Thread: effect of hypercapnia

Goodmorning everyone,
I can't understand why hypercapnia has a vasoconstrictor effect, considering that hypercapnia means that there is a higher level of Co2 in the blood and that Co2 is a vasodilator, shouldn't hypercapnia has a vasodilator effect ?
thank you for your time

Originally Posted by davprest

Goodmorning everyone,
I can't understand why hypercapnia has a vasoconstrictor effect, considering that hypercapnia means that there is a higher level of Co2 in the blood and that Co2 is a vasodilator, shouldn't hypercapnia has a vasodilator effect ?
thank you for your time

Yes, hypercapnia causes cerebral vasodilatation, that's why we keep CO2 on lower side to reduce ICP as low CO2 causes cerebral vasoconctriction and lowers ICP.

Cerebral blood flow is determined by a number of factors, such as viscosity of blood, how dilated blood vessels are, and the net pressure of the flow of blood into the brain, known as cerebral perfusion pressure, which is determined by the body's blood pressure. Cerebral blood vessels are able to change the flow of blood through them by altering their diameters in a process called autoregulation; they constrict when systemic blood pressure is raised and dilate when it is lowered. Arterioles also constrict and dilate in response to different chemical concentrations. For example, they dilate in response to higher levels of carbon dioxide in the blood and constrict to lower levels of carbon dioxide.
Acute changes in PCO2 between 20 and 60 mmhg have been shown to change CBF 1 to 2 ml/min/100 gm of brain per 1 torr change in PCO2.
However, during sustained alteration in PCO2 , the CBF and absolute carbon dioxide tension often fail to correlate closely. This has led to the hypothesis that alteration of the pH of the brain extracellular space mediates the cerebral vascular response to carbon dioxide and hence HV, and that brain interstitial fluid pH is a major regulator of CBF.