The carotid body, aortic body and medulla contain
chemoreceptors, which detect changes in CO2 level (H+ ion
concentration) of the blood.
These receptor cells send nervous impulses to the
respiratory centre in the brain.
The higher the CO2 concentration
(and therefore [H+]) the more frequently they send impulses to the
The respiratory centre in the brain contains two sections an
inspiratory centre and an expiratory centre.
Nerves impulses are sent from the inspiratory centre of the
respiratory centre in the medulla to the diaphragm and intercostals.
These impulses (from respiratory centre) cause muscle to
Contraction of intercostal muscles and contraction of
diaphragm causes ribs to move up and out and the diaphragm to move down which
increases volume of thorax/lungs thereby decreasing pressure (causing
Stretch receptors in the intercostal muscles are stimulated by
increase in size of thorax/lungs.
These receptors send impulses to the expiratory centre which
inhibits the inspiratory centre therefore inspiratory impulses are inhibited so
contraction of the diaphragm and intercostals ceases and elastic recoil of the
lung tissue causes expiration.