Holding your breath

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Does holding your breath cause a build up of carbon dioxide in the body?

Yes, holding your breath will cause the carbon dioxide levels in your blood to quickly rise. This will cause your blood to become more acidic (reduce it's pH). The body's respiratory drive goes off blood pH, as soon as the blood become slightly acidotic (for whatever reason) respiratory rate and depth increase.

As a conscious person you can only hold your breath so long before you're forced to draw a breath. In unconscious people the drive is still there but the airway may obstruct causing oxygen levels to fall, resulting in brain damage then death.

I've always been quietly amazed (but concerned!) with free-divers for this reason.

Yes it's amazing what the body can be trained to do.

Underwater divers have other issues to contend with along with the whole oxygen/carbon dioxide/bicarb situation. Increased pressures underwater reduces thoracic capacity and therefore haemodynamics. The solubility of gasses particularly nitrogen is altered the deeper one goes. Free diving certainly isn't something for the faint of heart!