Chemoreceptor

A molecular structure on the surface of a cell, sensitive to chemical substances, such as epinephrine, released by nerve cells.

CO₂ dissolves in blood, since it is lipid soluble, it can also diffuse easily across plasma membranes, including the blood-brain barrier. This means that CO₂ can diffuse in to and be detected in any part of the body. Carbonic anhydrase is present in cells, so with the addition of water (H₂O), carbonic acid (H₂CO₃) can form. Carbonic acid, like other strong acids, will dissociate rapidly, forming an equilibrium between the acid (H₂CO₃) and its corresponding ions (H⁺ and HCO₃^-). Therefore an increase in carbon dioxide levels will lead to an increase in hydrogen ions (higher pH); conversely, a decrease in CO₂ will lead to a fall in hydrogen ion concentration (lower pH).

Under normal physiological conditions, the PCO₂ (CO2 conc.) in arterial blood is 40 mm Hg. An increase of CO₂ concentration leads to a condition called hypercapnia, a decrease causes hypocapnia.

Normally pH buffers in the blood and cells will reduce the pH altering effects of varying CO₂ levels. In the cerebrospinal fluid however, there are fewer buffers than in blood and so the pH effect caused by CO₂ concentration can be much more keenly sensed. For this reason the central chemoreceptors are found in the medulla oblongata and will detect these fluctuations, stimulating the breathing centre. When respiration increases, the CO₂ concentration will fall and cerebrospinal fluid pH returns to normal, hence a negative feedback system governs your breathing.