Why don’t giraffes faint? Giraffes don’t faint because they have a very strong heart and a very high blood pressure.
If we bend over and then stand up again, we get a head rush. Sometimes it can be so bad that you faint. This is called orthostatic hypotension. Your blood pressure is set at the rate it is to push all of your blood around your circulatory system and back to the heart again. When you crouch down or sit down, gravity pulls blood down into your legs and lower abdomen, where it pools. Your blood pressure adjusts to the new amount of blood in the system, and everything is fine. When you stand up, the blood vessels should constrict to keep the blood pressure the same until all of the blood is moving again. Sometimes, for whatever reason, the blood vessels don’t constrict. That means that when you stand u,p your blood pressure suddenly drops and you don’t have enough blood in your head. That causes a head rush, and people can pass out if it is bad enough. The blood flow to the brain and eyes reduces, so you get tunnel vision and sometimes a tingling tongue. If the blood pressure isn’t restored quickly enough, the person will pass out, which brings the body back down to the horizontal and allows the blood pressure to return to normal. Unconsciousness usually only lasts a few seconds, if that. So, why doesn’t this happen to giraffes?
You would think that because giraffes have such long necks, and because they drop their heads down low a lot, that the same thing would happen to them. Giraffes have to lower their head to ground level when they drink, and the blood pressure in their heads should rise to such levels that they pass out. That obviously doesn’t happen because you don’t see passed-out giraffes all over the Serengeti. And if they did pass out, it would be an evolutionary reason why their necks wouldn’t have become so long. The fact that their necks are that long and they don’t pass out means they have evolved to cope. There are several ways that giraffes can do this.
The first is the way they cope with their blood pressure. The only way to pump blood from the heart of a giraffe to its head, which is several meters away, is to have an extremely high pressure. You will know this if you have tried to blow a liquid up a long pipe. To get the liquid to go higher, you need to introduce more pressure. In humans, high blood pressure can cause several problems. Blood vessels can get damaged, and the heart itself can become damaged. The left ventricle of our hearts becomes thicker and stiffer, making it harder and harder to pump blood. This is called diastolic heart failure. Giraffes don’t have these problems. They don’t get damaged in their arteries and veins because they have thick support structures that hold the arteries together and stop them from bursting. They also don’t get diastolic heart failure. The ventricle in their heart does get thicker, but it doesn’t stiffe,n and they have no trouble using it. This is because they have genes that keep it flexible. They have several other genes related to their circulation that we don’t have.
Another method that giraffes have evolved is the way their heart beats. They have a modified heartbeat rhythm. Our heart beats at the same rhythm when it is taking in blood and when it is pushing blood out. A giraffe’s heart takes longer to fill than it does to push blood out. This puts more blood in the heart and allows the giraffe to pump more blood with each heartbeat.
Giraffes also have big veins in their necks with valves that can contain blood. When the giraffe bends its head down to drink, the veins fill with blood and stop it all going to the brain. This lowers the blood pressure while the giraffe has its head down. When it picks its head back up again, the blood that is stored in the neck rushes straight back to the heart, and in one pump, the heart brings the blood pressure back to normal.
Giraffes also have dense connective tissue that tightly wraps around their legs. When a person has high blood pressure, water is forced out of the blood vessels and pools in the legs, making them swollen. This would hinder giraffes too much, and the strong, tight fibers they have compress their legs and stop any fluid from building up there. They also have thick arteries in their legs that can restrict the flow of blood to the legs and lower the blood pressure there.
Giraffes also have several other adaptations, some of which researchers have not yet worked out. One of those is how they can retain normal eyesight with such high blood pressure. People with high blood pressure slowly lose their eyesight due to damage, but this doesn’t happen with giraffes. And this is what I learned today.
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Sources
https://www.livescience.com/853-giraffes-dizzy.html
https://www.novartis.com/stories/giraffes-have-high-blood-pressure-why-dont-they-drop-dead
https://www.smithsonianmag.com/science-nature/cardiovascular-secrets-giraffes-180977785
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/dizziness-orthostatic-hypotension
https://www.mayoclinic.org/diseases-conditions/orthostatic-hypotension/symptoms-causes/syc-20352548
Photo by Anna Tarazevich: https://www.pexels.com/photo/giraffe-s-drinking-water-on-a-wooden-container-6796900/