PaCO2

Think of carbon dioxide as the exhaust your cells produce while burning fuel. Breathing is how the body vents that exhaust. PaCO2 is the pressure of carbon dioxide dissolved in arterial blood, and it serves as a direct, sensitive gauge of whether you are venting enough — that is, whether your breathing is keeping pace with the carbon dioxide being made.

What makes PaCO2 so useful is its tight, almost see-saw relationship with alveolar ventilation, the fresh air actually reaching the gas-exchanging regions each minute. Breathe more deeply or more often than needed, and you blow off carbon dioxide faster than it is produced, so PaCO2 falls; breathe too little, and it accumulates and PaCO2 rises. Because the body adjusts breathing minute by minute around a target, the value in health sits in a narrow band of roughly 35 to 45 mmHg.

PaCO2 therefore reads breathing adequacy more faithfully than oxygen does. A high value (hypercapnia) signals that alveolar ventilation is too low — exhausted respiratory muscles, severe airflow obstruction, a depressed breathing drive; a low value (with rapid breathing) points to overbreathing. It is also a pillar of acid–base interpretation, since carbon dioxide forms acid in the blood, so PaCO2 directly shapes the blood's pH.