When your body does not get enough water and other fluids, you can develop dehydration. Water is essential for nearly every body system: it helps maintain blood volume, regulate temperature, transport nutrients, remove waste, lubricate joints, and support normal brain, heart, kidney, and muscle function.
What happens during dehydration?
1. Your blood volume decreases
A large proportion of blood is water. When you lose more fluid than you replace, the amount of fluid circulating in your bloodstream falls.
As a result:
- Your heart may beat faster to maintain circulation.
- Blood pressure can fall, especially when you stand up.
- You may feel dizzy, weak, or light-headed.
- Your hands and feet may feel cold because the body prioritizes blood flow to vital organs.
With severe dehydration, circulation can become dangerously inadequate.
2. Your brain is affected
The brain is particularly sensitive to changes in hydration.
Even relatively mild dehydration can contribute to:
- Headache
- Difficulty concentrating
- Tiredness
- Irritability
- Poor mood
- Reduced alertness
- Dizziness
- Slower physical and mental performance
More severe dehydration can cause confusion, unusual behavior, extreme sleepiness, or loss of consciousness.
3. Your kidneys conserve water
Your kidneys normally filter your blood and produce urine to remove waste.
When you are dehydrated, the body releases more antidiuretic hormone (ADH), which tells the kidneys to conserve water. Consequently:
- You urinate less frequently.
- Your urine becomes darker and more concentrated.
- Your kidneys retain more water.
This is an important short-term survival mechanism, but prolonged or severe dehydration can put significant stress on the kidneys.
4. Your mouth and skin become dry
As the body tries to preserve its remaining water, less fluid is available for saliva and other secretions.
You may experience:
- Dry mouth
- Sticky or thick saliva
- Dry or cracked lips
- Dry-feeling skin
- Reduced sweating in significant dehydration
Interestingly, skin dryness by itself isn’t a reliable way to diagnose dehydration, because many other factors can cause dry skin.
5. Your body has trouble controlling temperature
Sweating is one of the body’s main cooling mechanisms.
When you’re dehydrated, your body may have less fluid available for sweating. This makes it harder to release heat, particularly during:
- Exercise
- Hot weather
- Fever
- Physically demanding work
This can increase the risk of heat exhaustion and, in extreme circumstances, heat stroke, which is a medical emergency.
6. Your muscles can be affected
Muscles require adequate fluid and electrolytes—such as sodium and potassium—to function normally.
Dehydration can contribute to:
- Muscle fatigue
- Weakness
- Reduced exercise performance
- Muscle cramps
- Reduced endurance
Heavy sweating can cause both water and electrolyte losses, so replacing only water may not always be sufficient after prolonged, intense sweating.
7. Your digestion can slow down
Water helps keep stool soft and supports normal movement through the digestive tract.
Insufficient fluid intake can therefore contribute to:
- Hard stools
- Constipation
- Difficulty passing stool
- Abdominal discomfort
Increasing fluid intake can help when inadequate hydration is contributing to constipation, although constipation has many possible causes.
What happens as dehydration becomes more severe?
Dehydration exists on a spectrum.
| Level | Possible effects |
|---|---|
| Mild | Thirst, dry mouth, darker urine, tiredness |
| Moderate | Significant weakness, headache, dizziness, reduced urination, rapid heartbeat |
| Severe | Very little/no urination, confusion, fainting, very rapid heartbeat, low blood pressure, inability to regulate body temperature |
| Life-threatening | Shock, organ dysfunction, loss of consciousness, potentially death |
Thirst is not always an early or reliable warning, particularly in older adults, young children, and some people with medical conditions.
Why electrolytes matter
Hydration isn’t simply about water. Your body also needs electrolytes, particularly sodium, potassium, and chloride, which help regulate:
- Fluid balance
- Nerve signaling
- Muscle contraction
- Heart function
For ordinary daily activities, food and normal beverages generally provide the necessary electrolytes. But during prolonged vomiting or diarrhea, heavy sweating, endurance exercise, or certain illnesses, substantial electrolyte losses can occur.
This is why oral rehydration solutions can be particularly useful for significant fluid loss from diarrhea or vomiting: they contain an appropriate balance of water, salts, and glucose that promotes intestinal absorption.
What can cause dehydration?
Common causes include:
- Not drinking enough fluids
- Heavy sweating
- Hot or humid weather
- Intense exercise
- Fever
- Vomiting
- Diarrhea
- Excessive urination
- Certain medications, including some diuretics
- Illnesses that increase fluid losses
- Difficulty accessing or drinking fluids
Alcohol can also contribute to fluid loss and impaired judgment about drinking enough water.
Who is at greater risk?
Some people can become dehydrated more easily, including:
- Infants and young children
- Older adults
- People exercising or working in heat
- People with vomiting or diarrhea
- People with certain kidney, heart, or metabolic conditions
- People taking medications that increase fluid loss
- People who cannot independently obtain or drink fluids
How much water do you need?
There isn’t one universal amount that every person should drink. Fluid requirements vary according to body size, diet, activity, climate, pregnancy, illness, medications, and other factors.
A useful approach is to drink regularly throughout the day and respond to thirst, while recognizing that thirst may be less reliable in some circumstances. Your urine color can also provide a rough indication: consistently very dark urine and infrequent urination can suggest that you’re not getting enough fluid, although urine color is affected by medications, supplements, foods, and medical conditions.
When dehydration becomes an emergency
Seek urgent medical attention if someone has signs such as:
- Confusion or altered mental state
- Fainting or inability to stay awake
- Very little or no urine
- Severe weakness
- Rapid breathing or a very rapid heartbeat
- Inability to keep fluids down
- Signs of shock, such as extreme weakness, clammy skin, or very low blood pressure
- Severe overheating or suspected heat stroke
For heat stroke, especially when there is confusion or loss of consciousness, emergency medical care is required rather than simply giving the person water.
The key idea
Your body constantly loses water through urine, sweat, breathing, and stool. If losses exceed replacement, the body first compensates by increasing thirst, conserving water through the kidneys, and reducing urine output. As dehydration progresses, blood circulation, brain function, temperature regulation, kidney function, digestion, and muscle performance can all deteriorate. Severe dehydration can ultimately lead to shock, organ damage, and death.
If you’d like, I can also explain the stages of dehydration from the first few hours through severe dehydration, including what happens to the brain, kidneys, blood pressure, heart, and cells step by step.