When a person holds his breath in for too long, the amount of carbon dioxide in his body begins to accumulate, according to The Science Creative Quarterly. He may start feeling a burning sensation in his lungs.
When the level of carbon dioxide becomes too high, painful contractions begin in the ribs and in the diaphragm. The pain is a signal that the person needs to exhale. The body and brain start to suffer from a lack of oxygen. The critical line refers to the moment when a person accumulates so much carbon dioxide in his body that he begins to feel pain. Hyperventilating is one way to delay the critical line moment, but hyperventilating is also dangerous, notes The Science Creative Quarterly. Hyperventilation can result in unconsciousness because it undermines the body's signals to breathe.
The effects of breath holding on the brain are not yet clear, according to The New York Times. Scientists from the University of Queensland conducted neuropsychological tests on free divers to examine the way their brains behaved compared to people who did not free dive. They found that the free divers' brains responded normally in visual, language and recall tests. A SPECT brain scan showed some abnormalities in five free divers' brains, but scientists were uncertain of the significance of the findings.