Question

Rebreathing from a closed bag results in arterial hypercapnia (raised partial pressure of carbon dioxide), which stimulates respiration. Briefly explain this neural mechanism.

Answer 1

Normal arterial pco2 is 40 mmHg. Hypercapnia occurs due to rebreathing in a bag. An increase in arterial pco2 causes a prompt increase in the pulmonary ventilation resulting in carbon dioxide washout and a near restoration of arterial pco2 to normal level. There exists a linear relation between increase in arterial pco2 and increase in pulmonary ventilation.

carbon dioxide increases pulmonary ventilation mainly by stimulating the central chemoreceptors.

Central chemoreceptors are the cells (neurons); that lie just beneath the ventral surface of the medulla oblongata and are therefore known as medullary receptors. The neurons forming central chemoreceptors project directly over to the respiratory centres which are located slightly deeper to the central chemoreceptors.

Carbon dioxide readily crosses the blood brain barrier, because it is a small, very soluble and uncharged molecule. In the CSF, carbon dioxide combines with water to form H2CO3 which decides into H+ and HCO3-. The increase in H+ concentration of CSF and interstitial fluid, stimulates the central chemoreceptors, whereas a decrease in H+ concentration inhibits the respiration.

Central chemoreceptors are not stimulated by hypoxia. Therefore the stimulation of Central chemoreceptors leads to an increase in rate and depth of respiration.