What happens to your body when you hold your breath?

When you hold your breath, your body immediately stops taking in oxygen and releasing carbon dioxide. Carbon dioxide builds up in your blood, which is actually what triggers the urge to breathe, not the lack of oxygen. Your heart rate initially slows slightly due to the diving reflex, and your spleen contracts to release extra oxygen-carrying red blood cells into circulation. As time goes on, oxygen levels drop while CO2 continues rising, causing increasing discomfort and the overwhelming urge to breathe. Your blood vessels constrict and blood pressure rises. If you hold your breath long enough, you can lose consciousness as the brain, which is extremely sensitive to oxygen deprivation, begins to shut down non-essential functions to protect itself.