Massive hemorrhage is necessary to bring the deer down quickly. A deer must lose at least 35 percent of its blood, or 2.75 pints in a 150 pound deer before falling. The better the hit, the quicker the loss. Deer blood carries high levels of vitamin K1 and K2 in early autumn. Vitamin K is an anti-hemorrhage agent, which greatly aids blood clotting.
Frightened whitetails produce high levels of B-endorphin, which supports rapid wound healing.
Endorphins consist of morphine-like chemicals from the pituary gland, allowing the animal to control pain. Deer, particularly in northern areas, have thick layers of tallow along the back and below the brisket. This can plug wounds, preventing a good blood trail. A string tracking device attached to a bow and arrow is sometimes usefull in recovering game. However, the string does affect the arrow flight on long shots.