How does the body measure dehydration?

How does the body measure dehydration?

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What, physically, does the body do to measure it? I assume it's measured by how how turgid or plasmolysed certain 'sample cells' are, or water concentration in the blood,

  • What is the way they use to measure it?
  • What do the receptor cells that do the measuring actually do that is variable with water concentration, etc.

From Wikipedia entry on thirst:

In the mammalian brain, the posterior surface of the hypothalamus forms the front wall of the third ventricle (a cerebrospinal fluid-filled cavity) and clusters of cells (osmoreceptors) on this surface, notably in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), signal this cellular dehydration to other parts of the brain, and thirst is experienced. Destruction of this part of the hypothalamus in humans and other animals results in partial or total loss of desire to drink even with extremely high salt concentration in the extracellular fluids.1

The entry on osmoreceptor says in part:

When the osmotic pressure of blood changes (i.e. it is more or less dilute), water diffusion into and out of the osmoreceptor cells changes. That is, they expand when the blood plasma is more dilute and contract with higher concentration.

It also describes how the kidney measures chlorine anion flow through some nephrons, which triggers a cascade of messenger molecules resulting in increased blood levels of the hormone angiotensin, which also results in thirst messages originating in the hypothalamus.

1Derek A. Denton (8 June 2006). The primordial emotions: the dawning of consciousness. Oxford University Press. pp. 118-19. ISBN 978-0-19-920314-7.

2Walter F., PhD. Boron (2005). Medical Physiology: A Cellular And Molecular Approach. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 872

Dehydration results in decreased blood volume (hypovolemia) and in slight decrease of blood pressure. The body recognises dehydration by these ways:

  1. Baroreceptors in the aorta detect the drop of blood pressure - this results in activation of the sympathetic nerves and the release of adrenaline from the adrenal medulla - this leads to constriction of the arteries ad increased heart rate.
  2. Volume receptors in the heart atria detect hypovolemia - this result in the secretion of the antidiuretic hormone from the pituitary gland - this results in water retention in the kidneys.
  3. Osmoreceptors in the hypothalamus detect increased blood osmolality - this triggers the release of the hormone ADH.
  4. Juxtaglomerular apparatus in the kidneys detect decreased perfusion, what results in the renin-angiotensin-aldosterone release cascade, which results in water retention in the kidneys.

All these mechanisms are activated in the compensated hypovolemic shock.


Monitoring hydration status before, during and after exercise is essential for both performance and safety during physical activity. Maintaining an appropriate level of hydration (a euhydrated state) has been shown to increase performance (aerobic exercise, anaerobic exercise, strength, power), allows athletes to exercise at lower body temperatures and heart rates, improves cognitive function, and has been shown to enhance immunological function.

Dehydration is influenced by exercise intensity, environmental conditions (temperature and humidity), and availability of fluids during exercise. Once an individual loses 2% of their body mass from fluid losses impairments in performance are noticeable and these impairments become more extreme with greater levels of dehydration. Also, it has been shown that with increasing levels of dehydration body temperature and heart rate increases over and above the levels of someone who is hydrated, which can increase the risk of heat illness in dehydrated exercising individuals.

An appropriate hydration strategy involves athletes to begin exercise hydrated, minimize fluid losses during exercise, and then replace fluid losses after exercise. Hydration needs are individualistic, so athletes should be aware of their own hydration needs to maximize performance and safety.

Possible Causes

What causes dehydration?

Dehydration happens when you don’t drink enough water, or when you lose water quickly through, for example, sweating, vomiting and/or diarrhea. Certain medications such as diuretics (water pills) can result in increased urination and dehydration.

Who’s at risk of becoming dehydrated?

Anyone can become dehydrated if they don’t take care of themselves and drink water. However, infants and children, especially when they’re sick, are at a higher risk because they may be unable to communicate that they’re thirsty. Monitor the amount of fluids your kids take in.

Older adults are also at a higher risk. Their body’s fluid reserves shrink and their body’s ability to tell them they’re thirsty doesn’t work as effectively. This means they don’t carry as much water in their bodies and they can’t tell as easily when they’re thirsty. If you’re a caretaker of an elderly individual, especially one who may have memory problems, offer them drinks frequently. Even if they’re enduring an uncomfortable infection like a UTI (urinary tract infection), they still need to consume liquids.

What are the signs of dehydration? What does dehydration feel like?

If you suspect that you or someone else is severely dehydrated, seek immediate medical attention.

Signs of dehydration include:

  • Headache, delirium, confusion.
  • Tiredness (fatigue).
  • Dizziness, weakness, light-headedness.
  • Dry mouth and/or a dry cough.
  • High heart rate but low blood pressure.
  • Loss of appetite but maybe craving sugar.
  • Flushed (red) skin. Swollen feet. Muscle cramps.
  • Heat intolerance, or chills.
  • Constipation.
  • Dark-colored pee (urine). Your pee should be a pale clear color.

The best way to beat dehydration is to drink before you get thirsty. If you wait until after you're thirsty, you're already dehydrated.

In what other ways does dehydration affect me?

Dehydration does more than you might expect. If affects you not only physically (note the signs stated above), but mentally and emotionally as well. If you’re dehydrated, you may feel:

Note that these symptoms may be worse in someone who has dementia.

How does dehydration affect the brain?

Severe hydration shrinks the blood vessels in the brain. When there aren’t high enough fluid levels in your brain, that affects your memory and coordination.

How does dehydration affect the heart? Can dehydration cause high blood pressure?

Your heart has to work harder when there’s less water in your blood.

How does dehydration affect the kidneys?

The average person urinates (pees) about six or seven times a day. If you’re dehydrated, you may urinate less. This is because less water in your blood causes your kidneys to hold on to the urine.

Does dehydration cause cramping?

Loss of electrolytes, like sodium and potassium, can cause cramping. They’re expelled through perspiration (sweating). Drink water, but also a sports drink to replenish your electrolytes if your fluid losses are extensive from sweating, vomiting or diarrhea.

Can medications cause dehydration?

Diuretic medications, which are prescribed to treat heart failure and high blood pressure, can increase your risk of dehydration.

Can dehydration cause shortness of breath?

Shortness of breath is not a symptom of dehydration. However, it may go alongside dehydration. For example, you might be playing a sport outside in the hot sun and get dehydrated from lack of water and also feel short of breath from all the activity.

Stage 2: Fainting

Stage 2: Fainting Michael Brandon Myers

Water Lost: Four percent of body weight. For a 170-pound person, that’s 7 pounds. This is roughly equivalent to riding a bike for three hours in extreme heat without rehydrating, or going without water for two days.

Effects: Your blood is so concentrated that the resulting decrease in blood flow makes your skin shrivel. Your blood pressure drops, making you prone to fainting. You’ve basically stopped sweating, and without this coolant, you start to overheat.


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Dehydration, in food processing, means by which many types of food can be preserved for indefinite periods by extracting the moisture, thereby inhibiting the growth of microorganisms. Dehydration is one of the oldest methods of food preservation and was used by prehistoric peoples in sun-drying seeds. The North American Indians preserved meat by sun-drying slices, the Chinese dried eggs, and the Japanese dried fish and rice.

Hot-air dehydration was developed in France in 1795, enabling the commercial production of dehydrated food products, particularly spaghetti and other starch products. Modern dehydration techniques have been largely stimulated by the advantages dehydration gives in compactness on the average, dehydrated food has about 1 /15 the bulk of the original or reconstituted product. The need to transport large shipments of food over great distances during World War II provided much of the stimulus to perfect dehydration processes. The advantages of reduced bulk later came to be appreciated by campers and backpackers and also by relief agencies that provide food in times of emergency and disaster.

Dehydration equipment varies in form with different food products and includes tunnel driers, kiln driers, cabinet driers, vacuum driers, and other forms. Compact equipment suitable for home use is also available. A basic aim of design is to shorten the drying time, which helps retain the basic character of the food product. Drying under vacuum is especially beneficial to fruits and vegetables. Freeze-drying benefits heat-sensitive products by dehydrating in the frozen state without intermediate thaw. Freeze-drying of meat yields a product of excellent stability, which on rehydration closely resembles fresh meat.

The dairy industry is one of the largest processors of dehydrated food, producing quantities of whole milk, skim milk, buttermilk, and eggs. Many dairy products are spray dried—that is, atomized into a fine mist that is brought into contact with hot air, causing an almost instant removal of moisture content. See also food preservation.

Dehydration Reaction Definition in Chemistry

A dehydration reaction is a chemical reaction between two compounds where one of the products is water. For example, two monomers may react where a hydrogen (H) from one monomer binds to a hydroxyl group (OH) from the other monomer to form a dimer and a water molecule (H2O). The hydroxyl group is a poor leaving group, so Bronsted acid catalysts may be used to help to protonate the hydroxyl to form -OH2 + . The reverse reaction, where water combines with hydroxyl groups, is termed hydrolysis or a hydration reaction.

Chemicals commonly used as dehydrating agents include concentrated phosphoric acid, concentrated sulfuric acid, hot ceramic and hot aluminum oxide.

A dehydration reaction is the same as a dehydration synthesis. A dehydration reaction may also be known as a condensation reaction, but more properly, a dehydration reaction is a specific type of condensation reaction.

Researchers use microwaves to measure signs of dehydration

Image showing microwave sensor technology system measuring drop of blood. Credit: Queen Mary, University of London

London and the National Physical Laboratory (NPL) has successfully used microwaves to measure blood-based molecules known to be influenced by dehydration.

The researchers are now looking to develop the technology into a wearable device that could be used by athletes, or in healthcare settings, to monitor hydration in real time.

For the study, the research team utilized a microwave-based measurement system, developed by NPL, to examine changes to salts and proteins such as hematocrit and albumin, which are present in the blood and influenced by dehydration.

They found they were able to accurately measure the levels of these biomarkers using dielectric sensor technology that recorded the microwave absorption through different blood solutions.

Dr. Tina Chowdhury, Senior Lecturer in Regenerative Medicine at Queen Mary, said: "The next step is to turn the microwave sensor into a non-invasive, wearable device that will track the dehydration biomarkers associated in real time. We hope the devices will be a portable and reliable way for athletes to detect dehydration, and in future, we will need to investigate the suitability our current prototype device to ensure it works for athletes during moderate to strenuous exercise.

"However the applications of this technology aren't just limited to the sporting community. The ability to track important molecules and proteins in human blood, which are well-known signs of dehydration, is also of importance for healthcare settings."

Using microwaves to monitor dehydration

Dehydration is a continuing problem in the healthcare sector and can lead to multiple organ failure, and even death in patients. The NHS estimates costs of around £13 billion in the UK due to malnutrition, which causes dehydration.

Current methods used to assess hydration can be time-consuming, often involving of the preparation and analysis of urine or blood samples that are unable to be re-used for further testing.

Other non-invasive devices that actively measure hydration have been proposed however, most depend on the use of chemical sensors to detect molecules in bodily fluids such as sweat or plasma. This type of approach is only able to provide an estimate of hydration levels and can be influenced by the physical attributes and daily behaviors of the individual.

One of the benefits of using microwave dielectrics is the ability to measure multiple factors simultaneously from a single sample.

Dr. Rob Donnan, Reader in Terahertz Engineering at Queen Mary, said: "Adapting conventional microwave instrumentation to biological study requires careful thought for meaningful data to be obtained. This work has begun to do this and we look forward to developments that may lead to a viable patch sensor for gauging how hydrated a person is."

Developing a simple, portable device

The research builds on previous work from Dr. Richard Dudley and Dr. Mira Naftaly at NPL, who developed a wearable device called Hydration Sensor, which uses microwaves to monitor hydration by measuring water levels in the body.

The team will use the Hydration Sensor technology, which attaches via the ear lobe, to transform the microwave sensor system into a simple, portable device that can track selected biomarkers of dehydration over time.

Dr. Richard Dudley, Science Area Leader at NPL, said: "The development of our on-body hydration monitoring device has been held back by two challenges a quantifiable relationship between blood and microwave measurement, and a measurement device that has good contact to a subject and remains stable for at least an hour.

"Working with Queen Mary has helped us address the first and we are now confidently working towards engineering a solution for the second. Our ambition is to see our 'hydration devices' sold alongside heart rate monitors and blood pressure systems."

Prevention for Babies and Young Children

Children lose fluids and electrolytes just like adults do, so make sure your child has access to plenty of water and other fluids, especially if they’re very physically active or if it’s a warm day. And make sure your child eats plenty of fruits and veggies -- that contain lots of water.

If your infant or young child is dehydrated, you can try the “baby” version of a sports drink, like Pedialyte or Equalyte. If an over-the-counter solution isn’t available, give them small sips of water. Don’t try to make up your own homemade version. Be sure to check with you pediatrician if your child doesn’t get better quickly.

Dehydration and blood glucose levels

If our blood glucose levels are higher than they should be for prolonged periods of time, our kidneys will attempt to remove some of the excess glucose from the blood and excrete this as urine.

Whilst the kidneys filter the blood in this way, water will also be removed from the blood and will need replenishing. This is why we tend to have increased thirst when our blood glucose levels run too high

If we drink water, we can help to rehydrate the blood. The other method the body uses is to draw on other available sources of water from within the body, such as saliva, tears and taking stored water from cells of the body.

This is why we may experience a dry mouth and dry eyes when our blood glucose levels are high.

If we do not have access to drink water, the body will find it difficult to pass glucose out of the blood via urine and can result in further dehydration as the body seeks to find water from our body’s cells.

Effects of hydration and dehydration on body composition analysis: a comparative study of bioelectric impedance analysis and hydrodensitometry

Since 1983, bioelectric impedance has been researched with respect to its validity and reliability in the determination of body composition. It continues to be compared to hydrostatic weighing, the anthropometric "gold standard". This study was designed to investigate the relationship between bioelectric impedance analysis (BIA) and hydrodensitometry (HW) under three conditions: control, hydration and dehydration. Caucasian males (aged 18-44 years) served as subjects (n = 10). Body composition was determined by BIA and HW before intervention, 30 minutes post-hydration, and following a combination of exercise and sitting in a steam room to decrease body weight by two to four percent (mean = 2.81%). Statistical treatment by two-way analysis of variance for repeated measures revealed that although there were no significant differences between the two techniques of body composition determination under any of the three conditions, there was a statistically significant decrease in percent body fat determined in the dehydrated state as compared to the control and hydrated conditions. Recommendations include the determination of hydration state prior to engaging in body composition analysis by either method.


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