The cells that respond to changes in blood gas concentration and generate the respiratory rhythm are found in several nuclei in the

The answer is; medulla oblongata and pons.

In this region of the brain, there are chemoreceptors that can detect changes in the pH of the blood. These are called central chemoreceptor. There are also chemoreceptors in the arteries called peripheral chemoreceptors. When these chemoreceptors detect a drop in the blood PH (due to increased dissolved carbon dioxide in the blood), the depth and rate of breathing is increased in the lungs.