Sympathetic Nervous System When stressed, sympathetic nerve activity rises, resulting in direct vasoconstriction of afferent arterioles (norepinephrine action) and stimulation of the adrenal medulla. This response increases blood pressure by reducing renal plasma flow and glomerular filtration rate. Afferent arteriole constriction leads to reduced inflow into the glomerulus and thus decreased sodium reabsorption from the filtrate. This mechanism helps to conserve sodium and water during times of increased need for these substances (e.g., when facing environmental dehydration or after a large meal). The net effect is a reduction in blood volume and blood pressure. The efferent arteriole also constricts in response to stress, leading to reduced outflow from the kidney and thus decreased sodium delivery to body tissues. This mechanism acts to restore blood pressure to normal levels. Sympathetic nervous system activation can occur in situations where there is no actual threat to life; rather, it is believed that this response serves to prepare the body for action or reaction to a future challenge. For example, when preparing for a fight or fleeing from danger, the sympathetic nervous system is activated, causing the body to tense up and pump more blood through the vessels.