Final answer:
The nervous system, which includes the brainstem's respiratory centers, controls breathing by monitoring blood carbon dioxide levels and maintaining blood pH, assisted by the chemical system involving bicarbonate ions.
Step-by-step explanation:
The body has two main systems that control breathing: a/an nervous system and a chemical system. The correct answer is b. Nervous.
The nervous system controls various functions in the body, including the regulation of breathing. It does so by monitoring the levels of carbon dioxide in the blood through respiratory centers located in the medulla and pons of the brainstem. These centers send signals to the respiratory muscles like the diaphragm and intercostal muscles to adjust the breathing rate. This biological feedback mechanism is crucial for maintaining blood pH levels within a narrow range. Additionally, the chemical system in our body, notably involving bicarbonate ions, also plays a role in managing the respiratory process, particularly through the renal system which affects the overall acid-base balance in the blood.