DEHYDRATION

the loss of water from the body; it is almost invariably associated with some loss of salt (sodium chloride) as well. The treatment of any form of dehydration, therefore, requires not only the replacement of the water lost from the body but also the restoration of the normal concentration of salt within the body fluid. Dehydration may be caused by restriction of water intake or by excessive water loss. The commonest cause of dehydration is failure to drink liquids. The deprivation of water is far more serious than the deprivation of food. The average person loses approximately 2.5 percent of total body water per day (about 1,200 millilitres [1.25 quarts]) in urine, in expired air, by insensible perspiration, and from the gastrointestinal tract. If, in addition to this loss, the loss through perspiration is greatly increasedas is demonstrated in the case of the shipwrecked sailor in tropical seas or the traveler lost in the desertwithin only a few hours the dehydration may result in shock and death. When swallowing is difficult in extremely ill persons, or when people cannot respond to a sense of thirst because of age or illness or dulling of consciousness, the failure to compensate for the daily loss of body water will rapidly result in dehydration and its consequences. In vomiting or diarrhea large volumes of water may be lost from the body, always with an associated loss of electrolytes (e.g., sodium, potassium). The exact nature of the resultant electrolyte deficit will depend in part on the site of the lesion causing symptoms and in part on whether the ill person is receiving other fluids. In uncomplicated, untreated vomiting or diarrhea, the dehydration that results is ordinarily a balanced loss of electrolytes (salt) and water. The symptoms of dehydration depend in part on the cause and in part on whether there is associated salt deprivation as well. When loss of water is disproportionately greater than loss of electrolytes, the osmotic pressure of the extracellular fluids becomes higher than in the cells. Since water passes from a region of lower to a region of higher osmotic pressure, water flows out of the cells into the extracellular fluid, tending to lower its osmotic pressure and increase its volume toward normal. As a result of the flow of water out of the cells, they become dehydrated. This results in the thirst that always accompanies pure water depletion. In those diseases in which there is loss of salt in excess of water, the decreased concentration of sodium in the extracellular fluid and in the blood serum results in decreased osmotic pressure, and water therefore enters the cells to equalize the osmotic pressure. Thus there is extracellular dehydration and intercellular hydrationand no thirst. Water deprivation produces distinctive symptoms in human beings. Gradual weight loss occurs, amounting to two to three pounds per day. Thirst is the most prominent symptom, along with dryness of the mouth, a craving for fluid, decreased production of saliva, and impaired swallowing. It is probable that thirst is the result of this subsequent intracellular dehydration and increased intracellular osmotic pressure. Experimentally, thirst can be produced when the cells have lost about 1 percent of their intracellular water. As dehydration progresses, the tissues tend to shrink, the skin becomes dry and wrinkled, and the eyes become shrunken and the eyeballs soft. Fever develops, which may be mild but may become marked as dehydration progresses. Dehydration itself probably affects the temperature regulatory centres in the brain. As dehydration and salt loss progress, however, the plasma volume and the output of the heart decrease, with consequent decrease of the blood supply to the skin. Sweating decreases and may stop completely, and the main avenue for heat loss is closed. The body temperature may then rise precipitously. There are marked changes in the volume of the extracellular and intracellular fluids, but the blood plasma volume changes the last and the least. The plasma volume is maintained more or less constant at the expense of the tissue fluids. If, however, the plasma volume does fall, the output of the heart also fails, and the pulse rate climbs, indicative of a dangerous physical state. The renal (kidney) changes that occur in humans during prolonged water depletion similarly tend to maintain a normal balance. If water deprivation continues and the plasma volume falls, however, the output of urine will be drastically reduced. As long as urine output of more than 30 millilitres per hour is maintained, the kidney can excrete nitrogenous and nonnitrogenous solids with maximum efficiency. Once the urine flow is decreased below this level, the kidney is unable to function efficiently, the substances are retained in the body, and their concentration in the blood rises. The final result of prolonged dehydration is now apparent. The normal distribution of salt and water in the body is destroyed, the plasma volume decreases, and the blood viscosity increases. As a result of these changes renal function is impaired, the urinary output falls, and waste products accumulate. Far more life-threatening, however, is decreased loss of moisture from the skin, with the subsequent rise in temperature, and the fall in cardiac output with the attendant irreversible shock. Once renal failure occurs, about 8 percent of the total body water has been lost (4 litres). When 5 to 10 litres of body water have been lost a person is acutely and severely ill, with a contracted plasma volume, increased concentration and viscosity of the blood, renal failure and excessive urea in the blood, and a falling blood pressure. In a previously healthy adult, death follows the loss of 1215 litres of body water. In the very young, the very old, or the debilitated, death occurs at a lower level of dehydration. The treatment of any form of dehydration depends not only on restoring the depleted water but also on the reestablishment of normal levels of body electrolytes (salt) and limitation of the production of nitrogenous waste products. Before any of these therapeutic measures can be applied, however, the initiating cause must be removed. The sailor or the desert traveler must be rescued, the vomiting infant must be cured, or the underlying disease must be treated. Then, after accurate biochemical determinations of the levels of various electrolytes and other blood components have been made and the plasma volume has been measured, the physician may give measured quantities of the appropriate mixtures of salt and water. Given the right amounts of salt and water, the human body will gradually restore the normal relationships between the cells, the extracellular fluid, and the plasma volume. That done, the complicated functions of the kidney will clear the circulating blood of the retained waste products, and the body will have restored its own normal balance. in food processing, means by which many types of food can be preserved for indefinite periods by extracting the moisture, thereby inhibiting the growth of microorganisms. Dehydration is one of the oldest methods of food preservation and was used by prehistoric peoples in sun-drying seeds. The North American Indians preserved meat by sun-drying slices, the Chinese dried eggs, and the Japanese dried fish and rice. Hot-air dehydration was developed in France in 1795. Modern dehydration techniques have been largely stimulated by the advantages dehydration gives in compactness; on the average, dehydrated food has about 1/15 the bulk of the original or reconstituted product. The need to transport large shipments of food over great distances during World War II provided much of the stimulus to perfect dehydration processes. The advantages of reduced bulk later came to be appreciated by campers and backpackers and also by relief agencies that provide food in times of emergency and disaster. Dehydration equipment varies in form with different food products and includes tunnel driers, kiln driers, cabinet driers, vacuum driers, and other forms. Compact equipment suitable for home use is also available. A basic aim of design is to shorten the drying time, which helps retain the basic character of the food product. Drying under vacuum is especially beneficial to fruits and vegetables. Freeze-drying benefits heat-sensitive products by dehydrating in the frozen state without intermediate thaw. Freeze-drying of meat yields a product of excellent stability, which on rehydration closely resembles fresh meat. The dairy industry is one of the largest processors of dehydrated food, producing quantities of whole milk, skim milk, buttermilk, and eggs. Many dairy products are spray driedthat is, atomized into a fine mist that is brought into contact with hot air, causing an almost instant removal of moisture content. See also food preservation.