Dehydration A. Gross Effect on Water Exchanges

During restriction of water intake, all pathways of water exchange are reduced. Figure 7 and Table X I I give d a t a on water exchanges in several species when dehydrated. Except for the donkey and cow, evaporation is the major loss. Usually the greatest absolute conservation of water is in the reduction of urine and fecal water loss. Reduction of I.W. provides less absolute saving, and I.W. becomes the critical loss. T h e camel, however,

TABLE XII WATER EXCHANGES OF SEVERAL SPECIES OF MAMMALS WHEN DEHYDRATED0 Water intake Water loss Species Diet Body Water Water Oxida- Urine Fecal weight drunk in food tion water water water gm. ml. ml. ml. ml. ml.

Evapo- rative water loss ml.

TA R.H. CO.) (%) Source Dipodomys merriami 35 White rat 288

0.0 0.0 1.34 0.34 0.06 0.0 0.03 5.3 3.5 1.8 Peromyscus leucopus 20.0 1.71 0.14 0.82 1.32 WU<* noveboracensis1 kg. I. Donkey 97 0.0 Camelus dromedarius

I. I. 1.33 3.0 In winter 243 0.0 0.34 0.54 0.56 0.88 In Summer 243 0.0 0.12 0.54 0.90 0.73 Dry Shorthorn cowsc 632 19.2 1.18 1.18 9.04 9.20

1.09 Pearled barley 13.2 Mixed diet 1.27 Purina chow I. 2.51 Hay, dates 1.51 Hay, dates 1.02 Hay, dates 3.39 Lucerne hay

25 0 the Schmidt-Nielsens (1951) 21 — Dicker and Nunn (1957) 28 48 Chew (1951) 10 — K. Schmidt-Nielsen et al. (1957) 10 — K. Schmidt-Nielsen et al. (1957) 30 — K. Schmidt-Nielsen et al. (1957) — — Balch et al. (1953) ° Absolute data for mammals compared on a metabolic basis in Fig. 7. summer. b Restricted to 20.5% ad libitum drinking. c Restricted to 60% ad libitum. d = fecal water loss measured with urine.

Animals were not subject to heat stress except the camel in

I. I

136 Robert Μ. Chew

IN FOOD OXIDATION DRUNK

IN FECES IN URINE I.W.

Ο 16 h WATER EXCHANGES

FIG. 7. Water exchanges of several species of mammals while on limited water intake, compared on a metabolic basis. Absolute data, experimental conditions, and sources of data are given in Table XII.

effects its greatest savings by reduction in evaporation. The camel in summer is unique in t h a t its urine volume is greater when it is dehydrated t h a n when hydrated (Fig. 7).

Urine volumes are quickly adjusted to a restricted water intake in Peromyscus (Chew, 1951) and the white r a t (Dicker and Nunn, 1957), b u t I.W. is reduced more slowly.

B. Changes in Water Compartments

T h e nature of changes in the fluid compartments depends on the cause of the dehydration, the speed with which it occurs, and the physiological characteristics of the animal.

When dehydration is caused by lack of water in the absence of heat stress, dogs, rabbits, humans, and "likely all m a m m a l s " show a gradual decline in both E C W and I C W volumes, the I C W loss being greatest (Elkinton and Taffel, 1942; Black et al, 1944). Na+ and Cl~ are retained in E C W , helping to hold water and maintain circulating volume, b u t also producing hypertonicity of E C W . K⁺ is selectively excreted, and, judging from increased blood urea concentration, protein catabolism is accelerated.

However, cellular water loss is more rapid t h a n K⁺ and protein loss, and cellular dehydration ( = concentration) occurs. This intracellular

hyper-tonicity m a y be the cause of death in prolonged water deprivation (Black et al, 1944). W h e n cellular dehydration is caused in dogs by giving saline loads, there is respiratory failure after loss of 2 0 - 4 0 % I C W , although the heartbeat is still vigorous (Winkler et al, 1944).

W h e n dehydration is produced rapidly in dogs by heating (Kanter, 1953, 1954; Adolph, 1947a), in short-term exposure the water loss is mainly from E C W ; if the loss is rapid enough the blood plasma loses water out of proportion to its volume, and circulatory failure m a y occur. I n less in-tensive longer-term exposure, the loss is gradually balanced between E C W and I C W . T h e changes in water compartments of Merino sheep t h a t have been deprived of water and exposed to the sun differ according to the rate of dehydration (Macfarlane et al, 1961). In the hottest periods 1 2 % of the total weight loss is plasma water, b u t in cooler periods only 3 % . Interstitial water loss is about 3 5 % of total weight loss in the hottest periods and 2 1 % in cooler periods. I n both situations more t h a n half of the loss is intracellular water. Survival in the sun depends upon water reserves in the rumen (up to 1 3 % of body weight) and extracellular water reserves.

T h e E C W of Merinos of tropical regions is 2 0 - 8 0 % greater t h a n t h a t of Merinos from temperate regions. I n instances of rapid dehydration, sheep excrete much more sodium t h a n potassium; this partly osmotically balances the loss of water without salt t h a t is occurring through the respiratory t r a c t of t h e heat-stressed animals. Then, during recovery, Na+ excretion is reduced while water is being ingested.

W h e n salt is lost in sweat, changes are somewhat different. I n horses dehydrated in a hot climate, E C W and I C W loses are equal, b u t the loss from plasma is proportionately less (Lemaire et al, 1957). All extracellular electrolytes are reduced in total quantity, b u t not quite as rapidly as water, so t h a t the plasma becomes somewhat hypertonic; calculations indicate the cells are also dehydrated. Dehydration with and without salt loss both undoubtedly occur in wild mammals.

A third type of dehydration, intracellular dehydration or "dyshydration,"

involves the loss of salt without water or with replacement of water by drinking (Mach, 1954). When produced in dogs by intraperitoneal dialysis (Cizek et al, 1951; Semple, 1952) there is a maintained shift of water into cells, hemoconcentration, and low plasma chloride. W a t e r and food intake are reduced for several days, b u t t h e n moderate polydypsia m a y occur.

I n humans the sensation of thirst is much less t h a n in other types of dehydration. White rats lose body fluid and K⁺ after a few hours and make some readjustment of fluid distribution (Semple, 1952). Intracellular dehydration can be provoked by corticoid therapy in humans (Mach, 1954) and it might be a complication in wild animals in stressful situations when A C H secretion is high.

138 Robert Μ. Chew C. Kidney Function

I n the white rat, a 24-hour fast without water results in reduction of urine volume by 8 4 % and a four- to ninefold increase in U : P concentration ratio. Clearances of Cl~, Na+ and urea are decreased, while t h a t of K⁺ is increased (Heller, 1949). After 3 days the plasma is more concentrated;

Dicker (1957) suggests t h a t this reduces R P F and G F R , the last change being the important factor in concentration of the urine. I n the dog ex­

posed to acute heat dehydration, urine volume is reduced to the minimum in the first 15 minutes (Kanter, 1953).

Merino sheep deprived of water in the summer do not have a significantly reduced urine volume until they have lost 1 0 - 1 2 % of their initial body weight (Macfarlane et al., 1961). Urine volume decreased from 1600 ml.

per day to less t h a n 300 ml. after 3 days' deprivation, and to less t h a n 100 ml. after 6 to 10 days. Initially, oliguria was due to increased reabsorption of water, b u t later there was renal failure. Filtration rate, renal plasma flow, and glucose and electrolyte reabsorption were only half of normal after 5 days.

D . Metabolism and Respiratory Water Loss

I n water-restricted Peromyscus the metabolic rate is reduced 2 5 % and heat loss by vaporization, 2 3 % (Chew, 1951). Reduction of I.W. during restriction is due at least partly to reduced respiratory volume. Howell a n d Gersh (1935) found a gradual reduction in breathing rate of Dipodomys mohavensis kept on dry food. T h e p a t t e r n of breathing was altered in t h a t awake animals took several rapid breaths and then paused 2-4 seconds. The slower rates of breathing were usually observed in awake animals.

Paducheva et al. (1957) found t h a t karakul sheep watered only once every third day have reduced respiratory volumes and I.W.^r as much as 2 4 % less t h a n those of animals watered daily; I.W.^r showed a significant increase during the 1-2 hours immediately after watering.

Jerboas (Jaculus orientalis) are less active on a dry diet t h a n a moist one and show a reduction in basal metabolic rate of 9 % (Kirmiz, 1962). How­

ever, Kozakevich (1960) found t h a t pygmy ground squirrels (Citellus pygmaeus) have a greater metabolic rate at Ta 5° to 10° after a period on a dry diet. The body temperature of the dry-diet group was always somewhat higher t h a n the moist-fed group at T^a 5° to 35°.

E . Limits of Tolerance

M a m m a l s show great ability to stabilize body weight on water intakes much below those they voluntarily take (see Section I I , D, 2, d). When

water balance cannot be reestablished, the camel can withstand a loss of water equal to 4 0 % B⁰ (B. Schmidt-Nielsen et al., 1956), cats 1 7 - 2 3 % , dogs 1 6 - 2 0 % (Adolph, 1947a), donkeys 1 2 % (Adolph and Dill, 1938), Merino sheep 3 1 % or more (Macfarlane et al., 1961), and wild rabbits (Oryctolagus cuniculus) 4 8 % (Hayward, 1961). B y their extreme toleration of dehydration and their use of burrows, some 0. cuniculus are able to sur­

vive two months on dry pastures in the summer without drinking water;

just a small amount of succulent vegetation would ensure the survival of all individuals.

Tolerable water losses are not known for rodents, since they dehydrate slowly and their weight loss involves considerable solids. White mice die after losing 39^12% B⁰, but the fatal point is dependent on initial weight and a percentage expression is not completely meaningful. Tolerable weight loss cannot be increased by previous conditioning by restriction of water intake (Chew and Hinegardner, 1957). Other species tolerate the following weight loss before becoming moribund: Peromyscus leucopus 3 2 - 5 2 % B⁰ (Chew, 1951); Microtus pennsylvanicus 3 2 % (Hall, 1922); white mice 3 5 - 4 0 % (Cullingham, 1960); Mus musculus and Reithrodontomys megalotis from salt marshes 3 2 % and 3 0 % , respectively (Catlett and Shell-hammer, 1962); Clethrionomys rutilus, Microtus oeconomus, and Μ. arvalis 2 7 - 3 5 % (Hermann, 1961); Meriones crassus 3 6 % or more (Misonne, 1959);

Jaculus orientalis 4 0 % (Kirmiz, 1962); Citellus leueurus 4 4 . 6 % (Hudson, 1962); Lagurus lagurus 5 0 % (Hermann, 1961); Neotoma albigula 3 0 % (B.

Schmidt-Nielsen et al, 1948); white rat 4 0 - 5 0 % (Barker and Adolph, 1953).

When Microtus pennsylvanicus are kept as 1 pair and 5 pairs per pen of 15 square feet and subjected to total dehydration, the survival time of t h e grouped animals is significantly lower t h a n of t h e single-paired animals.

Social interaction and increased general activity of the crowded animals apparently lowers their resistance to dehydration (Warnock, 1961).

The endocrine systems of a t least some desert m a m m a l s are hyperadapted to dehydration stress. The dromedary camel does not show the spectacular changes in endocrine glands when it is deprived of water, as do some mammals (Charnot, 1958). After a camel has been 21 days without drinking, its adrenal glands do not show any important depletion of lipids and the posterior pituitary does not show the slightest decrease of neurosecretion.

I n kangaroo rats (Dipodomys merriami), after a week on dry rolled oats mixed with 5 % NaCl and 5 % N a H C 0³, t h e adrenal glands have not changed with respect to their density of staining and distribution of lipids.

I n t h e adrenals of white rats, however, there is marked depletion of lipids (Nichols, 1949).

140 Robert Μ. Chew F . Recovery from Dehydration

T h e rate of replacement of a water deficit has been measured in only a few m a m m a l s ; these can be classified into two groups: (1) those t h a t replace their deficit immediately in one large drink, and (2) those t h a t replace more slowly, b u t usually within 1 day.

All the herbivores studied so far show a replacement ^ t h a n deficit in one quick drink. The camel can replace 2 0 % B⁰ in 10 minutes ( = 5 liters per minute), and is limited only by its capacity to drink 3 0 - 3 3 % B⁰ at one time; the donkey can drink at a rate of 8 liters per minute and t a k e u p to 2 0 % Bo in less t h a n 2 minutes (B. Schmidt-Nielsen et al., 1956). Merino sheep, after 5 days without water in the summer, take 7-9 liters at once, against a 11.8-kg. total weight loss (Macfarlane et al., 1956). Desert mule deer in summer average 5.8 liters in 2.8 minutes, with maximum rates of intake of 2.7 liters per minute (Clark, 1953).

For the desert bighorn sheep (Ovis canadensis nelsoni) in summer, the average drinking time for rams, ewes, and lambs is 5.3 minutes (Welles and Welles, 1961). I n this time, one ewe drank nearly 3 gallons and a 7-month-old nursing lamb drank half a gallon. Dehydrated bighorn sheep undergo a striking "metamorphosis" after drinking. Within a half hour an emaciated animal with dull and rough pelage becomes smooth and rounded, with the glistening hide of an animal in perfect health.

Ruminants have several advantages in making up a water deficit quickly ; they have a larger " s t o m a c h " capacity, and the water goes into the rumen, from which it is only slowly released into the abomasum and absorbed (Watson, 1944; Andersson and Persson, 1958). Rapid filling of the rumen is of obvious advantage to grassland and desert mammals t h a t must come to an exposed water hole for drinking.

The dog also can quickly make up a water deficit in 2-3 minutes, pro­

viding it is not too great. M a x i m u m drinking rates of 0.7 liters per minute have been measured for 17-28-kg. dogs. I n heat-dehydrated dogs, deficits of 8 % B⁰ or less are usually made up immediately, whereas greater deficits take longer (Adolph, 1939, 1947a). However, t h e degree of replacement m a y vary for unexplainable reasons. Adolph (1943) found t h a t a water deficit incurred during exercise on a cool day was only 6 5 % immediately replaced, whereas t h a t on a hot day was 9 5 % replaced. K a n t e r (1953) found t h a t dogs allowed to drink voluntarily while exposed to T^a 48°

replaced less of their deficit t h a n dogs t h a t were not allowed to drink until after exposure.

The cat does not immediately make up losses of as little as 4 % B⁰, and rabbits, guinea pigs, and white rats make up an even smaller percentage of a smaller deficit (Adolph, 1947a; Adolph et al., 1954).

Weight recovery of rodents depends on the composition of their weight loss. If the weight loss is largely water, it is made up within the first day.

White mice, after restriction to one-third normal intake, drink twice the normal amount on the first d a y of recovery and almost completely regain lost weight on t h a t day (Chew and Hinegardner, 1957). Peromyscus recovers from restriction similarly (Chew, 1951). The white rat recovers lost weight in 1 hour following 1-2 days of deprivation (Adolph, 1943).

R a t s t h a t are rapidly dehydrated, losing 10.2% of their body weight in 6 hours, recover their initial weight promptly in 3-5 minutes of continuous drinking (Lewis et al., 1960). Recovery is slower when solids are lost in addition to water. White rats take a week to recover weight after 6 days of deprivation of water and drink little more t h a n normal in the first days of recovery (Adolph, 1943). I n rats 6 days without water, intubation of water does not cause diuresis unless the water load is greater t h a n 1 0 % B⁰; this is more t h a n necessary to reestablish normal osmolarity and apparently indicates an inability of a concentrating kidney to cope immediately with a water excess.

T h e rabbit drinks very little during heat dehydration and, as analyzed by Fowler (1955), takes 5 days to 4 weeks to recover fully. T h e slow recovery involves a period of excessive fluctuations of body weight ( = water ?) following the initial water loss.

After 3 days without water in hot weather, horses replace 9 6 % of their water deficit in the first day, b u t only 6 0 % of total weight loss. Weight is not fully regained in 5 days, apparently because of nutritional difficulties (Lemaire et al., 1957).

T h e loss of salt in sweat m a y slow recovery of a water deficit, since the salt deficit m u s t be made u p simultaneously. Man, a prime example of this, makes u p only 3 0 - 5 0 % of a water deficit in the first period of drinking, and the balance is taken with meals (Adolph, 1943). Horse sweat contains salt (Jirka and Kotas, 1959; Buddenbrock, 1956), and salt loss is probably a factor in the slower recovery of this mammal.

T h e donkey and the dog lose little or no salt during heat dehydration (Adolph and Dill, 1938), and their rapid drinking m a y represent dilution of body fluids back to normal tonicity. Freeborn et al. (1934) found no Cl~

in water used to wash the skin of cattle after exposure to T^a 46°, a fact indicating t h a t their sweat is salt free.

Sodium excretion by kidneys during heating is a function of the method of evaporative cooling. I n men (sweating, salt containing sweat) sodium is retained by the kidneys, while in sheep (panting) there is excretion of salt in order to maintain osmotic homeostasis (Macfarlane et al., 1958b).

Certain tissue damage from dehydration must also be repaired. Barker and Adolph (1953) found necrosis of lymph nodes and hemorrhages of

142 Robert Μ. Chew intestinal mucosa in rats deprived 13-16 days. Garofeanu and Derevidi (1924) found lesions in dogs as early as the fourth day without water:

congestion and dilation of blood vessels, narrowing of lung alveoli by vascular distention, desquamation of alveolar epithelium, reduction in size of thyroid vesicles, and changes in staining properties of thyroid colloid and kidney tubules.

XI. Water Balance and Biological Processes

In document Water Metabolism of Mammals (Pldal 92-100)