When you hold your breath for half a minute the carbon dioxide concentration kn the blood?

When you hold your breath for half a minute, carbon dioxide (CO2) levels in the blood begin to rise because it is still being produced by cellular metabolism but not being expelled through breathing. This increase in CO2 leads to a corresponding decrease in blood pH, making the blood more acidic. The body detects this rise in CO2 and the drop in oxygen levels, eventually triggering the urge to breathe in order to restore normal gas exchange.