A 25-year-old machinist is injured by a metal sliver in his left hand. Over the next few days, the wounded area becomes reddened, tender, swollen and feels warm to the touch. Redness at the site of injury in this patient is caused primarily by which of the following mechanisms?

Vasodilation of precapillary aerioles increases blood flow at the site of tissue injury. This condition (active hyperemia) is caused by the release of specific mediators. Vasodilation and hyperemia are primarily responsible for the redness and warmth (rubor and calor) at sites of injury. The other choices do not regulate active hyperemia.