Sympathetic nervous system stimulation causes vasoconstriction of most of the blood vessels, including those in the skin, the digestive tract, and the kidneys. The net effect of this is a shunting of blood away from the organs not necessary to the immediate survival of the organism and an increase in blood flow to those organs involved in intense physical activity. Fibres from the sympathetic nervous system innervate tissues in almost every organ system, providing regulatory function as dilation of pupil, inhibition of gut motility, and erection of hair.