Under normal conditions the human body keeps very careful books on its water content so that daily losses will be exactly balanced by daily intake. So effective is the body's regulation of its fluid content that most of us will go through our whole lives without ever experiencing a serious disturbance in water balance.

         Like the hormonal system that regulates blood sugar, the body's control of its water can be compared to the thermostat of a home. Just as an air conditioner and a furnace act in opposite directions to maintain room temperature within a narrow range, so do the body's other control systems utilize pairs of antagonistic mechanisms to keep important blood conditions within acceptable bounds.For blood sugar, the paired opposites are the pancreatic hormones insulin and glucagons. In the case of water, they are thirst and urinary output. And both thirst and urine volumes are controlled by a tiny portion of the brain, the hypothalamus that continually keeps tabs on tile blood's sodium concentration. When your body's water content is failing, the concentration of sodium in your blood goes up a little, just as a puddle of seawater becomes saltier as the water gradually evaporates. When your hypothalamus detects this increased saltiness of your blood, it sends out thirst messages, and you begin thinking about the nearest water fountain. If you drink more water than your body needs to correct its saltiness, your blood will become more dilute than the hypothalamus likes. In that case, the hypothalamus sends out a hormonal message to the kidneys, instructing them to excrete a greater volume of urine. This back-and-forth interplay of regulatory forces goes on constantly, and we all stay nicely hydrated without giving it a thought.

Intake
         Beverages provide the major source of water intake, but "solid" foods also contain appreciable amounts of water, as Table illustrates. In addition, water is a chemical by product of energy-yielding metabolic reactions; this water, called water of oxidation, furnishes a small but significant addition to the body's total fluid intake.

Output
         Water is lost from the body in several ways. The major outflow is usually urinary, unless extraordinarily great water losses through other channels (excessive sweating, diarrhea, vomiting) predominate. Urine contains metabolic waste products, chiefly those resulting from the breakdown of protein. The volume of urine excreted is controlled by the hypothalamus, which secretes a hormone called ADH (the antidiuretic hormone). ADH has the effect of decreasing urine volume; it is sent out to the kidneys when the blood is becoming too concentrated and water needs to be conserved. When the blood becomes too dilute, the hypothalamus cuts back its ADH production, and more water leaves the body through the kidneys.
         The second largest route of water loss is ordinarily through the skin. As already noted, the volume of sweat can vary enormously, from the few hundred milliliters lost through insensible perspiration to the several liters that can be excreted during hot-weather exercise.
         The lungs constitute the third largest conduit for water loss, as the air breathed in is drier than the surface of the lung tissue it contacts. On a cold day you can "see your breath" in the air and quickly fog up the windows of a parked car, but you lose just as much water through breathing in warm weather it just doesn't show up as easily.
         Finally, water is also lost through the digestive tract. Saliva, stomach juices, intestinal and pancreatic secretions, bile, and lymph are all continually poured into the intestines; together they may amount to as much as 8 to 10 liters in a day. However, practically all of this fluid is normally reabsorbed as it passes through the gut, so as little as 200 ml may be finally excreted in the feces. But if the stool should hurry through too quickly to allow effective reabsorption, the potential for water loss is obviously enormous. Uncontrolled diarrhea, then, can bring on deadly dehydration.