In mammals, the left recurrent laryngeal nerve is longer than the right; in the giraffe it is over 30 cm (12 in) longer. These nerves are longer in the giraffe than in any other living animal;[53] the left nerve is over 2 m (6 ft 7 in) long.[54] Each nerve cell in this path begins in the brainstem and passes down the neck along the vagus nerve, then branches off into the recurrent laryngeal nerve which passes back up the neck to the larynx. Thus, these nerve cells have a length of nearly 5 m (16 ft) in the largest giraffes.[53] The structure of a giraffe's brain resembles that of domestic cattle.[33]:31 The shape of the skeleton gives the giraffe a small lung volume relative to its mass.[55] Its long neck gives it a large amount of dead space, in spite of its narrow windpipe. These factors increase the resistance to airflow. Nevertheless, the animal can still supply enough oxygen to its tissues.[55]
The giraffe's mouth while drinking
The circulatory system of the giraffe has several adaptations for its great height. Its heart, which can weigh more than 11 kg (25 lb) and measures about 60 cm (2 ft) long, 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).[16] Giraffes have unusually high heart rates for their size, at 150 beats per minute.[32]:76 The rete mirabile in the upper neck does not appear to prevent excess blood flow to the brain when the giraffe lowers its head as has often been believed. Instead the blood rushes to the head but when it raises again, the blood vessels constrict and direct the blood through a anastomosis between the occiput and vertebrae and into the brain so the animal doesn't faint.[56] 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.[57] 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.[17]
Giraffes have oesophageal muscles that are unusually strong to allow regurgitation of food from the stomach up the neck and into the mouth for rumination.[32]:78 They have four chambered stomachs, as in all ruminants, and the first chamber has adapted to their specialised diet.[15] The intestines of an adult giraffe measure more than 70 m (230 ft) in length[58] and have a relatively small ratio of small to large intestine.[58] The liver of the giraffe is small and compact.[32]:76 A gallbladder is generally present during fetal life, but it may disappear before birth.[15][59][60]