Water in the Body: Why Hydration Matters
We know that we need water to live. But what actually happens to water after we drink it—and why can losing too much become dangerous?
Water helps maintain blood volume, regulate body temperature, transport nutrients, remove waste, lubricate joints, and support the brain, kidneys, muscles and other organs.
What Happens After You Drink Water?
Water passes through the stomach and is absorbed primarily through the small intestine into the bloodstream.
From there it is distributed throughout the body. Much of our water is inside our cells, with the rest in the blood and fluid surrounding cells.
The brain and kidneys work together to keep this water in balance. When the body begins to run short, we become thirsty and the brain releases more vasopressin (ADH). This tells the kidneys to conserve water, producing less and more concentrated urine.
How Do We Lose Water?
We continuously lose water through urine, bowel movements, perspiration and breathing. Usually these losses are easily replaced.
Dehydration becomes more likely with:
Diarrhea or vomiting
Fever
Heavy sweating or hot weather
Increased urination
Certain medications, particularly diuretics
Not drinking enough during illness
Vomiting and diarrhea are especially important because we can lose electrolytes such as sodium and potassium along with water. When losses are substantial, replacing water alone may not be enough. Oral rehydration solutions contain a balance of water, sodium and glucose that helps the intestines absorb fluid and replace losses. Severe dehydration may require medical treatment.
What Happens When We Become Dehydrated?
Dehydration occurs when the body loses more fluid than it takes in.
Early signs can include thirst, dry mouth, darker urine, fatigue, weakness, headache and dizziness.
More severe dehydration can lower blood volume and blood pressure and interfere with kidney and brain function. It can cause confusion, fainting and serious electrolyte abnormalities.
Why Does Aging Matter?
Older adults are particularly vulnerable because the sensation of thirst may become less reliable with age.
Medications can increase fluid loss, illness can reduce the desire to drink, and mobility problems can make getting fluids more difficult.
Dehydration may also contribute to dizziness, falls, low blood pressure and confusion or delirium in older people.
Dehydration and Illness
Dehydration is often a consequence of another medical problem. Gastrointestinal illness can cause rapid losses through vomiting and diarrhea. Fever increases water needs, poorly controlled diabetes can cause excessive urination, and diuretics increase the loss of water and sodium.
Inadequate fluid intake can also contribute to kidney stones in susceptible people.
The relationship can go both ways: illness can cause dehydration, and dehydration can make an illness harder for the body to handle.
When to Get Help
Persistent vomiting or diarrhea, very little urination, marked weakness or dizziness, fainting, confusion, unusual sleepiness, or an inability to keep fluids down can indicate more serious dehydration.
These signs deserve particular attention in older adults. Severe confusion, loss of consciousness or seizures require prompt medical attention.
Bottom Line
The body carefully regulates water through thirst, hormones and the kidneys.
Most of the time this happens almost automatically. But vomiting, diarrhea, fever, heavy sweating, medications and illness can upset that balance surprisingly quickly.
As we get older, paying attention to hydration becomes especially important because thirst may no longer provide as strong a warning that the body needs water.
Learn More
MedlinePlus — Dehydration
A clear overview of the causes, symptoms and treatment of dehydration, including the increased risk for older adults.
NIDDK — Your Kidneys & How They Work
Explains how the kidneys filter the blood, regulate water and minerals, and remove excess water as urine.
Video
Dehydration in Older Adults: Symptoms, Treatment & Prevention - 30 Minutes
Geriatrician Dr. Leslie Kernisan explains why dehydration becomes more common with aging, how to recognize it, and when it may require medical attention.