Page 58 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

48 ELECTROLYTE DISORDERS

Interstitial sodium content is perceived as the fullness of the
Tubular fluid fluid Blood
circulating blood volume. The term effective circulating
Lateral volume has been used to refer to the relative fullness of
intercellular
space
Luminal membrane the circulating portion of the extracellular compartment
Basolateral
as perceived by the body. There are several sensors in the
membrane
Tight junction afferent limb of the body’s regulatory system for control
Na +
of sodium balance (see Table 3-1). Low-pressure
Glucose, amino acids, etc.
mechanoreceptors (i.e., volume receptors) in the cardiac
Na +
H + atria and pulmonary vessels and high-pressure baro-
Proximal 3Na +
tubule − ATP 2K + receptors (i.e., pressure receptors) in the aortic arch
Cl
Anion − and carotid sinus play a primary role in the body’s ability
to sense the adequacy of the circulating volume. Within
H +
Na + the kidneys, the juxtaglomerular apparatus responds to
Loop
diuretics changes in perfusion pressure with changes in renin
production and release. Less well characterized are
– Na + a +
K +
Thick 2Cl − receptors in the liver and the central nervous system that
ascending 3Na +
limb of ATP 2K + may contribute to sodium homeostasis.
Henle's loop Na + The kidneys constitute the primary efferent limb of
H +
sodium control and regulate sodium balance by excreting
an amount of sodium each day equal to that ingested.
Thiazide
diuretics There are several overlapping control mechanisms for
– Na + regulation of renal handling of sodium. This redundancy
Distal Cl − 3Na +
convoluted ATP 2K + serves to protect against sodium imbalance should one
tubule Na + control mechanism fail. The two points of control for
sodium balance in the kidneys are glomerular filtration
and tubular reabsorption. Autoregulation maintains renal
+
K -sparing
diuretics blood flow and glomerular filtration rate (GFR) relatively
constant despite fluctuations in systemic arterial pressure;
Collecting – 3Na +
Na + ATP
duct 2K + thus, the filtered load of sodium is also kept relatively
constant (see Chapter 2).
Glomerulotubular Balance
Figure 3-2 Renal tubular mechanisms for the reabsorption of
sodium along the length of the nephron. (Drawing by Tim Vojt.) Even slight changes in GFR have the potential to have
drastic effects on sodium balance if the absolute amount
of sodium reabsorbed by the tubules remains constant.
the nephron is responsible for altering sodium reabsorp- Consider a normal 10-kg dog in sodium balance with a
tion in response to dietary fluctuations. In the late distal serum sodium concentration of 145 mEq/L and a
tubule (so-called connecting segment) and collecting GFR of 4 mL/min/kg. The daily filtered load of sodium
þ
ducts, sodium enters passively through Na channels in in this dog would be 57.6 L/day 145 mEq/L ¼ 8352
the luminal membranes of the principal cells. 127,147 This mEq/day. If the kidneys reabsorb 99.5% of the filtered
movement of Na þ generates a lumen-negative load of sodium (8310 mEq/day), the amount excreted
transepithelial potential difference that facilitates Cl in the urine is 42 mEq/day. Consider what would happen
þ
reabsorption. The Na channel in the principal cells is if there was a primary (i.e., spontaneous) increase in GFR
blocked by the diuretics amiloride and triamterene. of only 1%, but the absolute amount of sodium
One of the main effects of aldosterone is to increase the reabsorbed remained unchanged. The filtered load of
number of open luminal Na þ channels in the cortical sodium would be 58.2 L/day 145 mEq/L ¼ 8439
collecting ducts, thus altering sodium reabsorption in mEq/day, but the amount reabsorbed would remain
response to changes in dietary sodium intake. The renal 8310 mEq/day. This would result in the excretion of
tubular mechanisms for sodium reabsorption are 129 mEq/day, an amount three times that normally
summarized in Figure 3-2. excreted. Under these conditions, the dog would develop
negative sodium balance. Glomerulotubular balance
RENAL REGULATION OF SODIUM prevents this scheme of events from occurring.
BALANCE If spontaneous (primary) fluctuations in GFR occur,

ECF volume is directly dependent on body sodium con- the absolute tubular reabsorption of filtered solutes
tent. The body is able to sense and respond to very small changes in a similar direction. Thus, the fraction of the
changes in sodium content. The adequacy of body filtered load that is reabsorbed remains relatively constant