Giraffe On The Loose

Modifications to the giraffe's structure have evolved, particularly to the circulatory system. A giraffe's heart, which can weigh up to 10 kg (22 lb) and measure about 60 cm (2 ft) long, must generate approximately double the normal blood pressure for an average large mammal to maintain blood flow to the brain. 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.
The jugular veins also contain several (most commonly seven) valves to minimise blood flowing back into the head and assist it getting to the inferior vena cava and right atrium in the same situation. 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 the same way as a pilot's g-suit.