Systems physiology of the baroreflex during orthostatic stress: from animals to humans

Abstract

The baroreflex is a key mechanism involved in the control of arterial pressure (AP) during orthostasis in humans. However, the baroreflex is a closed-loop feedback system, from baroreceptor pressure input to systemic AP, and therefore requires open-loop experiments to identify its system characteristics. The requirement limits our ability to identify baroreflex system characteristics in humans. Open-loop research in animals has revealed dynamic and static characteristics of the two baroreflex subsystems: the neural and peripheral arcs. The neural arc, from baroreceptor pressure input to sympathetic nerve activity (SNA), has high-pass dynamic characteristics, indicating that more rapid change in input AP causes greater response in SNA. In contrast, the peripheral arc, from SNA input to systemic AP, has low-pass characteristics. Orthostasis increases the gain of the neural arc, which compensates for the lower transfer gain of the peripheral arc and in turn maintains total baroreflex function. Here, I discuss the possibility that baroreflex subsystem characteristics identified in animals can be applicable to the human sympathetic response to orthostasis, with a focus on loading speed-dependence of orthostatic sympathetic activation.

Functional evidence of paraventricular nucleus involvement in cardiovascular and autonomic modulation in response to acute microgravity (head‐down tilt) in unanesthetized rats

“…However, because of the baroreflex system through the inputs coming from the arterial baroreceptors, BP is kept within narrow values (Guyenet, 2006). A key role of this system in humans is the maintenance of BP levels after an orthostatic challenge (Cooper and Hainsworth, 2002;Kamiya et al, 2014).…”

Effects of an Innovative Head-Up Tilt Protocol on Blood Pressure and Arterial Stiffness Changes

“…A significant increase in intravascular blood pressure causes a redistribution of blood in the vascular system, with 500–800 mL blood accumulating in the lower body parts [ 23 ]. This redistribution of blood reduces the venous return to the heart, the filling pressure in the atria and ventricles of the heart a tendency to decrease systemic BP and activation of cardiopulmonary baroreceptors [ 24 , 25 ]. Orthostatic activation of sympathetic baroreflex causes autonomous nervous system stimulation [ 26 ], activity of the sympathetic nervous system (SNS) [ 27 ].…”

Self-Organization of Blood Pressure Regulation: Experimental Evidence

“…Short-term blood pressure control by means of the baroreflex is a prime example of a homeostatic model in which a biological variable is maintained at its normal value by means of a physiological regulatory mechanism (Kamiya et al, 2014 ). Blood pressure drifts, such as those caused by blood volume shifts arising with the standing position, are compensated heart beat after heart beat by the baroreflex (Rowell, 1993 ).…”

“…Baroreflex inputs from blood vessel walls inform the central nervous system of these drifts. In return, the central nervous system alters heart and blood vessel functions to compensate for the drift and to return blood pressure to its reference value (Kamiya et al, 2014 ). The homeostatic model is based on causal relationships that draw a (curvi-) linear link between blood pressure and heart rate (Parlow et al, 1995 ).…”