Chemoreceptors

Chemoreceptors detect altered blood gas levels, and so send signals to the CNS to rectify this problem by altering the breathing. The relevant parts of the brain deal with this issue, particularly the medulla and the pons. The medulla oblongata has a major role to play in the regulation of breathing, as it does in the controlling many autonomic body functions – functions not under our conscious control (unlike many of the others, ventilation can be consciously controlled by the cerebral cortex, but this can be overridden). The medulla possesses the respiratory centre of the brain, and upon receiving the signals from chemoreceptors, it reacts by sending a signal to the respiratory muscles, altering their action to compensate for the blood gas

The CO2 in the blood is transported largely as bicarbonate (HCO3−) ions, by conversion first to carbonic acid (H2CO3), by the enzyme carbonic anhydrase, and then by disassociation of this weak acid to H+ and HCO3−. Build-up of CO2 therefore causes an equivalent build-up of the disassociated hydrogen ions, which, by definition, decreases the pH of the blood. The pH sensors on the brain stem detect this fall in pH, which the respiratory centre compensates for by increasing the rate and depth of breathing. The consequence is that the PCO2 does not change from rest going into exercise. During very short-term bouts of intense exercise the release of lactic acid into the blood by the exercising muscles causes a fall in the blood plasma pH, independently of the rise in the PCO2, and this will stimulate pulmonary ventilation sufficiently to keep the blood pH constant at the expense of a lowered

Respiration is regulated by groups of chemoreceptors:

• Central & peripheral chemoreceptors (pO2 & pCO2 (H+)) – Central chemoreceptors monitor blood pH and peripheral maintain relatively constant arterial pO2 and arterial pCO2

• Pulmonary stretch receptors (inflation &