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Every heartbeat, a giraffe has to push blood uphill to a brain held far above its chest. The giraffe's answer fits its body shape: high pressure, a thick-walled heart and one-way valves in the neck veins.
How does a giraffe pump blood to its head?
It makes far more pressure than it needs at heart level, so that enough is left after the climb. Giraffes have a mean arterial blood pressure of about 200 mm Hg, which appears necessary to keep a perfusion pressure of about 100 mm Hg at the head end of the carotid arteries, according to a 2021 review by Christian Aalkjær and Tobias Wang in the Annual Review of Physiology. Put simply, about half the pressure the heart makes is used up between the heart and the head.
That systemic blood pressure is twice that of mammals of the same body mass, found Graham Mitchell and John Skinner, who measured the hearts of 56 giraffes weighing from 18 kg to 1,500 kg (Comparative Biochemistry and Physiology, 2009).
Does a giraffe have a huge heart?
Not compared with its body. In the same study, giraffe heart mass was 0.51% of body mass, the same as in other mammals (Mitchell and Skinner, 2009). What differs is the wall. The walls of the left ventricle and between the ventricles are thickened, and their thickness rises in a straight line with neck length.
The pump's output is ordinary too: cardiac output matched the prediction for a mammal of that size, while peripheral resistance was double. The authors concluded that, as the neck lengthened, the weight of the blood column led to a thicker heart and thicker-walled arterioles, and the higher blood pressure followed.
Why does blood pressure fall as blood rises up the neck?
A column of fluid of height h presses down with pressure P = ρgh. Whole human blood has a density of about 1,060 kg/m³, according to this compilation of measured values; we use it as a stand-in for giraffe blood.
Pressure for each metre of blood: 1,060 × 9.81 × 1 ≈ 10,400 Pa.
Convert, using 1 mm Hg ≈ 133.3 Pa: 10,400 ÷ 133.3 ≈ 78 mm Hg per metre.
The giraffe loses about 200 − 100 = 100 mm Hg between heart and head (Aalkjær and Wang, 2021). That drop is the weight of about 100 ÷ 78 ≈ 1.3 m of vertical blood column.
Now imagine the giraffe had a pump making only 100 mm Hg at the heart. After the same climb, the pressure left at the head would be close to zero, with almost nothing to push blood through the brain.
This ignores friction, so it is only an estimate, but it shows why a long neck needs high pressure.
What happens when a giraffe bends down to drink?
The whole column flips. Giraffes lower their heads to drink and then lift them 5 to 6 m in one sweeping movement, as the Annual Review of Physiology review describes. With the head below the heart, gravity now pulls blood in the neck veins towards the head.
The jugular veins also carry one-way valves. Mitchell, van Sittert and Skinner examined 60 jugular veins from 30 giraffes and found an average of 6.4 valves in the left vein and 6.8 in the right, with 88% of them bicuspid (South African Journal of Wildlife Research, 2009). They concluded the valves' main job is to stop blood flowing back from the inferior vena cava and right atrium into the jugular vein.
With the head up, a 1996 modelling study found that pressures measured in the jugular veins of upright giraffes do not fit a rigid siphon, and are better explained by veins that are partly collapsed and resist flow (Pedley, Brook and Seymour, 1996).
How can you use the giraffe in biology and physics answers?
When a question asks how a tall animal moves blood, run four checks:
Height: how far above or below the heart is the organ? That gives h.
Cost: how much pressure does that height take away? Use ρgh, about 78 mm Hg per metre of blood.
Backflow: what stops blood running the wrong way when posture changes? Look for valves.
Strength: what has to be stronger to cope? Thicker walls, not just a bigger organ.
The same steps tie into the two loops of the human heart, explained in double circulation in humans, and into why blood pressure drops from arteries to veins. For how the neck itself evolved, see Darwin vs Lamarck on the giraffe's neck.
If ρgh still feels like a formula rather than a picture, the Fermi Concept Library gives every concept in physics and biology its own lesson and its place in the map, so you can see where pressure meets circulation. When you want to practise the numbers, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Next time you see a giraffe drink, run the four checks: height, cost, backflow, strength.
