vasodilation, partly by lowering pH inside smooth muscle cells, which reduces their constrictive response. When CO2 remains abnormally high, this vasodilation can overpower the usual myogenic and metabolic mechanisms that normally adjust resistance to keep blood flow steady. As a result, the autoregulatory ability is diminished or abolished, and cerebral blood flow becomes more directly influenced by pressure and CO2 levels rather than being kept constant by the vessels. This is why persistent hypercapnia disrupts autoregulation. The other statements don’t fit because autoregulation is not enhanced by high CO2, it is not unaffected by CO2, and it does not depend solely on blood pressure.