The act of breathing raises the blood oxygen level, lowers the blood carbon dioxide concentration, and raises the blood ph. according to the principles of negative feedback, sensors that regulate breathing should respond to

Levels of carbon dioxide and oxygen in the bloodstream.  This is detected by the sensors in the medulla oblongata in the brain. In response, signals are sent to the muscles of the heart to beat faster or slower and those of the diaphragm to increase or decrease breathing rate.