In a healthy individual, the strongest respiratory stimulus for breathing is

During normal breathing, the brain is stimulated to breath with increasing acidity as a result of CO2 concentration from basic metabolic processes. The brain is quite selfish and only really wants to maintain it's pH which should be at a range of 7.3-7.45, and will not tolerate any decrease. 

In patients who have a chronic respiratory disorder with things like COPD. The brain has become accustomed to excessive acidic content, and is now stimulated by the Hypoxic drive or by low oxygen content. 

If it doesn't have a nucleus its a prokaryote