<P> Central chemoreceptors of the central nervous system, located on the ventrolateral medullary surface in the vicinity of the exit of the 9th and 10th cranial nerves, are sensitive to the pH of their environment . </P> <P> These act to detect the changes in pH of nearby cerebrospinal fluid (CSF) that are indicative of altered oxygen or carbon dioxide concentrations available to brain tissues . An increase in carbon dioxide causes tension of the arteries, often resulting from decreased CO output (hypercapnia), indirectly causes the blood to become more acidic; the cerebrospinal fluid pH is closely comparable to plasma, as carbon dioxide easily diffuses across the blood--brain barrier . </P>

The ventilatory central chemoreceptors respond to changes in the