Page 25 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Physiology of Body Fluids in Dogs and Cats 15

Drinking Water in food Metabolic water function of both the amount of solute in the diet and the
osmolality of the urine. Diets with higher solute contents
require greater total water intake than do diets of rela-
Saliva
GUT tively lower solute content. Most small animals have free
Fecal water access to water and therefore ingest sufficient water to
support urinary excretion of dietary solutes. Sick animals
Evaporation often are inactive and have a poor appetite or are anorexic.
Skin ECF ICF
Sweat Water requirements to replace insensible losses related to
Evaporation activity and to support renal solute excretion thus are
Respiratory decreased, and maintenance water requirements presum-
Panting
ably are lower than those in healthy individuals. Increased
insensible water losses caused by fever or increased meta-
bolic rate during disease may offset this decrease in water
Free water loss: ADH
requirement. Basal needs must be defined accurately if
Obligatory urinary water loss: water requirements during disease are to be estimated
Renal solute load
Figure 1-7 Total body water: daily input and obligatory using increments of basal requirements. To address this
losses. (Adapted from Chew RW. Water metabolism of mammals. issue, the following discussion focuses on the relationship
In: Mayer WW, Van Gelder RG, editors. Physiologic mammalogy, Vol between basal water requirements and dietary solute in
II: mammalian reaction to stressful environments. New York: sedentary small animals in a thermoneutral environment.
Academic Press, 1965: 43–177.)
WATER LOSSES

Although the classic definition of insensible water loss in
healthy animals is water lost via the skin or lungs, in clini- URINARY AND FECAL WATER LOSS
cal veterinary medicine water lost in the feces and saliva Daily urinary water losses may be divided into obligatory
also is included in insensible losses. This approach is used water loss (i.e., water needed to excrete the daily renal sol-
because it usually is impractical to measure fecal and sali- ute load) and free water loss (i.e., water excreted unac-
vary water losses, which are small under normal companied by solute under the control of antidiuretic
conditions. This chapter uses the clinical definition of hormone [ADH]). Clearance of free water increases dur-
insensible water loss. Although evaporative losses may ing relative water excess, thus protecting the animal from
be great in heat-stressed, exercising, or active animals, the overhydration and hypotonicity that would result
the most important and predictable obligatory daily loss from retention of water in excess of solutes. Obligatory
of water in healthy, sedentary dogs and cats in a renal water loss must occur even in states of relative water
thermoneutral environment occurs via urine. Estimates deficit so that solute may be eliminated from the body.
for water input by drinking and water loss via urine, feces, Similarly, a small daily, obligatory fecal water loss is
or total insensible avenues in healthy dogs and cats are required for fecal excretion of solute. Obligatory fecal
variable (Table 1-6). water loss may increase if fecal solute increases (e.g., addi-
Daily maintenance fluid requirement may be defined tion of CaCl 2 or MgCl 2 to the diet). These ions increase
as the volume of fluid needed each day to maintain the fecal solute, because Ca 2þ and Mg 2þ are poorly absorbed
animal in zero fluid balance. Maintenance needs thus from the gastrointestinal tract. Maintenance water
are determined by daily sensible and insensible losses, requirements must include at least enough water to allow
by ambient temperature and humidity, by the animal’s renal and fecal solute excretion.
voluntary or forced activity, and by disease. A high ambi-
ent temperature, especially with low humidity, results in OBLIGATORY URINARY AND FECAL
increased insensible evaporative losses and, therefore, in WATER LOSSES
increased maintenance fluid requirements. Similarly, fever The amount of water required for elimination of the uri-
and increased metabolic rate associated with disease may nary solute load in theory depends on the maximal urine
increase fluid requirements. Estimates of maintenance osmolality that can be achieved by the animal (Table 1-7).
fluid requirements during thermal stress or disease usually However, solute usually is not excreted at maximal urine
are based on empirical adjustments of the estimated basal osmolality, especially when water is readily available for
fluid requirements. Maintenance fluid requirements also voluntary consumption. Urinary osmolalities from exper-
are determined partially by composition of the diet. In imental dogs at rest and in water balance ranged from
dogs and cats, most absorbed dietary nitrogen and 1000 to 2000 mOsm/kg. 17 In a study of client-owned
minerals not required to maintain zero balance or to pro- dogs, urine osmolality ranged from 161 to
vide for growth or tissue repair are excreted daily in urine. 2830 mOsm/kg, and urine osmolality was greater in
The volume of urine required for solute excretion thus is a the morning (mean, 1541 527 mOsm/kg; range,