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

How do CO2 levels in the blood get detected by chemoreceptors?

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.

Bibliography
Tortora Grabowski (2000) Principles of Anatomy and Physiology (9th Edition) John Wiley & Sons, Inc.

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