In mice, the retrotrapezoid nucleus (red) contains about 600 neurons and is directly connected to the preBötzinger complex (blue). Under normal levels of oxygen (normoxia), the activity of the preBötzinger complex controls the firing rate of neurons in the phrenic nucleus (green) by sending signals via premotor neurons. Phrenic neurons control the muscles involved in breathing and therefore control respiratory output. Ruffault et al. report that when levels of CO2 are high (which lowers blood pH), the retrotrapezoid neurons increase their firing rate: this adjusts the activity of preBötzinger complex and increases the firing rate of the phrenic neurons, which in turn increases respiratory output in order to eliminate more CO2.