Most vertebrates show respiratory and circulatory reflexes which can be traced to stimulation of various extero- or interoceptors. Widely distributed groups of exteroceptors, with a variety of stimulus modalities, are associated with defence reflexes which protect the respiratory passages and gas exchange surfaces. Other exteroceptors are associated only with the gas exchange surfaces. These are either mechanoreceptors or chemoreceptors and have a range of different dynamic characteristics, but are none-the-less amazingly uniform in their role in control of the normal breathing pattern. Intravascular receptors stimulated by the arterial blood pressure are located on all the gills in fishes, yet appear to be restricted to the pulmocutaneous arteries in anuran amphibians and to the truncal region or aortic root in reptiles and birds. On the other hand, the distribution of glomus-sustentacular-nerve cell complexes, associated with intravascular chemoreception, appears to be much more diffuse in higher than lower vertebrates. The wide distribution of these cell complexes may be attributed to their embryological origin from neural crest cells. Even so, the presence of these complexes does not appear to be essential for respiratory responses to environmental hypoxia since they have not been located in teleost fishes.
The role of peripheral receptors in controlling cardiovascular and respiratory functions, under steady state conditions in non-mammalian vertebrates, is not well defined and it may be that they are only modulators of circulatory and respiratory patterns arising from hormonal or humoral effects on the central nervous system.
Circulatory reflexes from carotid and extracarotid baroreceptor areas in man.