The Importance of Water

Water is important to living cells as a solvent, a transport medium, and a participant in metabolic reactions. The normal proportion of body water varies from 50% to 70% of a person’s weight. It is highest in the young and in thin, muscular individuals. In infants, water makes up 75% of the total body mass. That’s why infants are in greater danger from dehydration than adults. With increase in the amount of fat, the percentage of water in the body decreases, because adipose tissue holds very little water compared with muscle tissue. Various electrolytes (salts), nutrients, gases, waste, and special substances, such as enzymes and hormones, are dissolved or suspended in body water. The composition of body fluids is an important factor in homeostasis. Whenever the volume or chemical makeup of these fluids deviates even slightly from normal, disease results. The constancy of body fluids is maintained in the following ways:
* The thirst mechanism, which maintains the volume of water at a constant level;
* Kidney activity, which regulates the volume and composition of body fluids;
* Hormones, which serve to regulate fluid volume and electrolytes;
* Regulators of pH (acidity and alkalinity), including buffers, respiration, and kidney function.
The maintenance of proper fluid balance involves many of the principles discussed in other pages, such as pH and buffers, the effects of respiration on pH, tonicity of solutions, and forces influencing capillary exchange.

Fluid Compartments
Although body fluids have much in common no matter where they are located, there are some important differences between fluid inside and outside cells. Accordingly, fluids are grouped into two main compartments (Fig. 17-1):
* Intracellular fluid (ICF) is contained within the cells. About twothirds to three-fourths of all body fluids are in this category.
* Extracellular fluid (ECF) includes all body fluids outside of cells. In this group are included the following:
* Interstitial fluid, or more simply, tissue fluid. This fluid is located in the spaces between the cells in tissues all over the body. It is estimated that tissue fluid constitutes about 15% of body weight.
* Blood plasma, which constitutes about 4% of a person’s body weight.
* Lymph, the fluid that drains from the tissues into the lymphatic system. This is about 1% of body weight.
* Fluid in special compartments, such as cerebrospinal fluid, the aqueous and vitreous humors of the eye, serous fluid, and synovial fluid. Together, these make up about 1% to 3% of total body fluids.
Fluids are not locked into one compartment. There is a constant interchange between compartments as fluids are transferred across semipermeable cell membranes by diffusion and osmosis (see Fig. 17-1). Also, fluids are lost and replaced on a daily basis.

Figure 17-1 Main fluid compartments showing relative percentage by weight of body fluid. Fluid percentages vary but total about 60% of body weight. Fluids are constantly exchanged among compartments, and each day fluids are lost and replaced.

Water Balance
In a person whose health is normal, the quantity of water gained in a day is approximately equal to the quantity lost (output) (Fig. 17-2). The quantity of water consumed in a day (intake) varies considerably. The average adult in a comfortable environment takes in about 2300 mL of water (about 2 1/2 quarts) daily. About two-thirds of this quantity comes from drinking water and other beverages; about one-third comes from foods-fruits, vegetables, and soups. About 200 mL of water is produced each day as a byproduct of cellular respiration. This water, described as metabolic water, brings the total average gain to 2500 mL each day. The same volume of water is constantly being lost
from the body by the following routes:
* The kidneys excrete the largest quantity of water lost each day. About 1 to 1.5 liters of water are eliminated daily in the urine. (Note that beverages containing alcohol or caffeine act as diuretics and increase water loss through the kidneys).
* The skin. Although sebum and keratin help prevent dehydration, water is constantly evaporating from the skin’s surface. Larger amounts of water are lost from the skin as sweat when it is necessary to cool the body.
* The lungs expel water along with carbon dioxide.
* The intestinal tract eliminates water along with the feces.

Figure 17-2 Daily gain and loss of water.

In many disorders, it is important for the healthcare team to know whether a patient’s intake and output are equal; in such a case, a 24-hour intake-output record is kept. The intake record includes all the liquid the patient has taken in. This means fluids administered intravenously as well as those consumed by mouth. The healthcare worker must account for water, other beverages, and liquid foods, such as soup and ice cream. The output record includes the quantity of urine excreted in the same 24-hour period as well as an estimation of fluid losses due to fever, vomiting, diarrhea, bleeding, wound discharge, or other causes.

Sense of Thirst
The control center for the sense of thirst is located in the brain’s hypothalamus. This center plays a major role in the regulation of total fluid volume. A decrease in fluid volume or an increase in the concentration of body fluids stimulates the thirst center, causing a person to drink water or other fluids containing large amounts of water. Dryness of the mouth also causes a sensation of thirst. Excessive thirst, such as that caused by excessive urine loss in cases of diabetes, is called polydipsia. The thirst center should stimulate enough drinking to balance fluids, but this is not always the case. During vigorous exercise, especially in hot weather, the body can dehydrate rapidly. People may not drink enough to replace needed fluids. In addition, if plain water is consumed, the dilution of body fluids may depress the thirst center. Athletes who are exercising very strenuously may need to drink beverages with some carbohydrates for energy and also some electrolytes to keep fluids in balance.