13.7: Water Balance

Water balance is essential and disruptions from dehydration, overhydration, or environmental stresses can quickly impair normal body function.

Master this section and you'll be able to:

• Explain how the kidneys, thirst mechanism, and ADH work together to regulate water balance.
• Describe the effects of dehydration and overhydration on blood volume, temperature regulation, and brain function.
• Apply water balance concepts to situations such as aging, exercise, heat exposure, and alcohol consumption.

Water Balance and Why It Matters

Water balance means that the amount of water we take in matches the amount we lose. The kidneys and the thirst center in the brain work together to keep this balance steady. When we have not taken in enough water, we feel thirsty and the kidneys produce more concentrated urine. When we have more water than we need, we do not feel thirsty and the kidneys produce dilute urine.

In older adults, the thirst mechanism is not as reliable. They may not feel thirsty even when their body needs water, so they need to drink regularly throughout the day. Thirst is also not a good indicator when the body is losing water very quickly, such as with severe diarrhea, vomiting, fever, or heavy sweating during intense exercise. In these situations, dehydration can develop faster than the body’s normal control systems can respond.

Many body functions depend on water, so the balance between intake and loss must stay within a fairly narrow range. There is only a limited amount of flexibility before dehydration begins to interfere with normal function.

Figure \(\PageIndex{1}\):  Daily Water Balance in the Human Body. The body maintains water balance by matching daily water intake with water loss. Most water comes from food and drink, while a smaller amount is produced during metabolism. Water leaves the body through several routes, including “insensible losses” from the skin and lungs, urine produced by the kidneys, and a small amount in feces. In a healthy adult, total water in (about 2.5 L per day) equals total water out (about 2.5 L per day), keeping body fluid levels stable.
 

Dehydration

As dehydration develops, the body protects the organs and systems that are essential for survival. Maintaining blood volume is one of the highest priorities because circulation depends on it. To preserve blood volume, the body will reduce water loss in areas that are less immediately critical, such as sweating. This is why the body may allow only a certain amount of water to be used for cooling when conditions are hot and dry.

Figure \(\PageIndex{2}\): Effects of Progressive Dehydration on the Human Body.

Limits of the Body’s Water-Saving Response

The body can conserve water for only a short time. Holding back sweat helps preserve fluid, but it also allows body temperature to climb.

• Normal core temperature is about 37 °C (98.6 °F).

• An increase of 6–7 °C means a core temperature of 43–44 °C (about 109–111 °F), which is incompatible with survival for more than a very short time.

A real-world example occurred in April 2025, when 77-year-old guitarist Carlos Santana (musician, age 77) was getting ready for a concert in San Antonio, Texas. He developed a medical problem during preparations and had to be taken to the hospital. His manager later explained that he had been hospitalized for dehydration, and the show was postponed while he was monitored and rehydrated. He recovered and planned to resume his tour once stable.where doctors determined that he was dehydrated. The show was postponed while he was monitored and rehydrated, but after treatment and rest he resumed his Oneness Tour later that week.

A loss of only about five percent of body water may trigger early heat-exhaustion symptoms such as weakness, headache, and a rapid, weak pulse. When body water falls to around ten percent below normal, the risk of heat stroke becomes high. Heat stroke is a medical emergency and can be fatal if not treated quickly.

Even without heat exposure, dehydration lowers total blood volume. Reduced blood volume leads to lower blood pressure and a weak, fast pulse, as the heart works harder to maintain circulation. If the problem continues, decreased circulation can cause dangerous complications, including the inability to produce urine.

Can You Drink Too Much Water?

Although we often worry about drinking too little, there is also a condition in which people drink far too much and develop water intoxication. In some cases, compulsive water drinking is part of a psychological disturbance, leading a person to consume such large volumes of water that the kidneys cannot eliminate the excess quickly enough. This causes the body’s fluids to become overly diluted, allowing water to move into tissues and make them swell. The brain is especially vulnerable because it is enclosed within the skull and cannot expand safely.

Early symptoms include headache and vomiting. These can also occur with dehydration, which shows how headaches often serve as a general signal that something in the body is amiss.

Cases of water intoxication have occurred when fraternity pledges were forced to drink large quantities of water during hazing, sometimes with fatal outcomes. Your study guide mentions Matthew Carrington, who died in 2005 after a water-drinking hazing ritual at Chico State University. In spite of stronger laws meant to prevent this dangerous behavior, it is astonishing that such hazing tragedies continue. Not with water, but with alcohol and other noxious methods. In fact, alcohol-related hazing is the most common and most deadly form of hazing. 

Unfortunately there are other documented cases of water intoxication besides hazing deaths in college fraternities. So, while water intoxication is rare, it has been documented in several settings:

1. Radio contest: “Hold Your Wee for a Wii”: In 2007, Jennifer Strange, a 28-year-old mother of three, died after a radio contest in which participants drank large amounts of water without urinating. She consumed nearly two gallons in a few hours and later died of water intoxication.

2. Military training and endurance sports: Some military trainees and endurance athletes have developed fatal hyponatremia after drinking extreme amounts of water to prevent dehydration, overwhelming the kidneys’ ability to excrete the excess. 

3. Psychiatric conditions (psychogenic polydipsia): Certain psychiatric disorders, such as schizophrenia or bipolar disorder, can lead to compulsive water drinking. In severe cases, this can cause hyponatremia, brain swelling, and even death. 

Alcohol and Dehydration

Drinking alcohol leads to dehydration. If that seems strange, consider what alcohol does inside your body:

Alcohol readily crosses the blood–brain barrier, because ethanol is a small and lipid‑soluble molecule. In the brain, ethanol acts on the hypothalamic nuclei that normally synthesize antidiuretic hormone (ADH). As you have learned, these neurons send their axons to the posterior pituitary, where ADH is stored and then released into the bloodstream. ADH’s role is to increase water reabsorption in the kidney DCTs and collecting ducts, helping the body conserve water. When alcohol suppresses the activity of these hypothalamic neurons, less ADH is released from the posterior pituitary, so the collecting ducts become less permeable to water and reabsorb less of it, leading the kidneys to produce a larger volume of relatively dilute urine.

This extra urination pulls water out of the body more quickly than usual, decreasing total body water and lowering blood volume. The result is dehydration, which contributes to thirst, dizziness, fatigue, and the famous “morning-after” headache. In fact, a portion of a hangover is simply your body reacting to water loss. Drinking water during or after consuming alcohol helps replace some of the fluid your kidneys are rapidly sending out of the body.

In hot conditions, your body is already losing water through sweat. With less water available, alcohol becomes a larger proportion of your blood, so it will affect you more quickly. A beer, which has more water relative to alcohol, is usually a better choice in hot weather than a strong cocktail.

Add Exercise or High Altitude, and the Effect Intensifies. Heavy exercise and high altitude both increase water loss. At high altitude, the air is dry and thinner, so you lose more water when you breathe. After a tough tennis match in the sun or while on a mountain vacation, even a small amount of alcohol can feel stronger because there is less circulating blood volume to dilute it.

The same thing happens on airplanes. Cabin air is very dry, similar to high-altitude conditions on land. Dehydration contributes to jet lag, so staying hydrated and limiting alcohol makes a real difference during air travel.
 

What Controls Your Body’s Need for Water?

Adults need roughly 2 quarts of water each day to replace normal losses. About one quart is lost as urine, and the rest leaves the body through sweat, breathing, and feces. A general guideline is about one quart of water for every 1,000 calories consumed, while infants need about one and a half quarts per 1,000 calories. Daily needs rise in hot or dry climates or during strenuous activity because evaporation from the skin and lungs is a major cooling mechanism. Under extreme heat and exertion, such as in soldiers working in the desert, water loss can reach one quart per hour.

There are many misconceptions about water and sports. Some coaches still tell young athletes to limit drinking during a hot, intense game. In truth, unless someone drinks extreme amounts very quickly, water will not cause cramps or other problems. The real danger is poor performance or even serious illness when players do not drink enough. Thirst is usually a reliable warning that the body needs water, although the absence of thirst does not necessarily mean the body is fully hydrated.

Hot, strenuous activity creates special risks because athletes can lose water faster than they can drink it. During long events, athletes cannot rely on thirst alone and often cannot replace water losses quickly enough. This is why athletes should drink plenty of water before, during, and after activity lasting more than thirty minutes, even if they do not feel thirsty. Cold water may be absorbed slightly faster and helps cool the body, but room-temperature water works as well.