<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> <P> However, a change in plasma pH alone will not stimulate central chemoreceptors as H+ are not able to diffuse across the blood--brain barrier into the CSF . Only CO levels affect this as it can diffuse across, reacting with H O to form carbonic acid and thus decrease pH . Central chemoreception remains, in this way, distinct from peripheral chemoreceptors . </P> <P> The central chemoreception system has also been shown experimentally to respond to hypercapnic hypoxia (elevated CO, decreased O) and aqueous sodium cyanide injection into the whole animal and in vitro slice preparation . These methods can be used to mimic some forms of hypoxic hypoxia and they are currently being studied including the detection of variation in arterial CO tension acting as a quick - response - system for short term (or emergency) regulation . </P> <P> This system utilizes a negative feedback system, therefore if the pH of the cerebral spinal fluid does not compare to an ideal "set" level, then the receptor will send an error signal to the effectors and appropriate action may be executed . </P>

What causes the central chemoreceptors to be stimulated