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Internal systems

The giraffe's circulatory system has several adaptations to compensate for its great height. Its 11 kg (25 lb) and 60 cm (2 ft) heart must generate approximately double the blood pressure required for a human to maintain blood flow to the brain. As such, the wall of the heart can be as thick as 7.5 cm (3.0 in). Giraffes have relatively high heart rates for their size, at 150 beats per minute. When the animal lowers its head, the blood rushes down fairly unopposed and a rete mirabile in the upper neck, with its large cross-sectional area, prevents excess blood flow to the brain. When it raises again, the blood vessels constrict and push blood into the brain so the animal does not faint. The jugular veins contain several (most commonly seven) valves to prevent blood flowing back into the head from the inferior vena cava and right atrium while the head is lowered. Conversely, the blood vessels in the lower legs are under great pressure because of the weight of fluid pressing down on them. To solve this problem, the skin of the lower legs is thick and tight, preventing too much blood from pouring into them.