Why Is a Giraffe's Blood Pressure Twice Ours?
― The Head Is 2 Metres Up, and the Heart Lifts the Weight of the Blood
Blood is a liquid, so it has weight. The higher you send it, the more that weight presses back on the heart. A giraffe's brain sits about 2 metres above its heart. That is why a giraffe's heart is thought to pump blood at about twice the pressure of ours. So what happens when the giraffe lowers its head all the way to the ground for a drink?
You crouch down to tie your shoelaces, then stand up suddenly. Your vision goes whitish and you wobble. Has that ever happened to you?
That is a head rush. Your head has moved only a little higher above your heart, yet for a moment not enough blood reaches your brain. In a person, that height difference is at most 30 to 40 centimetres.
So how does a giraffe cope, with its head 2 metres above its heart? And every time it drinks, it lowers its head to the ground and then lifts it 2 metres back up. Even so, it doesn't stagger or fall over.
There are two main reasons
Lifting blood 2 metres takes extra pressure all by itself. A giraffe's heart has thick walls and is thought to generate about twice the pressure of a human heart.
Valves that stop blood flowing backwards when the head is lowered, skin that squeezes the legs tightly, and other features all over the body help it cope with strong pressure.
First, let's look at why "height" turns into pressure.
Lifting blood takes a force equal to its weight
Fill a hose with water and lift one end high, and the water only rises part of the way. The weight of the water above pushes back on the water below. Blood in a blood vessel works the same way. Blood travelling up from the heart has to push against the weight of the blood above it.
The pressure from the weight of blood is roughly proportional to height. For every metre you go up, the pressure drops by about 77 mmHg (millimetres of mercury, the unit of blood pressure). A human head is about 35 centimetres above the heart, so the drop is about 27 mmHg. In a giraffe, though, 2 metres means a drop of about 150 mmHg.
The brain needs blood to keep arriving at a certain pressure. So a giraffe's heart pumps at high pressure all the time. The blood pressure at the heart's outlet is thought to exceed 200 mmHg, about twice ours. In a person, that would be a level needing treatment right away.
Look at Figure 1. It shows how the blood pressure at the height of the brain changes as the giraffe raises and lowers its head.
When the head goes down, why is the brain safe?
Slide the control in Figure 1 all the way to the left and the pressure at brain height reaches about 340 mmHg. The heart's strong push and the weight of the blood above now both act toward the brain. If a person took this much pressure, their fine blood vessels could be damaged.
Real giraffes, though, drink without trouble. The large veins in the neck contain several valves that stop blood flowing backwards. They are thought to keep blood that should be heading back to the heart from rushing back into the head and pooling there when the head is lowered. In addition, mechanisms around the blood vessels of the head are thought to soften sudden changes in pressure. How well these mechanisms actually work is still being studied.
Another hard spot is the legs. A giraffe's feet are about another 2 metres below its heart. The weight of the blood adds on, so by calculation the pressure is far higher than at the heart's outlet. In a person, fluid would seep out of the vessels and the legs would swell. A giraffe's leg skin and the sheet of tissue beneath it are very tough, squeezing the vessels from outside like tight stockings. This squeezing is reported to hold back the seeping of fluid.
It is known that where a snake's heart sits depends on how it lives. Tree-climbing snakes spend long periods with the body upright and the head high. Their hearts are therefore close to the head, which shortens the height to the brain. Sea snakes, on the other hand, are held up by the water, so the weight of blood is hardly a problem, and the heart sits nearer the middle of the body. Living things answer the same physics, the weight of blood, in their own different ways.
If big long-necked dinosaurs held their heads high, their brains would have been several metres above their hearts. The blood pressure needed then is estimated to reach several times a giraffe's. So some have suggested that they did not raise their heads that high. Only bones remain, so the question is not settled.
Summary
Blood has weight, so the higher you send it, the more pressure it takes. A giraffe thickens the walls of its heart and delivers blood to the brain at about twice our blood pressure. On top of that, valves and tough skin take up the pressure so it does not rise too far when the head is lowered or in the feet. Our own head rush is the same physics peeking out, from a difference of just a few tens of centimetres.
Being tall means keeping heavy blood lifted.
A giraffe's high blood pressure is not a disease; it is body design.
How the blood in your feet gets back to the heart is explained in "Why do your feet swell when you stand all day?", and the body changes that happen at high altitude are covered in "the altitude sickness article".
- Fill a clear plastic hose (1 to 2 metres) with water and block both ends with your fingers.
- Sink one end in a bucket of water and take your finger off it. Keep the other end blocked, and lift it up little by little.
- Compare how strongly the fingertip blocking the upper end is sucked in at different heights.
The higher you lift it, the more strongly your finger is sucked in. That is the weight of the water above pulling downward. A giraffe's heart pushes back against this over 2 metres, every second. When working with water, do it outdoors or in a bathroom so nothing gets wet.
For those who want to know more ― terms, formulas and links to textbooksFrom middle-school science to university courses, each part is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics or biology
- High school+Advanced high-school material, or a textbook sidebar
- UniversityUniversity-level specialist subjects (comparative physiology, biofluid mechanics), not taught in high school
- ResearchNot yet taught even at university as settled fact; researchers are still investigating it
Middle schoolTerms: this phenomenon has names
- Water pressure (hydrostatic pressure): Pressure caused by the weight of a liquid. It grows larger the deeper you go. The same thing happens in blood.
- Blood pressure: How strongly the blood in a vessel pushes on the vessel wall. It is usually given as how many millimetres of mercury the pressure can push up (mmHg).
- Orthostatic hypotension (head rush): A condition in which the blood pressure at head height briefly drops when you stand up, making you unsteady.
Middle schoolHigh schoolChecking with a formula: the blood pressure a giraffe's heart needs
Suppose we want a blood pressure of about 70 mmHg to remain at brain height (an assumption based on the value at a human head's height). We estimate how much is needed at the heart's outlet using the formula for pressure in a liquid.
| In symbols | P_heart ＝ P_brain ＋ ρ × g × h |
| In words | Blood pressure at the heart's outlet ＝ blood pressure needed at the brain ＋ blood density × gravitational acceleration × height from heart to brain |
| Where it comes from | The balance of pressure inside a liquid (the hydrostatic pressure formula). The weight of a column of blood of height h becomes the pressure pushing on the blood below. |
| Symbol | Meaning and unit |
| ρ | Blood density (kg/m³). A little heavier than water |
| g | Gravitational acceleration (m/s²) |
| h | Height from heart to brain (m) |
| P | Blood pressure (mmHg). 1 mmHg is 133.3 Pa |
| Blood density | About 1050 kg/m³ |
| Gravitational acceleration | 9.8 m/s² |
| Height from a giraffe's heart to its brain (typical adult) | About 2 m |
| Height from a human's heart to brain | About 0.35 m |
| Blood pressure to keep at brain height (assumed) | About 70 mmHg |
| Pressure per 1 m of blood column (Pa) | 1050 × 9.8 ＝ 10290 |
| Pressure for the giraffe's 2 m (Pa) | 10290 × 2 ＝ 20580 |
| Convert to mmHg | 20580 ÷ 133.3 ≒ 154 |
| Blood pressure a giraffe's heart needs | 154 ＋ 70 ＝ 224 |
| Pressure for a human's 0.35 m (Pa) | 10290 × 0.35 ＝ 3601.5 |
| Convert to mmHg | 3601.5 ÷ 133.3 ≒ 27 |
| Blood pressure a human heart needs | 27 ＋ 70 ＝ 97 |
| How many times a human's is the giraffe's? | 224 ÷ 97 ≒ 2.3 |
The estimate for the heart's outlet is about 224 mmHg, a little more than double a human's. This roughly matches the values actually measured in giraffes (in the 200s of mmHg). Conversely, if the head is lowered 1.5 m below the heart, 10290 × 1.5 ＝ 15435 Pa, or about 116 mmHg, is added. At brain height that works out to 224 ＋ 116 ＝ 340 mmHg or so, the value at the far left of Figure 1.
High schoolHigh school+The heart is a pump, the vessels are hoses
High schoolIn high-school physics, the pressure at depth h in a liquid is taught as "ρ × g × h". It is the same formula that makes your ears hurt at the bottom of a pool. Blood in a vessel is also a liquid, so the formula applies directly. In biology you learn that the left ventricle of the heart pumps blood to the whole body. In giraffes, the wall of this left ventricle is very thick.
High school+A thicker-walled tube can withstand higher pressure, because the tension in a tube wall is proportional to the pressure and the radius (Laplace's law). A giraffe's heart is thought not to be especially large for its body size. Instead, it is thought to produce high pressure by thickening the wall and keeping the inner chamber on the small side.
UniversityHydrostatic pressure, and how swelling is prevented
The formula in this article is the basic equation of fluid statics (the hydrostatic pressure formula). Whether the legs swell is decided by a balance between the pressure difference across the vessel wall and the force with which proteins in the blood pull water back in (the Starling principle). In giraffe legs, it has been reported that tough skin and fascia keep the outside pressure high and so avoid upsetting this balance (research by Hargens and colleagues). As for how blood flow to the brain is maintained when the head is lowered, there is debate over whether a "siphon" (liquid flowing over the top of a tube) works.
📖 Derivations of the formula and further reading: Fluid statics (hydrostatic pressure) ― Wikipedia (Japanese) ／ Blood pressure ― Wikipedia (Japanese)
ResearchWhat is still not clear
- How the brain is protected when the head is lowered. It is known that the valves in the neck veins play a part, but how much the regulation around the brain's blood vessels contributes is still hard to measure.
- The hearts of long-necked dinosaurs. If they held their heads high, very high pressure would have been needed. There are several theories about neck angle and heart structure.
- Why high blood pressure doesn't ruin a giraffe's body. In people, sustained high blood pressure damages the heart and vessels. How giraffe cells withstand it is being studied, including through genetic research.
In other words, the contents of this article are also "an explanation of what is known so far". The mechanism for the lowered head in particular may be explained differently as research continues.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science Field 1 "Water pressure" / Field 2 "Blood circulation" | The weight of blood becoming pressure |
| High school | Physics "Pressure in liquids" / Biology "The heart and circulatory system" | Checking with a formula, the left ventricle wall |
| High school+ | Advanced physics "Tension in tube walls" | How a thick wall produces high pressure |
| University | Comparative physiology / biofluid mechanics | How leg swelling is prevented, the siphon debate |
| Research | Palaeontology / comparative physiology | Long-necked dinosaurs, regulation when the head is lowered |
| ― | Links to daily life | Head rush, swollen legs |
- Hargens AR, Millard RW, Pettersson K, Johansen K (1987) Gravitational haemodynamics and oedema prevention in the giraffe. Nature 329: 59–60.
- Mitchell G, Skinner JD (2009) An allometric analysis of the giraffe cardiovascular system. Comparative Biochemistry and Physiology Part A 154: 523–529.
- Lillywhite HB (1988) Snakes, blood circulation and gravity. Scientific American 259(6): 92–98.
- Seymour RS, Lillywhite HB (2000) Hearts, neck posture and metabolic intensity of sauropod dinosaurs. Proceedings of the Royal Society B 267: 1883–1887.
※This article is a general-audience science explainer. The figures given are rough estimates meant to help you understand the mechanism. Giraffe blood pressure values vary with the measurement method and the individual animal. If you get head rushes often, please consult a medical professional.