Question

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which triggers the central chemoreceptor to increase the rate and depth or breathing?
an increase in hydrogen ion concentration in the brains extracellular fluid in response to increasing carbon dioxide levels in the blood.
an increase in carbon dioxide concentration in the brains extracellular fluid in response to increasing carbon dioxide levels in the blood.
a decrease in carbon dioxide concentration in the brains extracellular fluid in response to decreasing carbon dioxide levels in the blood.

Explanation:

Central chemoreceptors are sensitive to changes in hydrogen ion concentration (\(H^+\)) in the brain's extracellular fluid. When blood carbon dioxide (\(CO_2\)) levels increase, \(CO_2\) diffuses into the cerebrospinal fluid (part of the extracellular fluid in the brain - related area). In the cerebrospinal fluid, \(CO_2\) reacts with water (\(H_2O\)) to form carbonic acid (\(H_2CO_3\)), which then dissociates into \(H^+\) and bicarbonate ions (\(HCO_3^-\)). The increase in \(H^+\) triggers the central chemoreceptors. An increase in \(CO_2\) concentration in the brain's extracellular fluid itself (without the conversion to \(H^+\) as described above) is not the direct trigger. A decrease in \(CO_2\) would lead to a decrease in \(H^+\) (opposite of the reaction when \(CO_2\) increases) and would not stimulate the central chemoreceptors to increase breathing rate and depth.

Answer:

An increase in hydrogen ion concentration in the brain's extracellular fluid in response to increasing carbon dioxide levels in the blood.