 The respiratory centres then determine the average rate of ventilation of the alveoli of the lungs, to keep the partial pressure carbon dioxide in the arterial blood constant.The respiratory center does so via motor neurons which activate the muscles of respiration (in particular the diaphragm).A rise in the partial pressure of carbon dioxide in the arterial blood plasma above 5.3 kPa (40 mmHg) reflexly causes an increase in the rate and depth of breathing.