Answer: Specialized cells in the ventral medulla are sensitive to the levels of carbon dioxide and hydrogen ions in the blood stream that then signal to the respiratory centers to increase breathing.
Central chemoreceptors are expressed on the surface of the medulla. At this area, it is in direct contact with the cerebrospinal fluid (CSF) and can detect various conditions of the body. For example, hypercapnia is an elevation of carbon dioxide in the body. The enzyme carbonic anhydrase converts carbon dioxide and a water molecule into carbonic acid, which releases free hydrogen ions into the CSF. A metabolic increase in carbon dioxide shifts the equation towards producing more H ions, which decreases the pH (more acidic). This change can be detected by the central chemoreceptors.
When the body is engaging in rigorous physical activity, the muscles utilize oxygen and produce carbon dioxide as a waste byproduct. This carbon dioxide dissolves into the blood stream and changes our physiology. The excess carbon dioxide must be expelled by the lungs.
One of the major functions of the brain stem is to help remind the body that it needs to breathe. More specifically, this function is controlled by a vaguely defined area referred to as the respiratory centers. These respiratory functions are divided across the ventral medulla and the pons.
The “older” parts of the brain, such as those structures closer to the brain stem, are generally responsible for the more essential basic functions of living. Getting feedback from dissolved carbon dioxide in the blood and responding by elevating respiratory rate is one example of such an essential function.