When a giraffe raises its head after bending down for a drink, blood pressure to the brain should drop precipitately – a more severe version of the dizziness that many people experience when they stand up suddenly. 'But we don't know.'Īalkjær has one more question about these remarkable animals. 'It would be fun to imagine that when the giraffe is standing still out there, it's closing off that sphincter just beneath the knee,' says Aalkjær. It remains unclear, however, whether giraffes open and close the arteries to regulate lower-leg pressure as needed. This could lower the blood pressure in the lower legs, much as a kink in a garden hose causes water pressure to drop beyond the kink. Giraffes also have thick-walled arteries near their knees that might act as flow restrictors, Aalkjær and others have found. 'Why don't we see giraffes with swollen legs? How are they protected against the enormous pressure down there?' asks Christian Aalkjær, a cardiovascular physiologist at Aarhus University in Denmark who wrote about giraffes' adaptations to high blood pressure in the 2021 Annual Review of Physiology. But you only have to look at the slender legs of a giraffe to know that they've solved that problem, too. People who suffer from hypertension are also prone to annoying swelling in their legs and ankles because the high pressure forces water out of blood vessels and into the tissue. Natterson-Horowitz and her team are hoping to study the placentas of pregnant giraffes to see if they have unique adaptations that allow this. In people, this can lead to severe complications that include liver damage, kidney failure and detachment of the placenta. Natterson-Horowitz is now turning her attention to another problem that giraffes seem to have solved: high blood pressure during pregnancy, a condition known as preeclampsia. 'All you have to do is look at a picture of a fleeing giraffe,' Natterson-Horowitz says, 'and you realise that the giraffe has solved the problem.' (Neither of her studies has been published yet.) This allows the heart to pump more blood with each stroke, allowing a giraffe to run hard despite its thicker heart muscle. In giraffes, the ventricular-filling phase of the heartbeat is extended, Natterson-Horowitz found. This type of heart failure is responsible for nearly half of the 6.2 million heart failure cases in the US today.Īnd the giraffe has another trick to avoid heart failure: the electrical rhythm of its heart differs from that of other mammals. The left ventricle of the heart becomes stiffer and less able to fill again after each stroke, leading to a disease known as diastolic heart failure, characterised by fatigue, shortness of breath and reduced ability to exercise.

In people, chronic high blood pressure causes a thickening of the heart muscles. It doesn't faze the giraffes, but a pressure like that would cause all sorts of problems for people, from heart failure to kidney failure to swollen ankles and legs. To have a blood pressure of 110/70 at the brain – about normal for a large mammal – giraffes need a blood pressure at the heart of about 220/180. Giraffes have sky-high blood pressure because of their sky-high heads that, in adults, rise about 6m (19ft) above the ground – a long, long way for a heart to pump blood against gravity. Their solutions, only partly understood by scientists so far, involve pressurised organs, altered heart rhythms, blood storage – and the biological equivalent of support stockings. Giraffes, it turns out, have solved a problem that kills millions of people every year: high blood pressure.

But to a cardiovascular physiologist, there's even more to love. To most people, giraffes are merely adorable, long-necked animals that rank near the top of a zoo visit or a photo-safari bucket list.

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