Breathing runs automatically from centers in the brainstem, yet you can also control it on purpose. The main signal that drives each breath is the level of CO2 in the blood.
Brainstem centers
Groups of neurons in the medulla generate the basic rhythm of breathing, and centers in the pons smooth the switch between breathing in and out. Signals travel down the spinal cord and through the phrenic nerves (C3 to C5) to the diaphragm.
Central chemoreceptors
Sensors near the surface of the medulla respond to the pH of the cerebrospinal fluid, which follows blood CO2. A small rise in CO2 strongly increases the rate and depth of breathing.
Peripheral chemoreceptors
The carotid and aortic bodies sense low oxygen, mainly when PaO2 falls below about 60 mmHg, and also react to acid in the blood. In some people with chronic CO2 retention, low oxygen becomes a larger part of the drive to breathe.
What depresses breathing
Opioids, sedatives, alcohol and brainstem injury blunt the response to CO2, so breathing becomes slow and shallow. Rising pressure inside the skull can cause irregular breathing patterns as the brainstem is compressed.
At the bedside
After giving opioids, check respiratory rate, depth and level of sedation, not just SpO2, because CO2 can rise while oxygen still looks fine on supplemental oxygen. Capnography shows rising CO2 early, and naloxone reverses opioid-induced respiratory depression.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).