Circulatory system of a giraffe:
By Sydney Larkin

Image:Namibie Etosha Girafe 01.jpg
Image:Namibie Etosha Girafe 01.jpg

The giraffe's heart is probably the most powerful among animals, because about double normal pressure is required to pump blood up the giraffe's long neck to the brain. With such high blood pressure, only special design features prevent it from 'blowing its mind' when it bends down to take a drink. Modifications to the giraffe's structure have evolved, particularly to the circulatory system.

A giraffe's heart, which can weigh up to 11 kg (24 lb) and measure about 2 feet long, has to generate around double the normal blood pressure for an average large mammal in order to maintain blood flow to the brain against gravity. In the upper neck, a complex pressure-regulation system called the rete mirabile prevents excess blood flow to the brain when the giraffe lowers its head to drink. A series of special one-way valves in the neck regulates blood flow, and there is a special net of elastic blood vessels at the base of the brain. Without these valves and elastic blood vessels, the blood pressure in the giraffe's head would be immense when it bends over, enough to cause brain damage. All of these features -- large heart, valves in the jugular vein, and wondernet of vessels -- must be in place simultaneously or the giraffe would die. They could not have evolved gradually.

Conversely, the blood vessels in the lower legs are under great pressure (because of the weight of fluid pressing down on them). In other animals such pressure would force the blood out through the capillary walls; giraffes, however, have a very tight sheath of thick skin over their lower limbs which maintains high extravascular pressure in exactly the same way as a pilot's g-suit. Equally marvelous is the fact the blood does not pool in the legs, and a giraffe does not bleed profusely if cut on the leg. The secret lies in an extremely tough skin and an inner fascia that prevents blood pooling. This skin combination has been studied extensively by NASA scientists in their development of gravity-suits for astronauts. Equally helpful to prevent profuse bleeding is that all arteries and veins in the giraffe's legs are very internal.

(most of the information above was taken from wikipedia)