Several receptor groups in the body regulate metabolic breathing. These receptors signal the respiratory center to initiate inhalation or exhalation. Peripheral chemoreceptors are located in the aorta and carotid arteries. They respond to changing blood levels of oxygen, carbon dioxide, and H+ by signaling the pons and medulla. Irritant and stretch receptors in the lungs can directly cause exhalation. Both sense foreign particles and promote spontaneous coughing. They are also known as mechanoreceptors because they recognize physical changes not chemical changes. Central chemoreceptors in the medulla also recognize chemical variations in H+. Specifically, they monitor pH change within the medullary interstitial fluid and cerebral spinal fluid.