Page 132 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Disorders of Water and Salt Balance
By decreasing osmolality (receptors in the liv- testinal lumen (in ileus; → p.156), into the ab-
er and brain) and via hypervolemia (stretch dominal cavity (ascites; → p.170) or in the pe-
receptors in the right atrium) an excess of wa- riphery (edema; → p. 234).
ter normally inhibits ADH release and thus An excess of water (hyperhydration) neces-
triggers diuresis (→ p.100). The blood pres- sarily leads to the enlargement of one body
sure, raised by the hypervolemia, inhibits the compartment (→ C). If there is NaCl excess at
renin–angiotensin–aldosterone system. At the the same time (isotonic or hypertonic hyper-
same time the release of ANF and possibly also hydration), the extracellular space is in-
Kidney, Salt and Water Balance ity. Excess NaCl increases ADH release via hy- NaCl content is normal or reduced (hypotonic
of ouabain is stimulated. The result is natriu-
creased. In hypertonic hyperhydration the ex-
resis which, after some delay, brings about the
tracellular space is increased, partly by osmot-
ic withdrawal of water from the cells. If the
correction of the plasma volume and osmolal-
hyperhydration), it is mainly the intracellular
perosmolality and thus leads to antidiuresis
and also an adjustment of osmolality.
space that is enlarged.
In lack of water (dehydration) the extracel-
An excess of water and NaCl (→ A) occurs,
for example, when fluid with greater osmolal-
lular space is reduced, especially when there is
a simultaneous lack of NaCl (isotonic or hypo-
ity than that of urine is ingested (e.g., ship-
the intracellular space is reduced (hypertonic
excretion of water and NaCl is also reduced in
dehydration), while it is increased in isolated
impaired renal function (GFR ↓). Uncontrolled
infusion of isotonic NaCl solution can then
lack of NaCl (hypotonic dehydration).
5 wrecked people drinking sea-water). The renal tonic dehydration). In isolated lack of water
lead to an excess of NaCl and water, while infu- Any reduction in extracellular space is espe-
sion of isotonic glucose solution results in an cially dangerous because of the decrease in
excess of water that remains in the body after plasma volume (hypovolemia). Signs of this
glucose has been metabolized. Even when kid- are reduced central venous pressure, tachycar-
ney function is intact, there will be an excess of dia, and a tendency to faint. If there is a drop in
water or NaCl if the release of mineralocorti- blood pressure, renal function is impaired and
coids or ADH is inappropriately increased (e.g., the release of ADH and aldosterone leads to
by hormone-producing tumors, → p. 260, oliguria (danger of urolithiasis). Conversely,
266). If the filtration balance in the peripheral an enlargement of extracellular volume leads to
vasculature is tipped, edemas occur at the ex- a rise in blood pressure when a part of the
pense of plasma volume (→ p. 234). This re- volume remains in the intravascular space
sults in a decreased plasma volume, which (→ p.114). On the other hand, the dilution of
stops the release of natriuretic factors (Atrial intravascular proteins promotes filtration in
natriuretic factor, ouabain) and stimulates the peripheral capillaries and edema forma-
that of ADH, renin, angiotensin, and aldoste- tion (→ p. 234) and, in the worst case, pulmo-
rone. The renal retention of NaCl then leads to nary edema (→ p. 80).
the correction of plasma volume, and thus to If the intracellular volume is enlarged, there
an increase in extracellular volume. is a particular danger that cerebral edemas
A lack of water and NaCl (→ B) can be the re- will develop (→ p. 358). Reduction in the intra-
sult of external fluid loss as is the case, for ex- cellular volume also leads mainly to disorders
ample, with excessive sweating (fever, heat), of the central nervous system that can pro-
in diarrhea, blood loss, burns or salt-losing gress to loss of consciousness and even death.
kidney (→ p.108). Renal water loss can occur
in ADH deficiency (central diabetes insipidus;
→ p. 260) and in lack of responsiveness of the
kidney to ADH (renal diabetes insipidus;
→ p.100). Even when the external balance is
122 kept, dangerous “internal losses” can occur,
such as a shift of plasma volume into the in-
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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