Peripheral chemoreceptors are localised in cervical, thoracic and abdominal regions of mammals with the cervical-located, carotid bodies appearing to be the most physiologically relevant for the initiation of cardiorespiratory reflexes in response to hypoxia. These organs have a characteristic morphology and receive an arterial blood supply in excess of their metabolic requirements, which may be important for their function, and they receive an afferent and efferent innervation. Type 1 cells are believed to contain the necessary transducing elements of these chemoreceptors and are pre-synaptic to afferent nerve terminals. Type 1 cells respond to falls in the partial pressure, but possibly also the O₂ content of blood, by inactivating species-dependent K channels to induce cell depolarisation, voltage-gated Ca²⁺ entry, neurotransmission and augmented afferent nerve discharge frequency. The identity of the protein sensor responsible for detecting hypoxia is not known with certainty but a number of candidates, including the enzymes AMPK and HO-2, have recently been proposed. In addition, these organs sense many other blood-borne, natural stimuli and are therefore most probably acting as polymodal receptors.