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Central chemoreceptors are most sensitive to: (AIIMS 2011, AIPG 2009)

A Increase in CO2 Tension
B Decrease in CO2 Tension

C Low O2 Tension
D Increase in H+

Ans. A Increase in CO2 Tension.(Ref: Ganong - Review of Medical Physiology 22nd Ed)

Central chemoreceptors :

I. The chemoreceptors that mediate the hyperventilation produced by increases in arterial PCO2 after the carotid and aortic bodies are denervated are located in the medulla oblongata and consequently are called medullary chemoreceptors.

II. They are separate from the dorsal and ventral respiratory neurons and are located on the ventral surface of the medulla.

III. The chemoreceptors monitor the H+ concentration of CSF, including the brain interstitial fluid.

IV. CO2 readily penetrates membranes, including the blood–brain barrier, whereas H+ and HCO3 penetrate slowly.

V. The CO2 that enters the brain and CSF is promptly hydrated.

VI. The H2CO3 dissociates, so that the local H+ concentration rises. The H+ concentration in brain interstitial fluid parallels the arterial PCO2.

VII. Experimentally produced changes in the PCO2 of CSF have minor, variable effects on respiration as long as the H+ concentration is held constant, but any increase in spinal fluid H+ concentration stimulates respiration.

VIII. The magnitude of the stimulation is proportionate to the rise in H+ concentration.

IX. Thus, the effects of CO2 on respiration are mainly due to its movement into the CSF and brain interstitial fluid, where it increases the H+ concentration and stimulates receptors sensitive to H+.

Regulation of respiratory activity

a. A rise in the PCO2 or H+ concentration of arterial blood or a drop in its PO2 increases the level of respiratory neuron activity in the medulla, and changes in the opposite direction have a slight inhibitory effect.

b. The effects of variations in blood chemistry on ventilation are mediated via respiratory chemoreceptors—the carotid and aortic bodies and collections of cells in the medulla and elsewhere that are sensitive to changes in the chemistry of the blood.

c. They initiate impulses that stimulate the respiratory center.

d. Superimposed on this basic chemical control of respiration, other afferents provide nonchemical controls that affect breathing in particular situations

Chemical control of breathing

a. The chemical regulatory mechanisms adjust ventilation in such a way that the alveolar PCO2 is normally held constant, the effects of excess H+ in the blood are combated, and the PO2 is raised when it falls to a potentially dangerous level.

b. The respiratory minute volume is proportionate to the metabolic rate, but the link between metabolism and ventilation is CO2, not O2.

c. The receptors in the carotid and aortic bodies are stimulated by a rise in the PCO2 or H+ concentration of arterial blood or a decline in its PO2.

d. After denervation of the carotid chemoreceptors, the response to a drop in PO2 is abolished; the predominant effect of hypoxia after denervation of the carotid bodies is a direct depression of the respiratory center.

e. The response to changes in arterial blood H+ concentration in the pH 7.3-7.5 range is also abolished, although larger changes exert some effect.

The response to changes in arterial PCO2, on the other hand, is affected only slightly; it is reduced no more than 30-35%.