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al o f P hy si ol og y Activation of the sympathetic nervous system produces many of the circulatory adjustments to stressors such as exercise, orthostatic stress, bleeding and hypoxia. Acute hypoxia stimulates oxygen sensitive chemoreceptors in the carotid body (Marshall, 1994; Prabhakar, 2000) and the brainstem (Reis et al. 1994; Solomon, 2000), and increases efferent sympathetic outflow in humans, as demonstrated by a number of studies using direct microneurographic recordings of sympathetic discharge to the skeletal muscl