You don’t have to think about breathing because your body’s autonomic nervous system controls it, as it does many other functions in your body. If you try to hold your breath, your body will override your action and force you to let out that breath and start breathing again. The respiratory centers that control your rate of breathing are in the brainstem or medulla. The nerve cells that live within these centers automatically send signals to the diaphragm and intercostal muscles to contract and relax at regular intervals. However, the activity of the respiratory centers can be influenced by these factors:

• Oxygen: Specialized nerve cells within the aorta and carotid arteries called peripheral chemoreceptors monitor the oxygen concentration of the blood and feed back on the respiratory centers. If the oxygen concentration in the blood decreases, they tell the respiratory centers to increase the rate and depth of breathing.
• Carbon dioxide: Peripheral chemoreceptors also monitor the carbon dioxide concentration in the blood. In addition, a central chemoreceptor in the medulla monitors the carbon dioxide concentration in the cerebrospinal fluid (CSF) that surrounds the brain and spinal cord; carbon dioxide diffuses easily into the CSF from the blood.

  If the carbon dioxide concentration gets too high, then both types of chemoreceptors signal the respiratory centers to increase the rate and depth of breathing. The increased rate of breathing returns the carbon dioxide concentration to normal and the breathing rate then slows down.