When you  hold your breath, which of the following gas changes in blood would first lead to the urge to breathe?

When you hold your breath, the ongoing accumulation of carbon dioxide in your cells, in your blood and lungs will eventually irritate and trigger impulses from the respiratory centre part of your brain. Rising levels of carbon dioxide signal the body to breathe and ensure our unconscious and autonomous respiration. The role of oxygen in the regulation of respiratory rhythm is quite insignificant.