Most of the regulation of respiration is handled by the medullary respiratory center, a section of the brain stem found in the medulla oblongata and the pons. The cerebral cortex can also override the brain stem and assert voluntary control over breathing, which is why we can hold our breath or intentionally breath rapidly.
Unless we're actively trying to control our breathing, respiration is handled almost entirely by the medullar respiratory center.
Homeostasis is maintained by a negative feedback loop; as sensor input comes in from the vagus and glossopharyngeal nerves about the status of the lungs and the concentration of oxygen and CO2 in the blood, the dorsal respiratory group adjusts the rate at which it sends rhythmic impulses through the phrenic nerves as well as the cervical, thoracic, and lumbar motor nerves, which cause the muscles of the diaphragm and intercostal muscles to trigger inhalation. The DRG doesn't seem to control exhalation directly; other nerve groups essentially trigger exhalation on the rhythm that the DRG sets. If oxygen gets too high, breathing is slowed; if oxygen gets too low, breathing is sped up.
This is the normal rhythm; if the body is placed under stress and requires more oxygen than normal (e.g. exercising, fight-or-flight response), then the ventral respiratory group takes over and triggers a faster breathing rhythm. There is a similar negative feedback loop to prevent breathing from getting too fast.
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