The system that the giraffe uses, called a rete mirable, is a truly amazing system.  It is a cluster of arteries and veins very close to each other, that can divert some of the blood flow under the right circumstances.  In the giraffe, it serves to equalize blood pressure when the animal lowers its head, serving as a kind of natural pressure relief system.

The same rete mirable system can also be found in many other animals.  In dogs, it protects their brains when their body overheats by diverting some of the overheated blood back to the body.  

Penguins have rete mirable systems that reduce blood flow to their feet, wings, and nasal passages, to reduce heat loss when they are swimming in icy water.  

Some whales and other diving mammals have rete mirable to adjust their blood pressure during deep dives.  

Fish use the same system to mix arterial and venous blood, causing it to give up dissolved gases to fill their air bladder, helping keep them just at the point where they don't float or sink.