South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Body fluid is distributed in several compartments as illustrated in Figure 1. Water is the most
abundant molecule in the body (total body water [TBW] accounts for 60-65% of body mass).
Because the body is made up of cells, it should not be surprising that most water (two-thirds of
TBW) is found inside cells, collectively called the intracellular fluid space (ICF). The remaining
one-third of TBW is outside cells, in the extracellular fluid space (ECF), which can be further
subdivided into the plasma space (25-30 litres), the interstitial spaces between cells (40-45 litres),
and the transcellular space (about 30 litres, primarily in the lumen of the gastrointestinal tract but
to a lesser extent in cerebrospinal fluid, pleural fluid, peritoneal fluid, and within joints). In
addition to water, body fluids are rich in electrolytes but the electrolyte composition varies greatly
between ICF and ECF (Figure 1).
Figure 1. Body fluid compartments and
exchangeable electrolyte contents of a 500
kg horse.
As with water balance, diet can also affect electrolyte balance. Most hays are rich in potassium
(K+) but low in sodium (Na+). Thus, horses typically consume diets that are excessive in K+ and
marginal in Na+. This is reflected by the fact that urine of normal, hydrated horses usually
contains high concentrations of K+ (200-400 mEq/L) and low concentrations of Na+ (0-50
mEq/L). Further, with disorders causing decreased feed intake and dehydration, the ability of the
kidneys to conserve (reabsorb) Na+ is much greater than that for K+. This point is particularly
important to remember for two reasons: i) most crystalloid fluids that are administered
intravenously are similar in electrolyte composition to plasma – that is, they are high in Na+ but
contain little K+; and ii) horses that are off feed will have continued, obligate losses of K+ in
urine. Without adequate supplementation of K+, horses that are receiving intravenous fluids for
more than a couple of days are at risk of further depletion of K+ from body stores and this could
contribute to muscle weakness (e.g. another complicating factor for post-operative ileus).
Another electrolyte that can be significantly affected by diet is calcium (Ca++). Horses are
somewhat unique among the large domestic species in that Ca++ metabolism is not well regulated
at the gut. As a consequence, horses absorb excessive amounts of Ca++ from feed and this excess
is eliminated in the form of calcium carbonate crystals in urine (a major cause for the turbid
appearance of normal equine urine). Thus, anorexia in horses is almost always accompanied by a
modest decrease in total serum Ca++ concentration (e.g., from a normal value of 12 mg/dL to 10
mg/dL [3 mmol/L to 2.5 mmol/L]) and decreased turbidity of urine. Fortunately, this trend toward
mild hypocalcaemia rarely complicates the primary disease process although it can exacerbate
muscle weakness and decrease cardiac contractility.
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