Page 26 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

16 APPLIED PHYSIOLOGY

TABLE 1-6 Measurements of Daily Water Intake and Output in Sedentary
Dogs and Cats
Measurement Species mL/kg/day* Condition or Diet Reference

Input
Water drunk Feline 71.3 Chew 9
Feline 50.6 Thrall and Miller 57
Canine 56.1-70.8 Chew 9
Canine 38.9 (19.5-84) O’Connor 40
Output
Urine volume Canine 13.3 (10.5-17.9) Caged O’Connor 40
Fecal water Feline 25-29 g/day Caged Jackson and Tovey 23
Feline 56 g/day Caged Thrall 57
Insensible loss Canine 20.5 69% H 2 O diet Smith et al 54
Feline 12.42 70% H 2 O diet Hamlin and Tashjian 16
Feline 29.0 Dry ration Thrall and Miller 57
Canine 26.2 (8.1-70.7) Beef and biscuit O’Connor 40
*Except as noted in table.

TABLE 1-7 Maximal Urine 1000
Osmolalities (mOsm/kg) 600 1200
Species mOsm/kg Reference 1600

Dog 2425 Chew 9 Urine volume (ml/day) 400 2000
17
Dog 2791 Hardy and Osborne *
Cat 3200 Chew 9
Cat 3420-4980 Thrall and Miller 57 200
50
Cat 2984 Ross and Finco *
*Values obtained after dehydration resulting in 5% body weight loss.
200 400 600 800
2Na + 2K + Urea (mmol/day)
273 to 2620 mOsm/kg) than in the evening (mean, Figure 1-8 Urine volume of a dog plotted against urinary
1400 586 mOsm/kg; range, 161 to 2830 mOsm/ excretion of solute (2Na þ 2K þ urea) during consumption of 320
kg). 58 There was no effect of sex on urine osmolality, (1), 385 (l), and 770 (s) grams of food. Each symbol represents data
but urine osmolality decreased significantly with age. from 1 day. The lines labeled 1000, 1200, 1600, and 2000 indicate
Figure 1-8 depicts urine volume and urine osmolality urine osmolality (mOsm/kg). (From O'Connor WJ, Potts DJ.
plotted as a function of urine solute in a dog fed varying Kidneys and drinking in dogs. In: Michell AR, editor. Renal disease in
dogs and cats: comparative and clinical aspects. Oxford, UK:
quantities of food. 40 Increased intake produced increased Blackwell Scientific, 1988: 35.)
renal solute and increased urine volume; however, urine
osmolality remained approximately 1600 mOsm/kg
(1200 to 2000 mOsm/kg). 41 Urine osmolalities did
not, as might be expected, increase toward the maximum from the diet of an animal in zero balance, all nitrogen
attainable (2400 to 2800 mOsm/kg) in water-deprived is assumed to form urea. Urea constitutes two thirds of
dogs. 9,17 Thus, urine osmolality is conserved in the pres- the urinary solute load in dogs. 40 The amount of solute
ence of increased urine solute load by an increase in urine in the diet is determined by the composition and the
volume. The physiologic mechanisms that conserve urine quantity of food and minerals ingested. Increasing dietary
osmolality as the renal solute load varies are not well protein results in increased urea production. Metabolism
defined. of carbohydrates and fats yields only CO 2 and H 2 O and
The renal solute load is derived from dietary sources of does not produce urea or other solutes that must be
þ
protein and minerals and comprises urea, Na ,K ,Ca 2þ , excreted in the urine. Diets high in minerals that are well
þ
þ

Mg 2þ ,NH 4 , and other cations and PO 4 3 ,Cl , absorbed from the gut (usually NaCl) provide more
2 solute for excretion.
SO 4 , and other anions. When estimating solute load