[The circadian rhythmicity of human orthostatic tolerance]

The results of studying the daily rhythmicity of orthostatic tolerance of blood system in practically healthy men are presented. The orthostatic test was performed 6 times a day with a 4-hour interval. The parameters of systemic, cerebral and peripheric hemodynamics have been recorded. It is found that a degree of a change in an intensity of cerebral blood supply experiences the marked variations depending on time of day. The day-night cycling of orthostatic tolerance of cerebral circulation is greatly determined by the biorhythm of the responses of arterial vessels of the lower extremities, i.e., by the natural periodization of blood redistribution processes. The highest orthostatic tolerance is noted at 3 to 11 a.m. and the lowest one at 3-7 p.m.