Final answer:
The buildup of carbon dioxide in the blood is the main trigger for resuming breathing after holding your breath, as involuntary breathing control mechanisms in the brain stem intervene to maintain gas exchange and homeostasis.
Step-by-step explanation:
The most important cue that prompts you to breath again after holding your breath is the buildup of carbon dioxide in your blood. When you hold your breath, oxygen levels decrease and carbon dioxide levels increase, leading to acidity changes in your blood. This is detected by chemoreceptors in the body, which then send signals to the brain. The brain responds by signalling the respiratory muscles to resume breathing to expel the excess carbon dioxide and take in oxygen.
Breathing is both a voluntary and involuntary process, controlled by the respiratory center in the brain stem. The diaphragm and intercostal muscles play a crucial role in this process — the relaxation of these muscles causes pressure changes that result in inspiration and expiration. Although you can consciously control your breathing to some extent, involuntary control will take over in situations where there is an imbalance, like excessive carbon dioxide levels from holding your breath too long.
Conscious control of breathing is important in various activities such as swimming, where swimmers need to coordinate their breaths with their strokes, and when performing activities like singing or playing wind instruments that require breath control. However, there is a limit to how long and to what extent you can control your breathing before involuntary mechanisms override your efforts to ensure proper respiration and gas exchange within the body's tissues.