A 38-year-old man was brought by his wife as his snoring becomes louder soon after falling asleep, and is then interrupted by a long silent period during which no breathing (apnea) occurs. He was diagnosed as having Obstructive sleep apnea. Individuals with OSA awaken when the aerial hypoxemia and hypercapnia stimulate both peripheral and central chemoreceptors. The activity of the central chemoreceptors is stimulated by which of the following?