Effects of Aging on the Vestibulo-Cardiovascular Reflex

Gravity for the human body has a major effect on the cardiovascular system. During the postural change from a supine to a standing position, gravity generates a hydrostatic pressure gradient according to the longitudinal direction, decreasing venous return and decreasing cardiac output. Thus, arterial pressure (AP) should be decreased, but healthy individuals usually maintain AP. For this AP control, baroreflex control is considered to be important. However, this control is a negative feedback system and not a maintenance system. The importance of the vestibular system, which is the sensor of acceleration force or gravity, has been investigated to maintain AP or raise AP in advance at the onset of postural change. With the change in gravity, sympathetic nerve activity increases, and AP similarly increases regardless of the direction of gravity. At the onset of head-up tilt in human subjects, vestibular systems, especially otolith organs, sense the changes in the direction of gravity for the body and raise AP in advance reflexively. The raise is balanced with a decrease in AP due to the footward fluid shift, and AP is maintained. A type of orthostatic hypotension that AP decreases at the onset of postural change might be induced by an imbalance or dysfunction of the vestibulo-cardiovascular reflex. In the aged subjects and astronauts after long-term space flight, the control function might be diminished.