DRINKING PLENTY OF WATER and keeping your blood pressure in a healthy range are two bits of health advice you probably hear often from your doctor. Both hydration and lowering your blood pressure are necessary for your overall health, but they’re also interconnected.
“Water makes up 90 percent of your blood volume,” says Mary Greene, M.D., a cardiologist with Manhattan Cardiology. “When you’re deficient in the main component of your blood—water—the blood pressure can be affected.”
When you lose more fluid than you take in, such as from sweating, vomiting, or diarrhea, your body doesn’t have enough fluids to carry out its normal functions, she explains. If you don’t replace those fluids, by drinking water or other liquids, you can get dehydrated.
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Dehydration can cause fluctuations in your blood pressure. It can both raise and lower your blood pressure—depending on how dehydrated you are, how long you’ve been dehydrated, and if you have other health conditions, explains Oral Waldo, M.D., a cardiologist at Palm Beach Gardens Medical Center and Good Samaritan Medical Center.
That’s why it’s so important to drink plenty of fluids throughout the day, Dr. Waldo emphasizes. It keeps your blood flowing so that it can efficiently deliver oxygen and nutrients throughout your body.
“Blood pressure fluctuations can be potentially dangerous because they can affect other organs such as the brain, kidneys, and heart, among others,” he says.
Here’s how dehydration can both raise and lower your blood pressure and why dehydration-related blood pressure fluctuations can be dangerous.
What Is Dehydration?
Dehydration happens when you lose so many fluids that your body can’t function normally. Drinking water and staying hydrated keeps that from happening and helps maintain normal blood pressure, says family physician Kim Yu, M.D.
Water is the best fluid for maintaining hydration, but tea, coffee, and sports drinks can also increase your fluid intake, she adds. Alcohol can be dehydrating and should be limited.
A general rule of thumb is that men drink 15.5 cups of fluids a day and women drink 11.5 cups. But that’s not a rule. Yu says you might need more if you exercise frequently or spend time outside in hot weather.
Basically, she says, “Drink enough to not feel thirsty.”
When you drink enough water, you’ll maintain an optimal blood volume, says Robert Pilchik, M.D., a cardiologist with Manhattan Cardiology.
“This allows your blood vessels to dilate, which reduces the resistance to blood flow,” he says. “This helps keep your blood pressure normal.”
Dr. Waldo says symptoms of dehydration can be mild, including dizziness and sleepiness, to severe, such as muscle damage or altered organ functioning. Other signs of dehydration include:
- Extreme thirst
- Less-frequent urination
- Dark-colored urine
- Dry mouth
What is Blood Pressure?
Blood pressure refers to the force of blood inside your arteries, according to the Cleveland Clinic. When your heart beats, it pumps blood into your arteries, which carry blood through your body to deliver oxygen and nutrients that your body needs to function.
Blood pressure readings contain two measurements:
- Systolic blood pressure, also known as the top number. This refers to the pressure in your arteries when your heart beats and sends blood into them.
- Diastolic blood pressure, or the bottom number, is the pressure in your arteries when your heart rests between beats.
Blood pressure is measured in millimeters of mercury (mm Hg).
How Dehydration Lowers Your Blood Pressure
Dehydration can cause your blood pressure to drop and then sometimes rapidly increase in response.
Lack of water lowers blood volume, which leads to lower blood pressure, Dr. Waldo says.
“It’s similar to when a small amount of water is flowing through a pipeline,” he explains. “The amount of fluid in the blood vessels is reduced. When an individual is dehydrated, this lack of water supply is felt throughout the entire body, including their blood volume.”
When the body doesn’t have enough blood, it has to make adjustments to keep blood pressure and blood flow so that your organs keep functioning, Dr. Pilchik says. Your blood pressure might increase as a result. Dehydration also can put stress on your heart and increase your risk of fainting.
Signs of Low Blood Pressure
Low blood pressure is considered 90/60 mm Hg or lower, according to the National Heart, Lung, and Blood Institute.
Signs of low blood pressure include:
- Dizziness or lightheadedness
- Blurry vision
- Heart palpitations
“In severe dehydration, rapid and severe decreases in blood pressure can cause shock and even death,” Dr. Yu says.
How Dehydration Raises Your Blood Pressure
When your blood volume drops from not drinking enough water, your body has to work to conserve blood pressure to ensure the body can carry out its normal functions and maintain homeostasis, Dr. Greene says.
Less water in your blood makes it more concentrated and increases its sodium levels. When your kidneys detect higher sodium, your body releases the hormone vasopressin, which tells your kidneys to retain water instead of eliminating it in your urine.
“Vasopressin also causes the blood vessels to constrict to maintain perfusion pressure to your tissues when blood volume is low,” Dr. Greene says. “This results in an elevated blood pressure.”
Dehydration also makes the sympathetic nervous system work harder, which puts stress on the body, triggering it to release hormones, like adrenaline, Dr. Pilchik says. This causes your blood vessels to constrict and the heart rate to rise, raising your blood pressure.
Signs of High Blood Pressure
Your blood pressure is considered high when it’s 130/80 mm Hg or higher, according to the National Heart, Lung, and Blood Institute.
The Dangers of Dehydration-Related Blood Pressure Fluctuations
“Dehydration-related changes in blood pressure, in particular, can be harmful because they can have an impact on the heart and the body as a whole,” Dr. Pilchik says.
“Understanding the link between dehydration and high blood pressure is important for keeping your heart healthy,” he adds. “Dehydration can have big effects on controlling blood pressure and on your general health.”
For instance, blood pressure fluctuations can cause kidney problems and increase your risk for a heart attack or stroke.
“Hydration should be a top priority as part of a complete plan for heart health,” Dr. Pilchik says.
Erica Sweeney is a writer who mostly covers health, wellness and careers. She has written for The New York Times, HuffPost, Teen Vogue, Parade, Money, Business Insider and many more.