The Difference Between Dehydration and Thirsty .

If you think about being thirsty at all, it seems like a fairly simple thought process: Find water. Drink it. Move on.
But, in fact, there is something rather profound going on as you take that long, refreshing drink after a run or a hot day in the garden.

As you become dehydrated, there is less water in your blood, and neurons in your brain send out the word that it is time to look for water.

Then, once you take a drink, you feel almost instantly satisfied. But if this is obvious, it is also mysterious.
You are not pouring water directly into your bloodstream, after all. It will take at least 10 or 15 minutes, maybe longer, for the water in your stomach to make its way into the blood. And yet somehow, the brain knows.

Sometimes that process is not as straightforward as it should be: People with a syndrome called polydipsia feel excessive thirst and drink enormous quantities of water. That can be dangerous, because if the blood is diluted too much, a person can die - a victim of water intoxication.

As neuroscientists ponder how and why we thirst, researchers at the California Institute of Technology (Caltech) have shed light on one small corner of the problem.

Interested in how the brain keeps track of what the body is drinking, they have identified a set of neurons that receive messages as thirsty mice gulp down water.

Passed around in the brain's thirst centres, these messages seem to be behind the sensation of swift satisfaction that comes after a drink and also suggest that it is not just what is drunk, but how it is slurped down as well, that affects the brain. If the circuits work the same way in people, it may be key to understanding the neuroscience of what happens as we feel thirsty.

In the last few years, biologists have been mapping the neurons within an area in the brain that regulates thirst, said Dr Yuki Oka, a professor at Caltech and senior author of the paper, which was published in Nature. Cells in this region had been observed going quiet after an animal had water, but it was not clear exactly why.

A graduate student in Dr Oka's laboratory, Mr Vineet Augustine, did a series of experiments with mice that had been genetically modified to make tracing the connections between their neurons easier. In these experiments, when a neuron caused another neuron to turn off, it got tagged, leaving a trail of breadcrumbs through the brain.

What Mr Augustine found was certain neurons in a region called the median preoptic nucleus were responsible for telling other cells in the thirst centre that drinking was occurring. Further experiments showed that mice without functioning versions of these neurons drank twice as much as normal mice.

In the opposite scenario, when the cells were artificially activated, even dehydrated mice did not experience thirst.
Intriguingly, what these cells are responding to is not the presence of water itself, Mr Augustine said.

The researchers discovered that letting a mouse take big gulps of water would spur the neurons into action. But giving it water in a gel form, which had to be chewed before it could be swallowed, did not. Neither did providing water in tiny, two-second-long sips, even when the animals consumed the same total amount of water. In fact, giving the mice oil to drink had just the same effect on the neurons as gulping water.

"That indicated to us that it's probably the speed - the speed of this ingestion - to which these neurons are responding," Mr Augustine said.

Apparently, a series of quick swallows is an evolutionarily acceptable shorthand for drinking water - reliable enough for the body to use it as a way to signal when enough has been consumed.

Aside from the spectre of water intoxication, there are good reasons to drink only the minimum amount necessary. When an animal lowers its head to drink, Dr Oka speculated, it is in quite a vulnerable position. "If you double the time of ingestion, that should double the risk of being prey," he said.
The researchers are now hoping to investigate other ways the body monitors water intake.

For instance, they want to see whether there are sensors in the gut that keep the brain apprised of the arrival of water. It seems likely, Mr Augustine said, that the brain gets some notice and adjusts accordingly.
NewYorkTIMES

Health Check: what happens to your body when you’re dehydrated?

Water is essential for human life. It accounts for for 50-70% of our body weight and is crucial for most bodily functions.
Any deficit in normal body water – through dehydration, sickness, exercise or heat stress – can make us feel rotten. First we feel thirsty and fatigued, and may develop a mild headache. This eventually gives way to grumpiness, and mental and physical decline.

We continually lose water via our breath, urine, faeces and skin. Most healthy people regulate their body’s water level remarkably well via eating and drinking, and are guided by appetite and thirst. But this is more difficult for infants, the sick, the elderly, athletes, and those with strenuous physical occupations, especially in the heat.

What happens when you dehydrate?

By the time you feel thirsty your body is already dehydrated; our thirst mechanism lags behind our actual level of hydration.

Research shows that as little as 1% dehydration negatively affects your mood, attention, memory and motor coordination. Data in humans is lacking and contradictory, but it appears that brain tissue fluid decreases with dehydration, thus reducing brain volume and temporarily affecting cell function.

As you “lose” body water without replacing it, your blood becomes more concentrated and, at a point, this triggers your kidneys to retain water. The result: you urinate less.
The thicker and more concentrated your blood becomes, the harder it is for your cardiovascular system to compensate by increasing heart rate to maintain blood pressure.

When your dehydrated body is “pushed” – such as when exercising or faced with heat stress – the risk of exhaustion or collapse increases. This can cause you to faint, for instance, when you stand up too quickly.

Less water also hampers the body’s attempts at regulating temperature, which can cause hyperthermia (a body temperature greatly above normal).

At a cellular level, “shrinkage” occurs as water is effectively borrowed to maintain other stores, such as the blood. The brain senses this and triggers an increased sensation of thirst.

How much should I drink?

Normal water needs range drastically due to a number of factors, such as body composition, metabolism, diet, climate and clothing.

Surprisingly, the first official recommendation about water intake was made as recently as 2004. According to the Institute of Medicine, the adequate water intake for adult men and women is 3.7 and 2.7 litres per day, respectively.
Around 80% of total daily water should be obtained from any beverage (including water, caffeinated drinks and alcohol!) and the remaining 20% from food.

But of course, this is just a rough guide. Here’s how to monitor your own hydration:

1. Track your body weight and stay within 1% of your normal baseline. You can work out your baseline by averaging your weight (just out of bed, before breakfast) on three consecutive mornings.
2. Monitor your urine. You should be urinating regularly (more than three to four times per day) and it should be a pale straw or light yellow colour without strong odour. If less frequent, darker colour or too pungent, then drink more fluids.
3. Be conscious about drinking enough fluids. Your fluid consumption should prevent the perception of thirst.