Changes in the mechanism of breathing that occur as a result of interventions on the outside or on the body-world boundary. Plots show classic results in respiratory physiology, in which the frequency of impulses of chemoreceptor centers varies as a function of the proportions of oxygen (O2) and carbon dioxide (CO2) in the air external to the organism. The manipulation of air concentrations of O2 and CO2 changes the arterial pressure (Pa) of these gases, which is the variable shown in the plots. Data are from anesthetized cats. From \citet{Biscoe_1970}