Cardiovascular and blood gas responses to shivering produced by external and central cooling in the pigeon

Cardiovascular and blood gas responses of pigeons to spinal cord cooling (35–36°C) were measured at thermoneutral (28°C) and low (5°C) ambient temperatures. Spinal cord cooling at thermoneutral temperatures caused immediate shivering and increases in heat production (223%), heart rate (152%) and cardiac output (169%), but blood pressure and stroke volume did not change. \(Pa_{{\text{CO}}_{\text{2}} } \) and \(P\bar v_{{\text{CO}}_{\text{2}} } \) increased slightly during the cooling; \(P\bar v_{{\text{O}}_{\text{2}} } \) and \(C\bar v_{{\text{O}}_{\text{2}} } \) decreased slightly while \(P\bar v_{{\text{O}}_{\text{2}} } \) and \(C\bar v_{{\text{O}}_{\text{2}} } \) decreased considerably (10 Torr and 1.7 mmol·l⁻¹, respectively), resulting in a greater a-vv difference in O₂ content. Ambient cooling produced responses comparable to spinal cord cooling. Simultaneous spinal cord and ambient cooling produced similar responses that were generally greater in magnitude than either kind of cooling alone. Consequently, heart rate, cardiac output and O₂ extraction from the blood were all significantly, linearly related to heat production over the wide range studied. Comparisons are made between cardiovascular responses of birds to shivering and exercise in regards to the relative importances of increases in heart rate, stroke volume and blood pressure. It is suggested that exercise and shivering may effect cardiovascular responses through similar receptor mechanisms.