Let us start with animal breathing. In this process, the animal actively exchanges components with the environment, specifically air volumes with different oxygen and carbon dioxide partial pressures. Neurons located in certain areas of the brain stem send a volley of activity that travels through the neurons' axons and reaches neurons in the phrenic nucleus, located in the spinal cord. Phrenic neurons, in turn, get activated by the incoming volley and send their impulses down the phrenic nerve, delivering acetylcholine onto the muscle cells that constitute the diaphragm. The acetylcholine-triggered activation of the diaphragm muscle causes it to contract, which expands the thoracic cavity and increases lung volume. This expansion decreases intra-alveolar pressure, drawing air from the organism's surroundings into the lungs. The diaphragm then relaxes, and the air is pushed from inside the lungs back to the exterior of the animal's body. Accompanying the volume exchange there is a substance exchange: inspired air is more enriched in oxygen than expired air, which in turn is more enriched in carbon dioxide. At the molecular level, we can conceive the mechanism as a continuous exchange of molecules. From an outside reservoir enriched in oxygen molecules, the organism draws oxygen inside and pushes out carbon dioxide. As long as the animal lives, this mechanism is operating. Let us now consider what happens when we intervene on the external side, lowering the oxygen concentration. This causes a relative increase in carbon dioxide concentration in the bloodstream, which in turn activates peripheral and central chemoreceptor neurons. The activation of the latter triggers an increase in drive to the diaphragm, resulting in stronger and more frequent breathing cycles. This phenomenon was used routinely by respiratory physiologists to elicit respiratory changes in anesthetized animals (See Figure). Something similar happens if we prevent the molecules from crossing the boundary, say by occluding the airway. This indicates that by manipulating external oxygen concentrations, or by preventing physical exchanges across the body-world boundary, we manipulate the internal mechanism.