Why does a drink of water quench your thirst so fast?
― Before the water reaches your blood, your throat tells the brain how much you drank
Thirst starts when the brain senses that your blood has become slightly more concentrated. But drinking water takes more than ten minutes to enter the blood. Even so, thirst fades as soon as you drink. The body does not wait for the result. It estimates that "this much is enough."
You have walked a long way in the heat, and your throat is parched. You pour barley tea from the fridge into a glass and drink it down in one go.
By the time the glass is empty, that fierce thirst has already slipped away. One or two glasses, and you really do feel "that's enough."
But think about it. The water must still be in your stomach. It has not reached every corner of your body.
Thirst fades fast for two reasons
When your body loses water, the salts in your blood become a little more concentrated. The brain has a spot that watches this concentration, and a change of just 1–2% is said to trigger thirst.
The act of swallowing, plus the amount and saltiness of the water reaching the gut, is passed to the brain at once. The brain uses this to stop thirst without waiting for the blood to change.
In other words, the system that starts thirst and the system that stops it look at different information. Let's take them in turn.
The brain measures blood concentration to create thirst
When you sweat, water leaves your body. Sweat is more dilute than blood, so mostly water goes out. The blood left behind becomes slightly more concentrated in salts and other substances.
Deep in the brain there is a small region that checks blood concentration directly. Normally the brain is shielded so that substances in the blood cannot get in. This one spot has thin protection, so it can touch the blood and measure how concentrated it is.
When the concentration rises, this spot decides "we are short of water." It creates thirst and, at the same time, signals the kidneys: "make urine more concentrated and save water."
If a 60 kg person loses three glasses' worth (0.6 litres) of water in sweat, the blood concentration rises by nearly 2%. The calculation is in the fold-out at the end. That is just about the change that makes you feel strongly thirsty.
Thirst is stopped by an advance report on "how much you drank"
So why does thirst vanish before the blood recovers? Look at Figure 1. The gap between the dotted and solid lines is the star of this article.
In experiments on mice, researchers recorded the activity of the brain neurons that create thirst. These neurons went quiet within seconds of the mouse starting to drink. At that moment, the blood concentration had not changed at all.
The first messengers are the mouth and throat. How many times you swallowed, and how much cold water passed through. This becomes a quick bulletin: "roughly this much has been drunk."
Next, the stomach and intestines check the amount and saltiness of the liquid arriving. With plain water, thirst stays switched off. But when mice were given salt water, the neurons that had been quieted by the mouth-and-throat bulletin started firing again.
This "stop on the bulletin, check the answer later" system matters. If thirst lasted until the blood recovered, people would keep drinking for more than ten minutes. They would end up drinking several times more than they need.
It has long been known that in animals whose water leaks out partway down the gullet, the drinker feels satisfied for a short while. But the water never reaches the stomach, so before long they start drinking again. The mouth's bulletin lasts only when it is paired with the gut's check.
In older people, thirst is known to be weaker even when the blood becomes more concentrated. Waiting until you feel thirsty tends to mean drinking too late. It is safer to set times and drink a little at a time.
Summary
Thirst begins when the brain senses that the blood has become slightly more concentrated. But when it comes to stopping thirst, the body does not wait for the blood's result. It stops thirst ahead of time, based on reports of "how much you just drank" from the mouth, throat and gut. That is why a drink of water brings relief even before the water has spread through your body.
The blood starts thirst; the throat stops it.
The body estimates "enough" before the water arrives.
For more on how bodies handle water, read why camels can walk for days without drinking and why sea fish can drink salt water and be fine, and you will see how different animals solve the problem differently. For the story of why you need the toilet more when it is cold, see this article.
- When you are thirsty after exercise or a bath, drink one glass of water while watching a clock.
- Right after finishing, then 1 minute and 5 minutes later, write down how strong your thirst is on a scale of 10.
- On another day, drink the same amount slowly over 10 minutes, and compare how the score falls.
Even when you drink it all at once, your score should drop sharply right after you finish. Please skip this observation if you are unwell or if a doctor has given you instructions about how much to drink.
For those who want to know more ― terms, formulas and links to textbooksWe state clearly which level each topic belongs to, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology
- High school+Advanced high-school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (physiology, neuroscience)
- ResearchNot taught even at university as settled fact; being studied by researchers right now
Middle schoolTerms: this phenomenon has names
- Osmotic pressure: A quantity that expresses how concentrated the dissolved substances in water are. A rise in the osmotic pressure of blood triggers thirst.
- Thirst (medical term: kōkatsu): The medical word for the feeling of thirst.
- Homeostasis: The body's work of keeping its internal conditions within a fixed range. Regulation of blood concentration is a classic example.
Middle schoolHigh schoolCheck with a formula: how much more concentrated does blood become when sweat removes water?
We calculate the change in blood concentration when a 60 kg person loses 0.6 litres of water in sweat. Sweat is much more dilute than blood, so here we treat it as "only water was lost."
| In symbols | C ＝ n ÷ V |
| In words | Concentration ＝ amount dissolved ÷ amount of water |
| Where it comes from | It is simply the definition of concentration. If the dissolved amount (n) stays the same and only the water (V) shrinks, the concentration rises by the inverse of the fraction of water left |
| C | Concentration of the body's water (a guide to osmotic pressure) |
| n | Amount of salts and so on dissolved in body water (treated as constant here) |
| V | Amount of water in the body (in litres) |
| Body weight | 60 kilograms |
| Share of body weight that is water (adult guide) | 0.6 |
| Water lost in sweat | 0.6 litres |
| Usual osmotic pressure of blood (guide) | about 290 milliosmoles per kilogram |
| Rise in concentration at which thirst starts | said to be 1–2% |
| Time for half the water you drink to enter the blood | about 12 minutes (said to be a little over 10) |
| Water in the body | 60 × 0.6 ＝ 36 litres |
| Water after sweating | 36 − 0.6 ＝ 35.4 litres |
| How many times more concentrated (n is the same, so it is the ratio of water amounts) | 36 ÷ 35.4 ≒ 1.017 times |
| Osmotic pressure after sweating | 290 × 1.017 ≒ 295 |
| Difference between thirst vanishing (about 1 minute) and blood recovering halfway | 12 ÷ 1 ＝ 12 times |
Sweating out three glasses' worth raises blood concentration by about 1.7%. That falls right in the 1–2% range where thirst begins. And thirst vanishes about 12 times faster than the blood recovers halfway. The numbers also support the view that the body is not waiting for the blood's result.
High schoolHigh school+Two-track control: thirst and urine
High schoolWhen blood osmotic pressure rises, the hypothalamus in the brain senses it, and the posterior pituitary releases a hormone called vasopressin. Vasopressin increases water reabsorption in the kidneys and makes urine more concentrated. This is the side of control that "reduces the water going out."
High school+Thirst is the side of control that "increases the water coming in." Urine control alone cannot win back lost water, so thirst is needed. Interestingly, observations in people have reported that once drinking starts, vasopressin also falls ahead of any change in the blood.
UniversityThe subfornical organ and predictive control
The sensors for blood osmotic pressure are thought to be the "circumventricular organs," which lack the blood–brain barrier, such as the subfornical organ and the organum vasculosum of the lamina terminalis. Neurons here raise their activity when osmotic pressure rises and create thirst. A 2016 study directly recorded in mice that these neurons go quiet within seconds of drinking starting. It is drawing attention as an example of predictive (feedforward) control: instead of waiting for the body to drift off course and then correcting it, the system estimates the outcome of an action in advance and stops.
📖 Derivations and further reading: Osmotic pressure (Wikipedia, in Japanese) ／ The paper showing that thirst neurons anticipate the outcome of drinking (in English)
ResearchWhat is still not clear
- How does the gut measure saltiness? It seems that concentration is sensed by blood vessels and nerves near the intestine, but which cells do this is still being investigated.
- Why does cold water quench thirst more easily? It has been suggested that cold-sensing nerves in the mouth contribute to stopping thirst, but the detailed mechanism is not yet known.
- Why does thirst dull with age? Whether the sensor becomes less sensitive or the stopping signal becomes stronger, there is no clear answer.
So the content of this article, too, is "an explanation within what is known today." In particular, many of the nerve recordings are results from mice, and whether the speed is the same in people is still being checked.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science year 2 "Structure and function of animal bodies"; year 1 "Concentration of solutions" | Losing water in sweat makes blood more concentrated |
| High school | Biology "Maintaining the internal environment (hormones and the autonomic nervous system)" | Vasopressin and water reabsorption in the kidneys |
| High school+ | Advanced biology (regulation of behaviour) | Two-track control by thirst and urine |
| University | Physiology and neuroscience (regulation of body-fluid osmotic pressure) | The subfornical organ and predictive control |
| Research | Neuroscience of feeding and drinking behaviour | Salt sensors in the gut; ageing and thirst |
| ― | Links to daily life | For elderly family members, set times to drink rather than waiting for thirst |
- Zimmerman CA, et al. (2016) Thirst neurons anticipate the homeostatic consequences of eating and drinking. Nature 537
- Augustine V, et al. (2019) Temporally and spatially distinct thirst satiation signals. Neuron 103
- Péronnet F, et al. (2012) Pharmacokinetic analysis of absorption, distribution and disappearance of ingested water labeled with D2O in humans. European Journal of Applied Physiology 112
- Thirst (口渇) ― Wikipedia (Japanese)
- Ozawa S. and Fukuda Y. (chief eds.), Standard Physiology (標準生理学), Igaku-Shoin (医学書院), chapter on regulation of body fluids
* This article is a general-audience science explainer. The figures given are guides and rough estimates meant to help you understand the mechanism. If you have an illness or other restriction on fluid intake, follow your doctor's instructions.