Hypercapnia is the single greatest stimulant of the respiratory center which is capable of exciting it to increase both rate and amplitude of respiration. Lowering of pH of blood has direct excitatory response on the respiratory centre. Lack of oxygen has no direct action on respiratory centre and extreme oxygen lack depresses the center. Stimuli affecting respiratory center: Chemical control: CO2 ( CSF and brain interstitial fluid H+ concentration) Oxygen and H+ ( carotid and aoic bodies) Non chemical control: Vagal afferents from receptors in airways and lungs Afferents from the pons, hypothalamus and limbic system Afferents from proprioceptors Afferents from baroreceptors : aerial, atrial, ventricular, pulmonary Ref: A Concise Textbook Of Physiology By S. And Kutty, K.M. page 102, Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 36. Regulation of Respiration. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.