Page 27 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Physiology of Body Fluids in Dogs and Cats 17

Not all solute produced by metabolism of ingested and evaporative losses usually were less than 1 mL/kg/hr.
absorbed food is necessarily excreted in the urine. Fecal During periods of activity, evaporative losses were
excretion of solutes does occur. In most healthy dogs, estimated at almost 7 mL/kg/hr. 40
þ

however, daily fecal Na ,K , and Cl excretion is sub- The total water intake per day of the dogs in Table 1-8
þ
stantially lower than urinary mineral excretion. The daily was quite variable from dog to dog, ranging from approx-
renal solute load is thus a function of the quantity of food imately 20 to 91 mL/kg/day. If insensible loss (primarily
ingested and of diet composition. Assuming a range of composed of respiratory evaporative loss) for each dog is
urine osmolalities in healthy dogs between 1000 and subtracted from total daily water intake, the range of
2000 mOsm/kg and a urine solute load of approximately water intake unrelated to insensible losses is narrower
400 mOsm in a 10-kg dog, the range of urine output (11 to 20 mL/kg/day) than the range for total water
would be 200 to 400 mL or 20 to 40 mL/kg/day. Urine intake. This emphasizes the profound effect that insensi-
volume is thus a function of renal solute load. Another ble losses may have on daily water balance in dogs.
important factor that determines urine volume is the Increases in ambient temperature, especially in associ-
total quantity of water ingested per day. Total water con- ation with low relative humidity, may result in marked
sumption depends both on water in the diet and on water increases in respiratory water evaporation in dogs. The
voluntarily consumed by drinking. panting response to heat is more efficient in dogs than
in cats. At an ambient temperature of 40 C, cats increase

URINARY FREE WATER their respiratory rate 4.5 times, whereas dogs can increase
9
Excretion of urinary free water is controlled by stimulation their respiratory rate 12 to 20 times. The estimated

or inhibition of secretion of ADH and by thirst. Urinary respiratory water loss in a panting dog at 41 C was
free water increases when enough water has been ingested 469 mL/day, whereas that for a cat under the same
to dilute body solute and result in hypotonicity. A 1% to 2% conditions was 41 mL/day (Table 1-9).
decrease in serum osmolality inhibits secretion of ADH
and abolishes thirst in humans. 45,46 During water deple- WATER INTAKE
tion, body water osmolality increases and ADH secretion
isstimulated.Anincreaseinserumosmolalityof1%to2%is
sufficient to provoke maximal ADH secretion in WATER IN FOOD
humans. 45,46 In dogs, increases in osmolality of 1% to The percentage of water in pet foods is variable. In gen-
3% stimulate thirst. 39,41 A water loss of 5 mL/kg of body eral, canned foods are more than 70% water, semimoist
weight provoked drinking in experimental dogs. 47 There- foods 20% to 40% water, and dry foods less than 10%
fore, daily urinary free water losses are very small during water. 29 Two representative cat diets are described in
water deficiency in otherwise healthy dogs and cats. Table 1-10. Therefore water in food makes up a variable
proportion of total daily water consumption, depending
RESPIRATORY AND CUTANEOUS on what type of diet is fed (Figs. 1-9 and 1-10). Cats can
EVAPORATIVE LOSSES exist without drinking water if fed a diet of cod, salmon,

Cutaneous evaporative water losses usually are small in or beefsteak. 43 If the beefsteak or salmon was partially
dogs and cats. Cats in hot environments are reported desiccated, cats became hydropenic (increased serum
to lick themselves with saliva to promote evaporative osmolality and serum sodium concentration), anorexic,
9
cooling. This phenomenon is rarely observed in clinical and cachectic. Thus, cats may meet their water needs
practice, but if it occurs, salivary water losses could signif- solely from the water in some foods.
icantly increase water need. Evaporative water loss from
the skin is minimal in dogs and cats because eccrine sweat DRINKING
glands (which are limited in distribution to the foot pads) The volume of water voluntarily ingested each day by
do not participate in thermoregulation in these species. healthy, sedentary dogs and cats in a thermoneutral envi-
Evaporative water losses usually are less in healthy, seden- ronment depends on the composition and the quantity of
tary cats in a thermoneutral environment compared with the diet ingested. Water intake decreases in experimental
dogs (see Table 1-6), probably because cats rarely pant. dogs if food intake is limited (Fig. 1-11). 9,10,33,34 After a
Evaporative losses in caged, sedentary laboratory dogs 1-day fast, drinking decreased to 25% to 50% of the nor-
are quite variable from dog to dog, and some individuals mal volume in dogs. After a 14- to 18-day fast, drinking
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experience significant daily losses via this route (Table was 45% of the normal volume in dogs. Conversely, if
1-8). Dogs in the study summarized in Table 1-8 fell into water intake is limited, food intake decreases in dogs
two categories: those that remained quiet in their cages all and cats. 43 As mentioned earlier, cats continue to eat
of the time and those that ran in circles, barking for and survive on some diets without drinking water. In
several hours each day. The mean evaporative loss for dogs that are chronically deprived of food, a basal level
2
all dogs was 27 mL/kg/day. This value overestimates of drinking is maintained. In sick, anorexic small animal
evaporative loss in quiet dogs. For dogs at rest, patients, drinking may decrease, and because such