<P> Peripheral chemoreceptors are put under stress in a number of situations involving low access to oxygen, including exercise and exposure to high altitude . Under sustained hypoxic stress, regardless of the cause, peripheral chemoreceptors show a great deal of plasticity; they will both swell the size of chemosensing cells and increase their number . Though researchers were previously unsure how carotid and aortic bodies came to increase their numbers so rapidly, recent findings point to the type II cells, which were previously thought to have only a supportive role and are now believed to retain properties of stem cells and can differentiate into type I transducer cells . </P> <P> Several studies suggest peripheral chemoreceptors play a role in ventilation during exercise . However, there is disagreement about whether they perform an excitatory or inhibitory role . Several studies point to increased circulation of catecholamine or potassium during exercise as a potential effector on peripheral chemoreceptors; however, the specifics of this effect are not yet understood . All suggestions of peripheral chemoreceptor involvement conclude that they are not solely accountable for this response, emphasizing that these receptors are only one in a suite of oxygen - sensing cells that can respond in times of stress . Collecting information on carotid and aortic body activity in live, exercising humans is fraught with difficulty and often only indicates indirect evidence, so it is hard to draw expansive conclusions until more evidence has been amassed, and hopefully with more advanced techniques . </P> <P> In addition to ventilatory effects, peripheral chemoreceptors may influence neuroendocrine responses to exercise that can influence activities other than ventilation . Circulation of the glucose - promoting hormone, glucagon and a neurotransmitter, norepinephrine, is increased in carotid - and aortic - body - enervated dogs, suggesting that peripheral chemoreceptors respond to low glucose levels in and may respond to other neuroendocrine signals in addition to what is traditionally considered to be their sole role of ventilatory regulation . </P> <P> Peripheral chemoreceptors work in concert with central chemoreceptors, which also monitor blood CO2 but do it in the cerebrospinal fluid surrounding the brain . A high concentration of central chemoreceptors is found in the ventral medulla, the brainstem area that receives input from peripheral chemoreceptors . Taken together, these blood oxygen monitors contribute nerve signals to the vasomotor center of the medulla which can modulate several processes, including breathing, airway resistance, blood pressure, and arousal, with central chemoformation about medullary oxygen levels and peripheral chemoreceptors about arterial oxygen . At an evolutionary level, this stabilization of oxygen levels, which also results in a more constant carbon dioxide concentration and pH, was important to manage oxygen flow in air - vs. - water breathing, sleep, and to maintain an ideal pH for protein structure, since fluctuations in pH can denature a cell's enzymes . </P>

Where would you find peripheral chemoreceptors that help control respiration