Answer:
Automatically start to breathe when the CO₂ levels in the blood reach a high enough value
Explanation:
When a baby holds his breath for long, the accumulatio of carbon dioxide occurs in tissues and blood and as a result the partial pressure of the carbon dioxide increases and that of oxygen decreases in the blood. The body is sensitive towards the levels of carbon dioxide and thus if it increases too much above the normal amount it leads to irritation in the lungs and lungs muscles are triggered form the respiratory part of the brain to take breath.
