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While at rest young subjects have lower mean arterial blood pressure than old subjects and lower circulating levels of the neurotransmitter of the sympathetic nervous system, norepinephrine (NE). After standing and performing an isometric exercise, young subjects increase their circulating levels of NE less than old subjects. The cold pressor test is commonly used as a stimulus to the sympathetic nervous system, and young subjects have a much smaller pressor response than old subjects. In response to uniform stresses young subjects secrete less NE and have a smaller blood pressure response than old subjects.

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