<prompt> Central Chemoreceptors are most sensitive to which of the following changes in blood?p CO2p CO2Hp O2  <response>: Central chemoreceptors are stimulated mainly by the H concentration caused by increased pCO2 levels. Since H cannot easily cross the blood brain barrier, increase in the blood H concentration have little effect in stimulating central chemo receptors. Increase in blood p CO2 causes CO2 to diffuse the blood brain barrier with ease where it reacts with water to form carbonic acid. This carbonic acid in turn dissociates into H and bicarbonate ions. This H ions then have a potent stimulatory effect on the central chemo receptors. This question asks about the changes in blood that stimulates the central chemo receptors. So the correct answer is p CO2 levels as H concentration in blood has little effect in stimulating the central chemo receptors. Ref: Textbook Of Medical Physiology By Khurana, Page 458 