Science Puzzle
What Makes You Gasp for Breath?
Take a deep breath and hold it. For a while it's easy. Then a feeling builds, stronger and stronger, until you can't hold on any longer and you gasp for air. Most people manage somewhere between 30 seconds and a minute and a half.
What is your body actually reacting to?
The Answer
Carbon dioxide building up in your blood. The urge to breathe is driven mainly by carbon dioxide, not by a lack of oxygen.
Every cell in your body makes carbon dioxide as it uses food for energy. Normally your lungs get rid of it as fast as it's made. When you hold your breath, it builds up in your blood, which makes the blood slightly more acidic. Sensors in your brainstem, the part of the brain that runs your breathing, pick up the change and send stronger and stronger signals to breathe.
When you finally gasp after a minute, your blood still has plenty of oxygen in it. Oxygen sensors do exist, in your neck and chest, but they only take charge when oxygen falls a long way, for example high up a mountain.
Breathing very fast before swimming underwater is dangerous for exactly this reason. It flushes carbon dioxide out of your blood without adding much oxygen, so the warning comes much later. A swimmer can run out of oxygen and black out underwater before feeling any need to breathe. Divers call it shallow water blackout, and it has drowned strong swimmers.
The principle: Control of breathing. The urge to breathe comes mainly from carbon dioxide building up in the blood, which sensors in the brainstem detect; oxygen running low plays a much smaller part.