Every cell in the body needs water to survive, but it's possible to get too much or too little of the liquid, and that can cause some pretty serious problems.

The body's fluid balance is not only affected by water that's taken in by consuming food and beverages and released in urine and sweat, but also by the concentration of sodium, an electrolyte. Electrolytes are minerals that carry an electrical charge when dissolved in a liquid such as blood. In the body, sodium is mainly found in the fluid outside of cells and plays an important role in the movement of water into and out of them.

Two different disorders, known as hyponatremia and hypernatremia, may result from changes in the balance of water in the body and levels of sodium in the blood.

Hyponatremia and hypernatremia are primarily disorders of water metabolism, said Dr. David Mount, a kidney specialist and clinical chief of the renal division at Brigham and Women's Hospital in Boston.

In hyponatremia, an excess of water in the body can lead to a low concentration of sodium in the blood, he said. And in hypernatremia, a deficit of water in the body can lead to a high concentration of sodium in the blood.

Hyponatremia

Hyponatremia is a low concentration of sodium in the blood because of an excessive retention of water, Mount said. In this electrolyte abnormality, there is too much water in the body and this dilutes sodium levels in the bloodstream, he noted.

Hyponatremia occurs when blood sodium goes below normal levels, which is 135 milliequivalents/liter (mEq/L).

When sodium levels in the blood are too low, extra water goes into body cells causing them to swell. This swelling can be especially dangerous for brain cells, resulting in neurological symptoms such as headache, confusion, irritability, seizures or even coma.

The symptoms of hyponatremia may be more serious when blood sodium levels drop very quickly and may be milder when they dip gradually, as that allows the body more time to adjust to the change. Other symptoms of the disorder include muscle cramps or weakness, nausea, vomiting, tiredness and a lack of energy.

Hyponatremia can result from an illness or from taking certain medications. According to the National Kidney Foundation, some of the causes may include:

• Severe vomiting or diarrhea.
• Excessive fluid intake, such as during endurance activities or from excessive thirst.
• Taking diuretics, medications that help flush excess water and sodium from the body.
• Kidney failure, a condition in which the kidneys have difficulty eliminating extra fluid from the body.
• Congestive heart failure, which can lead to a build-up of excess fluid in the body.
• Burns affecting a large area of the body.
• Small cell lung cancer.
• Taking antidepressants, including some commonly used selective serotonin reuptake inhibitors, particularly in older adults.
• Syndrome of inappropriate antidiuretic hormone secretion, a condition in which the body makes too much antidiuretic hormone, causing the body to retain too much water and diluting levels of sodium.

People can also consume excessive amounts of water during exercise and develop hyponatremia, Mount told Live Science. Exercise-associated hyponatremia is more likely to affect endurance athletes, such as marathoners, triathletes and ultra-distance race competitors.

Hyponatremia is not a permanent condition, although certain people may be more prone to the disorder than others because of lifestyle habits or a medical condition.

Treatment for hyponatremia depends on its cause and seriousness. In moderate cases of hyponatremia that are the result of diet, taking diuretics or drinking too much water, a person may need to restrict water intake, consume salty foods — such as bouillon or pretzels — or adjust diuretic intake to raise blood sodium levels.

A person with severe hyponatremia may be given a very concentrated saline solution intravenously. But sodium levels need to be corrected slowly and in a controlled fashion, to prevent swollen brain tissue, Mount said.

Hypernatremia

In hypernatremia, the body contains too little water relative to the amount of sodium, Mount said. This causes sodium levels to become abnormally high in the blood — more than 145 mEq/L — which causes water to move out of body tissues and into the blood in an attempt to equalize the concentration between the two. Water can be lost from brain cells, causing them to shrink, which can be dangerous.

Too much sodium in the blood is a common problem in older adults, especially those who have been hospitalized or are in long-term care facilities, Mount said. The disorder can also affect much younger people: Infants may experience hypernatremia if they have severe diarrhea, for example.

Besides thirst, many of the symptoms of hypernatremia, such as irritability, restlessness and muscle twitching, affect the central nervous system and stem from a loss of water content from brain cells. In some cases, hypernatremia can be life-threatening. Similar to hyponatremia, other symptoms of hypernatremia include feeling tired or lacking energy, confusion, seizures or coma.

The main cause of hypernatremia usually involves dehydration due to an impaired thirst mechanism or limited access to water, according to the Merck Manual. The disorder can also result from diarrhea or vomiting, taking diuretics or having a high fever.

People who aren't always able to provide water for themselves may be more at risk for hypernatremia. This includes people on tube feedings and those with altered mental status (stroke or dementia), plus people who are very young or very old and frail, according to a review in the New England Journal of Medicine.

Older people are more prone to hypernatremia because their thirst mechanism, kidney function, and hormones regulating salt and water balance may not work as effectively.

The main treatment for hypernatremia is simply to replenish fluids. A person with a mild case of hypernatremia can usually just drink fluids to recover. But in more severe instances, water and a small amount of sodium are given intravenously in controlled amounts over a 48-hour period to slowly reduce sodium levels to a normal range.

Fluid levels are corrected slowly to avoid the risk of cerebral edema, a dangerous condition in which there is swelling of the brain, Mount said.

Hypernatremia can be fatal, and may cause permanent brain damage if not treated properly. Some studies suggest the mortality rate may be more than 50% in hospitalized patients affected by the disorder.

By Cari Nierenberg, Live Science