Chemical Regulation of Breathing

Chemical regulation of breathing is part of the involuntary (autonomic) control of breathing. This mechanism is part of the bodies homeostasis to maintain an appropriate balance and concentration of CO2, O2, HCO2- and pH. There are two types of chemoreceptors that react strongly to a change in the blood gases: central and peripheral chemoreceptors.

Chemoreceptors

 * detects arterial pCO2 and pH
 * increase in alveolar pCO2 causes an increase in breathing rate
 * pO2 normally stimulates ventilation when it falls below ~8kPa (60 mmHg). When the fall is accompanied by an increase in PCO2, the increase in ventilation is greater

Central Chemoreceptors
Central chemoreceptors are located on the ventrolateral surface of the medulla oblongata. They respond indirectly to blood pCO2 but not to pO2. CO2 diffuses across the blood-brain barrier from blood to cerebral spinal fluid (CSF) while H+ and HCO3- are unable to.

As the blood CO2 readily passes the blood-brain barrier into the CSF it will react with H2O to make H2CO3, that will split into HCO3- and H+.

CO2 + H2O → HCO3- + H+

An increase in H+ concentration will directly stimulate the chemoreceptor neurons in the medulla oblongata. They will relay this information and cause an increase in ventilation which will lead to a decrease in CO2.

The central chemoreceptors are responsible for ~80% of the response to CO2 concentration.

Peripheral Chemoreceptors
Peripheral chemoreceptors are located in carotid and aortic bodies that have neuro-epithelial cells that contact with sensory nerve terminals. They respond to changes in pO2, pCO2 and pH. When they are stimulated, K+ channels close and Ca2+ channels open. This causes an increase in initiation of dopamine, impulses to respiratory center via the glossopharyngeal nerves (N IX) and an increase in ventilation.

The peripheral chemoreceptors are responsible for ~20% of the response to an increase in pCO2.