The respiratory center in the brain is stimulated primarily by:

 # The respiratory center in the brain is stimulated primarily by:

A. High concentration of carbon dioxide in the blood

B. High concentration of oxygen in the blood

C. Low concentration of oxygen in the blood

D. Low concentration of carbon dioxide in the blood

The correct answer is A. High concentration of carbon dioxide in the blood.

The respiratory center in the brain, located in the medulla oblongata and pons, is primarily stimulated by elevated levels of carbon dioxide (CO₂) in the blood. This is detected through changes in blood pH (caused by CO₂ forming carbonic acid), which triggers the respiratory center to increase breathing rate and depth to expel excess CO₂ and restore pH balance. While low oxygen levels (C) can also stimulate breathing, this is a secondary mechanism primarily mediated by peripheral chemoreceptors, not the primary driver. High oxygen (B) or low CO₂ (D) typically do not stimulate the respiratory center.