Figure 44

Circadian rhythmicity of steroid production and major stimulatory factors. Aldosterone and cortisol and their respective major stimulatory factors, plasma renin activity (PRA) and adrenocorticotropic hormone (ACTH), demonstrate circadian rhythms. The lowest values for all of these components are normally seen during the sleep period when the need for active steroid production is minimal. ACTH levels increase early before awakening, stimulating cortisol production in preparation for the physiologic changes associated with arousal. PRA increases abruptly with the assumption of the upright posture, followed by an increase in aldosterone production and release. Both steroids demonstrate their highest values through the morning and early afternoon. Cortisol levels parallel those of ACTH, with a marked decline in the afternoon and evening hours. Aldosterone demonstrates a broader peak, reflecting the postural stimulus of PRA.

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