Health Tooty Fruity

To maintain just the right water balance, the intake of water must exactly match the output of water. The body loses or disposes of water in several ways, as displayed in Figure 7-1. The kidneys are the main regulators of water. The kidneys account for about half of the total water output. Our skin loses water in two ways. The loss of water by diffusion from skin accounts for about 15 percent of the total water output, even when we are not sweating. Water loss from sweating varies considerably depending on activity, temperature, and humidity.

The lungs continuously lose water as water vapor and account for about 12 percent  of water output. The lungs lose extra water vapor during exercise and when the humidity is low. And, finally, about 5 percent of water is lost through the feces. Under normal conditions, the total amount of water in the body is regulated by the kidneys. Inside the kidneys are tiny tubules that can retain more water when necessary. When blood volume falls or blood pressure falls, or the extra-

Figure 7-1 How water can leave the body.

cellular fluid becomes too concentrated, the body responds by retaining more water from the urine. The amount of water retained by the kidneys is regulated by hormones, as seen in Figure 7-2. Antidiuretic hormone is secreted by the pituitary gland to keep more water in the body. Antidiuretic hormone limits the amount of water lost through the kidneys. Another hormone, aldosterone, also directs the kidneys to retain water. Aldosterone is secreted from the adrenal cortex, which is a gland located on the top of each kidney. 

These two hormones also trigger thirst. The minimum amount of water that must be excreted each day is about half a quart (500 milliliters). This amount is needed to carry metabolic waste products away in urine. More water than this is needed to allow for sweating, diffusion of water through skin, losses through lungs, and in the feces. Average daily losses run about two and a half quarts (about 2.5 liters). More water is useful to dilute the urine and ensure waste removal. Inadequate water intake has been correlated with increased rates of bladder cancer because of the longer time that the bladder may be exposed to potential carcinogens. 

One half-gallon to one gallon is a reasonable range of liquids for most people to drink each day. Hard work in the hot sun greatly increases water needs. Coffee, 

tea, sodas, and alcoholic beverages are not good water substitutes as their diuretic action causes losses of about half of the amount taken.

The obvious way that we take in water is through drinking water and other beverages, as shown in Figure 7-3. About 55 percent of our water intake is from drinking water and beverages. Food also contains water in varying amounts. Strawberries, watermelon, and broccoli contain about 90 percent water. Bread and cheese contain about 35 percent water. Water from food accounts for about 35 percent of our water intake. 

Together, water from food and beverages accounts for 90 percent of our water intake. When food is metabolized for energy in the cells, one byproduct is water. This metabolic water accounts for about 10 percent of our water. When carbohydrates, fats, and protein are burned for energy, their carbon and hydrogen atoms combine with oxygen to produce carbon dioxide and water. Metabolic water may find its way into plasma and can be regarded as intake. There are mechanisms to adjust the intake of liquids when the total water in the body is high or low. When body fluid stores get low, the mouth feels dry. This thirst encourages drinking to build up body stores of water. When body stores of

fluids are high, it is thought that stretch receptors in the stomach signal the brain to stop drinking. Stretch receptors in the bladder signal when it is time to release water. Sensors in the heart also monitor blood volume. Too much water is rarely a problem because it is so easily eliminated. Too little water can lead to dehydration, which can cause weakness and delirium if severe.