The remarkable influence of external stimuli in modulating this cardiovascular response during the transition from sleep to an active state is underscored by the observation that individuals who arouse from sleep but remain in bed (nonambulatory) exhibit a blunted hemodynamic response. The change to an erect position and possibly the mental stress associated with initiating the daily activities is an important component of this arousal phenomenon. In addition to enhancing catecholamine secretion, other hormonal systems with vasoactive properties are activated on standing (renin-angiotensin-aldosterone, antidiuretic hormone) and may contribute to this cardiovascular integration.
Some of the consequences of circadian rhythmicity in physiology are exemplified by the differences in integration of a given response in certain phases of the circadian cycle that may render the individual more vulnerable to challenge. Likewise, time-dependent variations in compensatory mechanisms activated to counteract ongoing disease processes may create abnormal rhythms of susceptibility to additional stimuli. buy asthma inhalers
For instance, the physiologic increase in many cardiovascular variables (ie, blood pressure, heart rate, left ventricular contractility, conduction system dro-motropic states, etc) which occurs in early morning as a consequence of the return to physical activity after sleep and the resulting increase in cardiovascular work and stress substantiate the early morning hours as a potentially vulnerable period for individuals with underlying cardiovascular disorders. Moreover, added to the paramount clinical relevance of circadian variability in the integration of bodily function in health and disease, several pharmacologic agents have circadian rhythms of drug effectiveness or toxicity. Like rhythms of disease susceptibility, these variations in the profiles of safety or effectiveness of many drugs appear to be a function of multiple component physiologic rhythms.
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