It is high levels of carbon dioxide, not low levels of oxygen, that stimulate the need to breathe. The beginning diver is very sensitive to carbon dioxide levels. These levels build even with a breath-hold of 15 seconds, causing the lungs to feel on fire. The trained diver has blown off massive amounts of carbon dioxide with hyperventilation, thus outsmarting the brain's breathing center. Normally metabolizing body tissues, producing carbon dioxide at a regular rate, do not replace enough carbon dioxide to stimulate this breathing center until the body is seriously short of oxygen.
Hyperventilation causes some central nervous system changes as well.
Practiced to excess, it causes decreased cerebral blood flow, dizziness and muscle cramping in the arms and legs. But moderate degrees of hyperventilation can cause a state of euphoria and well-being. Pressure changes in the freediver's descent-ascent cycle conspire to rob him of oxygen as he nears the surface by the mechanism of partial pressures. Gas levels, namely oxygen and carbon dioxide, are continuously balancing themselves in the body. Gases balance between the lungs and body tissues.