The hydrostatic pressure exerted by gravity on the column of blood in the neck, the heart is about 2m away from the head; necessitates an average systemic blood pressure of ~200 mm Hg compared with the standard of 100 mm Hg in other land mammals.
Although widely reported as having a very large heart relative to overall mass, heart mass in Giraffeis only 0. 5% of body mass – the same as that in other mammals. Giraffe’s heart weighs up to 11 kg and is up to 0.6 m long,
Instead of cardiac enlargement or an increase in cardiac output, cardiac hypertrophy of the left ventricular and interventricular heart wall muscles (their thickness is linearly related to neck length) is the key to maintaining cerebral blood flow.
Enlargement of the arteries and arterioles at or below the heart helps control blood flow, to the organs and, during drinking, the thick-walled arteries help prevent blood from rushing to the head.
Valves in the jugular vein direct the large amounts of blood returning to the heart via the inferior vena cava into the right atrium and prevent it from regurgitating into the jugular.
When a Giraffe raises its head, a momentary pause during head lifting and intense extracranial vasoconstriction help to prevent fainting. Microcirculation hemodynamics, the thick skin, thickened arteries, an autonomic nervous system, and innervation of limbs help to prevent fluid retention.