CO2 is the main dial
Ask most people what drives breathing and they will say oxygen. In a healthy person, the real answer is carbon dioxide. The brainstem watches the level of CO2 in the blood — the PaCO2 — moment by moment, and adjusts breathing to hold it almost perfectly steady. A tiny rise in CO2 produces a large increase in breathing; a tiny fall slows it down.
The reason is subtle but elegant. CO2 itself is not what the sensors detect. CO2 dissolves and reacts with water to form acid, so a rising CO2 makes the fluid around the sensors more acidic — it lowers the pH. The body is really defending its acidity, and breathing is its fastest tool for doing so. This is the key link between *breathing* and *acid–base balance* that the rest of this track builds on.
Two kinds of chemoreceptor
The body has two sensor systems. The central chemoreceptors sit in the medulla itself, bathed in the fluid around the brain. They respond to the pH change caused by CO2 and account for the great majority of the moment-to-moment CO2 drive. The peripheral chemoreceptors live in the carotid bodies at the fork of the neck arteries (and in the aorta). They sense CO2 and pH too, but they are uniquely able to detect a fall in oxygen.
Oxygen is the backup alarm
Oxygen only becomes a strong stimulus when PaO2 drops well below normal — roughly below 60 mmHg. Above that, the haemoglobin stays nearly fully loaded, so modest swings in oxygen barely move the carotid bodies. This is the hypoxic drive: a backup that takes over only when oxygen is genuinely scarce, such as at high altitude or in severe lung disease.
- Normal CO2 and oxygen → central sensors run the show, breathing is steady.
- CO2 rises a little → central sensors detect the acid shift → breathing speeds up to blow CO2 off.
- Oxygen falls below ~60 mmHg → carotid bodies fire hard → the hypoxic drive adds to breathing.