The level of carbon dioxide in the blood is monitored by nerve tissue. If you exercise strenuously, the increase in cellular respiration consumes oxygen from the blood and produces surplus carbon dioxide removed in the blood. Since this physical exertion requires faster breathing to replenish oxygen supplies and carry away carbon dioxide, the carbon dioxide sensors result in signals from the brain to increase breathing rate. After you have rested, and carbon dioxide levels are back to normal, your breathing rate decreases too. This is an example of