Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 17 Fluid, Electrolyte, and Acid-Base Balance
the volume dilution procedure used. Regulation of ECF
volume is a complex process in which a variety of factors
interact. The ECF consists of all the fluids located outside
the cells and includes the plasma (0.05 l/kg), interstitial
fluid and lymph (0.15 l/kg), and the transcellular
fluids ( Edelman and Leibman, 1959 ; Rose, 1984 ; Saxton
and Seldin, 1986 ). The transcellular fluids, which include
the fluid content of the gastrointestinal tract, are generally
considered a subcomponent of the ECF. In small animal
species, the fluid content of the gastrointestinal tract
is relatively small ( Strombeck, 1979 ). In the large animal
herbivore species, a substantial volume of fluid is normally
present within the gastrointestinal tract. In the horse, this
may amount to 30 to 45 l ( Carlson, 1979a ), and in cattle,
the forestomach may contain as much as 30 to 60 l of fluid
( Phillipson, 1977 ). During periods of water restriction and
certain other forms of dehydration, this gastrointestinal fluid
reservoir can be called on to help maintain effective circulating
volume ( McDougall et al ., 1974 ). All of the fluids
of the ECF contain sodium in approximate concentrations
of 130 to 150 mEq/l H 2 O. Sodium provides the osmotic
skeleton for the ECF, and the sodium content is the single
most important determinant of ECF volume ( Rose, 1984 ).
Sodium deficits result in decreases in ECF volume, whereas
sodium excess is most often associated with water retention
and results in edema ( McKeown, 1986 ; Rose, 1984 ).
C . Intracellular Fluid Volume
The ICF volume represents the fluid content within the
body’s cells. This volume cannot be measured directly but
is calculated as the difference between the measured TBW
and the measured ECF volume. Potassium provides the
osmotic skeleton for the ICF in much the same way that
sodium provides the osmotic skeleton for the ECF. Because
water is freely diffusible into and out of the cell, changes
in the tonicity of the ECF are rapidly reflected by similar
changes in ICF tonicity ( Saxton and Seldin, 1986 ). This is
largely the result of the movement of water across the cell
membrane with resultant changes of ICF volume. Thus,
whereas plasma sodium concentration decreases in response
to water retention, ICF volume increases ( Humes, 1986 ).
On the other hand, with water depletion resulting in hypernatremia,
ICF volume decreases ( Humes, 1986 ). Relatively
little is known about the organization of intracellular water
into the various subcellular compartments and organelles.
IV . REGULATION OF BODY FLUIDS AND
ELECTROLYTES
A . Effective Circulating Volume
The effective circulating volume refers to that part of the
ECF that is within the vascular space and is effectively
perfusing the tissues ( Rose, 1984 ). Effective circulating
volume tends to vary with ECF volume, and both parameters
vary with the total body sodium stores ( Rose, 1984 ).
Sodium loading produces volume expansion, whereas
sodium depletion leads to volume depletion.
Effective circulating volume is not a quantitatively measurable
entity but refers to the rate of perfusion of the capillary
circulation. Effective circulating volume is maintained
by varying vascular resistance, cardiac output, as well as
renal sodium and water excretion ( Rose, 1984 ). Decreases
in effective circulating volume result in decreased venous
return, decreased cardiac output, and decreased blood pressure.
Decreased volume and pressure are recognized by
special volume receptors in the cardiopulmonary circulation
and kidney, which trigger increased sympathetic tone
resulting in increased arterial and venous constriction as
well as increased cardiac contractility and heart rate. These
responses tend to correct for the volume deficit by increasing
cardiac output and systemic blood pressure. Volume and
pressure changes associated with decreases in effective circulating
volume also result in activation of the renin-angiotensin
system with subsequent enhancement of aldosterone
secretion by the adrenal cortex ( Brobst, 1984 ). Aldosterone
acts to enhance renal sodium resorption, which is a critical
factor for maintaining and eventually restoring effective circulating
volume. Additional factors that influence sodium
resorption in response to changes in fluid volume include
alterations in glomerular filtration rate, renal hemodynamics,
atrial natriuretic factor, and plasma sodium concentration.
B . Antidiuretic Hormone
Antidiuretic hormone (ADH) plays a primary role in the
regulation of the osmolality of the body fluids. Antidiuretic
hormone is synthesized in the hypothalamus, stored in the
neurohypophysis, and released in response to changes in
plasma osmolality. Because sodium concentration is the
primary determinant of plasma osmolality, ADH-release is
closely correlated to plasma sodium concentration. Special
sensors in the hypothalamus recognize increases in plasma
osmolality, and the normal response is increased thirst
to enhance water intake and the release of ADH, which
increases water reabsorption by the renal collecting tubules.
Antidiuretic hormone exerts its activity on the collecting
tubules by activating adenyl cyclase; this results in the generation
of cyclic adenosine monophosphate (cyclic AMP)
and protein kinases, which in turn alter the permeability of
the tubules to water ( Rose, 1984 ). Antidiuretic hormone also
is released in response to decreases in effective circulating
fluid volume, although the renin-angiotensin system exerts
primary control over volume changes. Antidiuretic hormone
acts extrarenally as an arterial vasoconstrictor, thus increasing
blood pressure. Plasma osmolality decreases in response
to a water load, and ADH release is inhibited. The resultant
reduction in ADH-mediated reabsorption of water in the collecting
tubules allows for appropriate renal excretion of the
water load and a return of plasma osmolality toward normal.