Is there anything more satisfying than quenching your thirst? You wake up at night or finish a long workout, notice you are thirsty, and then have a big drink of water. It feels nice, and it's something that we often do without thinking. Why does quenching our thirst feel good, and why are we thirsty in the first place?
The answer is actually more complicated than you might think because the body experiences two types of thirst! Both "thirsts" feel the same way but are a sign of different imbalances in the body. Processing and triggering thirst is one of the many ways the body aims to maintain homeostasis, or balance, in the body.
Why Do We Get Thirsty?
Humans experience thirst because receptors in the body detect an imbalance of fluid and want to regain homeostasis. Whether we experience osmotic or hypovolemic thirst, the hypothalamus ultimately learns about it. This part of the brain also triggers our thirst response, encouraging us to grab some water.
Osmotic and hypovolemic are slightly different and are caused by different everyday activities. You might experience osmotic thirst after having a salty snack. At the end of a good night's sleep, you may wake up and experience hypovolemic thirst.
Let's learn about the difference between these two types of thirst.
What Is Osmotic Thirst?
Osmotic thirst, also known as intracellular dehydration, is the type of thirst we experience when our blood cells lose water. That loss of water puts us out of balance. To reach homeostasis, the hypothalamus sends out a message to the body and we begin to feel thirsty.
Before we talk about the osmotic thirst process, it's important to know what is in our bloodstream. Blood contains white and red blood cells, platelets, and plasma. A majority of plasma is water, but it also contains proteins, salts, and more. We need water in the blood, just like we need water in other parts of the body.
How do our blood cells lose water? Salt draws water out of the cell. A healthy amount of salt is beneficial for the body; we need sodium to perform a lot of functions. But taking in a lot of salt without replenishing it with water can cause an imbalance. Thirst is all about restoring that balance!
If our blood cells lose water, our brain's osmoreceptors will need to pick up on it to begin restoring balance. Osmoreceptors are sensory receptors located throughout the body and brain. When they detect that there is low water in the blood, they send a message out to the hypothalamus. The hypothalamus sends a message to release hormones; specifically, antidiuretic hormones (ADH). (ADH is also known as vasopressin.) ADH encourages the kidneys to retain water.
The hypothalamus also sends out the thirst response, and we feel thirsty.
There are many ways that the body loses water, and not all of it comes through the bloodstream. To understand how the body responds to that type of fluid loss, we must learn about hypovolemic thirst.
What is Hypovolemic Thirst?
Hypovolemic thirst, or volumetric thirst, is what we experience when we lose extracellular fluids (ECF). This includes a loss of blood. As the body recognizes this loss, it also sends a message to the hypothalamus and the thirst response is triggered.
We can lose extracellular fluids in many ways:
- Blood donation or blood loss
In other words, we are always at risk of losing extracellular fluids. And that is okay, as long as we continue to drink water and hydrate properly.
If we don't hydrate properly, the body gets to work. Neuroscientists believe that baroreceptors, as well as the hormone angiotensin II, pick up on the loss of ECF. Similar to osmoreceptors, they send a message to the hypothalamus and the thirst response is triggered.
Why Do We Get Thirsty at Night?
At night, we lose water and extracellular fluids in many ways (including ways you might not expect!) We are sleeping, so we cannot replenish our water intake. This causes an imbalance and often triggers hypovolemic thirst.
How do we lose water at night? Dry air may evaporate any moisture that is found in the nose and mouth, or on the body. Sweating causes us to lose water. If you have a glass of wine, smoke a cigarette, or exercise and sweat before you go to bed, your risk of waking up dehydrated is even higher. Don't forget to have a nice glass of water before you go to sleep.
How Do We Stop Feeling Thirsty?
Now you know why we experience thirst. Just from living life as a human being, you probably know that we don’t feel thirst forever. Quickly after having a glass of water, your thirst goes away and you continue on with your day. Why? Scientists can’t say for sure, but they have some guesses that might surprise you.
The body can’t detect when our thirst is quenched in the same way it detects that we have lost ECF or have lost water in the blood cells. When we drink water, it takes 15 minutes for the water to be absorbed properly. If we stopped feeling thirsty after 15 minutes of drinking water to quench our thirst, we would actually put ourselves in danger. (Yes, you can drink too much water - and it can be fatal!) So the body has other ways of detecting that our thirst has been quenched.
Swallowing and Thirst
One of the ways that neuroscientists have identified is swallowing. The mouth and throat have many receptors that can detect when we are eating and drinking. These receptors can send a message to the brain that we are quenching our thirst. Studies were done on mice as they drank that suggest these receptors could be why we no longer feel thirsty after drinking. Other studies suggest that biting onto cold metal can “quench thirst” briefly, as the receptors “confuse” the cold object for a cold glass of water.
If this still leaves you with a lot of questions, you are not alone. There are still no definitive answers on why we might feel more or less refreshed after drinking water or less hydrating beverages like beer or tea.
The Intestines and Thirst
Another place where the body checks up on our hydration is the intestine. Once we swallow water or any other liquid, it makes its way down to the stomach and the intestines. Throughout the gut, as liquids enter the intestine and later the liver, cells detect liquid and also how salty the liquid has made the intestines. All of this information goes to the brain as well, where the brain will respond accordingly. This could offer more insight into how we know that we have had hydrating water or less-hydrating liquids, but more research needs to be done on this part of the process before we are sure.
Dopamine and Thirst
We can rule out that communication from hormones like dopamine tells the brain that the body is fully hydrated. Dopamine is released after drinking a cool beverage. You could probably guess that already! The feeling of having a refreshing beer or glass of juice instantly perks you up. But studies show that dopamine has nothing to do with hydration in the body, but just the act of actually drinking.
There are still plenty of studies to be done on how we “turn off” the thirst button in our brains. But we do have many pieces of the puzzle, and know why we feel thirsty in the first place! The next time you feel yourself wanting a tall glass of water, you can thank your osmoreceptors, baroreceptors, and your hypothalamus.