The body maintains fluid balance by moving electrolytes to where more water is needed. However, the level of electrolytes in your body can become too low or too high. That can happen when the amount of water in your body changes, upsetting fluid balance and causing dehydration or even overhydration.

Approximately 60% to 70% of our bodies are made up of water, but the ratio of water to other substances in our tissues varies, depending primarily on the ratio of fat mass to lean body mass (body composition), which is strongly influenced by age and sex. The body of an adult man is approximately 60% water, and an adult woman is about 55% water—with water composing about 75% of the mass of muscle, and 15% of adipose tissue mass. As people age, they typically lose muscle mass and gain fat, which will decrease the percent of body mass that is water. Organs have different water percentages, too: The brain and heart are made of approximately 73% water, but bone is only 10% water.

Water has many important roles in the body. It helps regulate body temperature within a very narrow range, which is important because even slight variations can affect body functions and damage organs. (Think about how a fever of 102°F—which is slightly higher than three degrees above normal—makes you feel.) The temperature of water rises slowly, so, because there is so much water in your body, it provides an important “brake” for unwanted body temperature fluctuations.

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Water transports nutrients and oxygen to tissues in your body. It is also present in the mucus and salivary juices of the digestive system, which help to move food through the digestive tract. Water also lubricates joints and mucous membranes in our noses, eyes, and the gastrointestinal tract. Since water can’t be compressed, it helps to protect delicate tissues like the brain, eyes, and spinal cord against injury and shock.

Water acts as a solvent—a liquid substance that is capable of dissolving another substance, and participates in the body’s biochemical reactions. For instance, without water our bodies can’t break down proteins, carbohydrates, or fats to extract energy. Water also helps to remove waste from the body via urination, perspiration, and bowel movements.

Approximately two-thirds of the water in our bodies is intracellular, or found inside cells. The rest is extracellular, found outside cells, primarily in the blood and the fluid that surrounds our cells (interstitial fluid). Water may move from outside to inside cells and vice versa, in a process called osmosis. The direction of water movement depends on the concentration of dissolved substances in intracellular and extracellular fluids called solutes. Some of the most important solutes are the electrolytes sodium, potassium, and chloride. Water moves across a membrane toward areas where there is a greater concentration of solutes, and, therefore a lower concentration of water molecules. (INFOGRAPHIC 13.10)

Question 13.5

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In the case of our marathoner, as he was rapidly drinking the low-solute water, his cells were absorbing dangerous amounts of water. In essence, the water passed, via osmosis, from his blood into his cells to try to equalize the concentration of solute particles between the intracellular and extracellular compartments. The sudden flood of water into the cells of his brain (the neurons in particular) caused his brain to swell and press against the inside of his skull, which prevented blood, and thus essential oxygen, from reaching his brain.

One of the reasons we need to drink fluids is because we lose water throughout the day and our bodies can’t store extra to fill the void. But how much water we need to drink depends on many factors. We lose, on average, about 350 ml, or 1¹⁄₂ cups, of water each day through respiration, the process of transporting oxygen from the air to the cells within tissues (inhalation) and, conversely, transporting carbon dioxide to the air (exhalation). When we breathe, water evaporates from the lungs and the skin. How much water we lose while respiring depends on the air temperature and our body temperature, how active we are, how humid the air is, and how big our bodies are. The more we respire—for instance, when we exercise and breathe hard—the more water we lose. (INFOGRAPHIC 13.11)

Question 13.6

We lose 140 ml to 150 ml of water, or about two-thirds of a cup, through our bowel movements, too; the amount is small because most of the water in our stools is re-absorbed into the body before the stool is eliminated. Diarrhea, however, can dramatically increase water loss, which can be dangerous: Diarrhea-associated dehydration is a major cause of child death in areas of the world that do not have access to clean water, as contaminated water can cause diarrheal illnesses. When we suffer diarrhea, we lose from 3,300 ml to 7,300 ml of water a day—as much as 70 times what we would lose through normal bowel movements.

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Our water needs can fluctuate because of other factors, as well. We need more water when we are ill, because we respire more with a fever, and our body perspires to cool itself; vomiting also increases water loss. We also need more water when we eat high-protein diets, in part because water is necessary to remove the byproducts of protein metabolism that accumulate in the blood. High-fiber diets increase our water needs, too, because fiber increases the water content of our stools, making it easier for us to pass them. Water needs are increased when we drink alcohol, take certain medications, and spend time at high altitudes.

The body achieves water balance by attempting to ensure that the amount of water we consume in food and drinks and the amount we produce through metabolism equals the amount of water we excrete. We can thank our kidneys and our brain for achieving this balance. The kidneys conserve water by reducing urine volume when necessary, and they excrete excess water by increasing the volume of urine and making it more dilute when we have excess fluid.

When the concentration of solutes in the blood increases, or when blood volume drops, the brain responds by stimulating the pituitary to release antidiuretic hormone (ADH). ADH tells the kidneys to conserve water to bring more water back into the bloodstream, which will also lower the concentration of solutes. When the brain detects that water volume has increased again, it decreases the production of ADH so that the kidneys stop conserving water. Without ADH, urine becomes very dilute. (INFOGRAPHIC 13.12)

Question 13.7

Thirst is typically a powerful and rapid barometer that tells us when we need to drink (and conversely, lack of thirst tells us we don’t need to drink). The same factors (an increase in solutes in the blood or a drop in blood volume) that stimulate the release of ADH also stimulate the brain to generate the sensation of thirst. So, in the end, ADH and thirst work in concert to increase body fluids and decrease the concentration of solutes in blood. However, in those over the age of 65, the thirst sensation is diminished when they become dehydrated or experience increased solute concentrations in blood, which slows the restoration of fluid balance.