EVOLUTION OF CARDIOVASCULAR BARORECEPTOR CONTROL

During animal evolution the circulatory system has shown a progressive modification in structure, function and short-term control. Short-term circulatory control has evolved from the limitation of a rising blood pressure via a reflex bradycardia to bidirectional control of blood pressure by appropriate reflex changes in heart rate, vascular resistance and impedance. Relevant experimental data ranges from extensive in mammals to nugatory in invertebrates. Baroreceptor research in intervening animal groups is varied, being particularly sparse in birds. This research is reviewed. There are few interspecies comparisons of baroreceptor physiology. Available data is complicated by variation in the techniques employed for assessing baroreceptor function. In non-mammalian research the correlation of heart rate changes to pharmacologically induced changes in blood pressure predominate. In mammalian baroreceptor research methods based upon the ability of discrete baroreceptor sites to effect changes in the peripheral vasculature are more prevalent. All methods are susceptible to modification by other experimental variables, particularly the anaesthetic state of the animal. Available evidence shows a consistent response of a decreasing heart rate to baroreceptor loading throughout the vertebrates, with a progressive increase in the ability of the baroreceptors to change peripheral vascular resistance. These findings are consistent with the known, progressive trend from cholinergic to adrenergic control of the vascular system during evolution. Known baroreceptor sites appear to be located so as to protect the end-organ or-organs primarily at risk from inappropriate blood pressure changes; namely the gill vasculature in the fish, pulmonary circulation in the Amphibia and Reptilia, and the brain and heart in higher animal groups. It is postulated that the carotid sinus baroreceptors have developed in the Mammalia as a second functional baroreceptor site to provide extra protection against hypoperfusion of vital organs, particularly the heart and brain. In humans the dynamic aspects of cardiovascular carotid sinus control, particularly of skeletal muscle flow and integration with cardiopulmonary baroreceptors, may represent a specific response to the adoption of an upright stance. Extremes of environmental stress encountered in contemporary life may exceed the limitation of baroreceptor function in humans, as, for example, during gravitational loading particularly following periods of weightlessness and modification by endurance training.