Question

Describe how the central chemoreceptors act to regulate ventilation. Be sure to describe the integrating centers and sensors involved, their locations, and explain their roles in determining ventilation rate. Be sure to include any chemical equations (and enzymes!) that are necessary to explain ventilation regulation and explain how the mechanisms work. Then explain how the system responds when arterial blood pH is elevated, and explain how the response corrects arterial CO2. Explain how the system responds when arterial blood pH is decreased, and explain how the response corrects arterial CO2.

Answer 1

The respiratory activity is regulated by the peripheral chemoreceptors and central chemoreceptors. Peripheral chemoreceptors includes the aortic and carotid bodies. Central chemoreceptors include the medullary neurons.

CENTRAL CHEMORECEPTORS

The central chemoreceptors are located on the ventral surface of medulla oblangata. The central chemoreceptors work by sensing the changes in blood pH. They indirectly sense the pCO2 changes based on the H+ concentration of the CSF.

i.e, when CO2 concentartion rises in blood, the CO2 readily crosses the blood brain barrier and reaches the CSF. In the CSF, CO2 combines with water to form Carbonic acid which dissociates into H+ and HCO3-.

The H+ is now sensed by the chemoreceptors sensitive to H+ concentration. Upon stimulation the central chemoreceptors work by increasing the rate and depth of breathing, causing the CO2 to be washed out.

To summarise,

Integrating centre : Medulla oblangata

Sensors : Rostral and caudal chemoreceptors sensitive to H+ concentration

Location : Ventral surface of medulla oblangata.

Stimulation : Leads to increase in respiratory rate and depth of respiration.

-------------------------------------------------------------------------------------------------------------------

Qn : How the system responds when arterial blood pH is elevated, and explain how the response corrects arterial CO2 ?

When there is an elevation of the arterial blood PH as in metabolic or respiratory alkalosis, H+ concentration is low and hence, the stimulatory effect of H+ on the peripheral and central chemoreceptors are abolished. As a result, the respiration is depressed leading to a retention of CO2, the arterial CO2 raises, raising the H+ concentration towards normal.

-------------------------------------------------------------------------------------------------------------------

Qn : How the system responds when arterial blood pH is decreased, and explain how the response corrects arterial CO2 ?

When there is a decrease in the arterial blood PH as in metabolic or respiratory acidosis, H+ concentration is high and hence, H+ ions stimulate the peripheral and central chemoreceptors. As a result, there is stimulation of respiration resulting in hyperventillation and CO2 washout. So, the arterial CO2 falls, decreasing the H+ concentration towards normal.

NB: A primary change in plasma pH is sensed by the peripheral chemoreceptors than the central chemoreceptors, as H+ ions are not able to diffuse across the blood brain barrier at physiological range of PH. However, larger changes in PH can affect both.