The rate and depth of breathing are regulated mainly by the level of

Explanation

Chemoreceptors in the medulla and in the great arteries respond to the fall in pH that a rise in carbon dioxide produces, and the respiratory centre then increases the rate and depth of breathing until the excess is blown off. Oxygen level acts only as a weak backup stimulus and matters mainly at high altitude. This is why holding the breath becomes unbearable from the build up of carbon dioxide, not from the lack of oxygen.