Why do we get a headache when we are dehydrated? You get a headache when you are dehydrated because your brain shrinks and pulls away from the sides of your skull.
The human body is about 60% to 70% water. Some parts of the body contain more water than other parts. The blood and the lungs are 83% water. The muscles and kidneys are 79% water. The brain and heart are 73% water. The skin is 64% water and the bones are 31% water. The water is a building material in every cell. It is used to regulate our body temperature through sweating and respiration. It transports energy and oxygen around our body in the blood. It carries blood to the kidneys and then is used to flush waste and toxins out of our bodies through urine. It is a shock absorber around the brain and spinal cord. It is the main component of saliva and the mucus in our noses. It lubricates the joints. Basically, without water we don’t work.
Our body uses water for many purposes, but we also lose water through sweat and urine. If we cannot replenish the water that we lose, we become dehydrated and, in the end, die. A human can survive for weeks without food, but only for about three days without water.
When we lose about 2% of our body weight in water, the brain detects the lack in the blood and triggers thirst. Ideally, this would make us drink and the problem would be solved. If water loss continues, the body starts to conserve water. The kidneys reduce the amount of water they release, making the urine darker and thicker. Your sweat reduces, which raises the body temperature. The blood thickens, which raises blood pressure. Your heart rate increases to keep oxygen levels up. Most people can undergo this amount of dehydration without a lot of trouble, so long as they can drink water again quickly.
As we lose more than 4% of our body weight in water, the kidneys further reduce the amount of water they use. Blood pressure rises because the blood gets even thicker and the person can faint. Organs start to shrink as the water is pulled out of them. The blood is thicker and the brain doesn’t receive as much oxygen, meaning dehydrated people can become confused and irrational.
As we get closer to 7% of our body weight lost in water, things start to get serious. Blood flow to non-essential organs is slowed, conserving water. This means that the waste the kidneys usually clean out start to build up in the body and your kidneys begin to fail.
When you reach 10% of body weight in water lost, death becomes likely. The heart cannot pump the thick blood enough to push oxygen around the body and the kidney cannot clean the blood. The toxic waste that the kidneys usually remove builds up to a fatal level and we die of kidney failure. An average-sized person can probably go three or four days before this happens, so long as the climate isn’t hot.
So, why do we get a headache when we are dehydrated? As the brain begins to shrink, it pulls away from the sides of the skull and this is where the headache comes from. It is a general headache that is not located in one part of the brain. The brain doesn’t have any pain receptors, but when it shrinks and pulls away from the skull it puts pressure on the nerves that are there, causing pain. It is easy to treat if you have access to water. Replacing the water you have lost will refill the brain, which will return to its original shape and the headache will dissipate. However, if you don’t have access to water, it can lead to seizures and, ultimately, death.
Living in a developed country with easy access to water, I forget that dehydration is one of the biggest killers in the world. Why? Because of diarrhea. Diarrhea kills 2,195 children every day, which is more than any other cause of death. When people get diarrhea, they lose fluids. In a developed country, it is easy to replace the fluid, but it is not so easy in poorer countries, and an easily preventable disease ends up killing so many people. So, next time you have a dehydration headache, drink some more water, and remember how lucky you are. And this is what I learned today.
Sources
https://www.bbc.com/future/article/20201016-why-we-cant-survive-without-water
https://my.clevelandclinic.org/health/diseases/21517-dehydration-headache
https://coastaluc.com/health-blog/drink-up-this-summer-a-quick-guide-to-the-stages-of-dehydration/
https://www.sciencealert.com/here-s-what-happens-to-your-body-when-you-re-dehydrated
https://www.usgs.gov/special-topics/water-science-school/science/water-you-water-and-human-body
https://www.popsci.com/dehydration-death-thirst-water/
https://my.clevelandclinic.org/health/diseases/21517-dehydration-headache
https://www.cdc.gov/healthywater/pdf/global/programs/globaldiarrhea508c.pdf