Page 5 - Medicine and Surgery

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Principles and practice of 1

medicine and surgery

Fluid and electrolyte balance, 1 Perioperative care, 13 Infections, 19

Intravascular–interstitial fluid balance: The capillary
Fluid and electrolyte balance
wall is semi-impermeable to plasma proteins, whereas
sodium passes freely across the capillary wall. This
Water and sodium balance meansthatproteins(throughoncoticpressure),rather
than sodium, exert the osmotic effect to keep fluid
Approximately 60% of the body weight in men and 55%
in the intravascular space. The hydrostatic pressure
inwomenconsistsofwater.Mostofthisexistswithintwo
generated across the capillaries offsets this, driving
physiologicalfluid‘spaces’orcompartments:abouttwo-
intravascular fluid out into the interstitial fluid. If
thirds within the intracellular compartment and one-
there is a reduction in plasma protein levels (hypoal-
thirdintheextracellularcompartment.Theextracellular
buminaemia), the low oncotic pressure can lead to
compartment consists of both intravascular fluid (blood
oedema; this is where there is excess interstitial fluid
cells and plasma) and interstitial fluid (fluid in tissues,
at the expense of intravascular fluid.
which surrounds the cells). Additionally a small amount
Wateriscontinually lost from the body in urine, stool
of fluid is described as in the ‘third space’, e.g. fluid in
and through insensible losses (the lungs and skin). This
the gastrointestinal tract, pleural space and peritoneal
waterisreplacedthroughoralfluids,foodandsomeisde-
cavity. Pathological third space fluid is seen with gas-
rivedfromoxidative metabolism. Sodium is remarkably
trointestinal obstruction or ileus and pleural effusion or
conserved by normal kidneys, which can make virtu-
ascites.
ally sodium-free urine, e.g. in hypovolaemia. Obligatory
Waterremains in physiological balance between these
losses of sodium occur in sweat and faeces, but account
compartments because of the concentration of osmoti-
for <10 mmol. The average dietary intake of sodium in
cally active solutes. Osmosis is the passage of water from
the United Kingdom is ∼140 mmol/day, which is the
alow concentration of solute through a semipermeable
equivalent of8gof salt. The recommended sodium in-
membrane to a more concentrated solution. A propor-
take for a healthy diet is 70 mmol/day. Normal kidneys
tion of the total osmotic pressure is due to the presence
can easily excrete this sodium load, and in a healthy per-
of large protein molecules; this is known as the colloidal
son the body is able to maintain normal fluid balance by
osmotic pressure or oncotic pressure.
sensing the concentration of sodium and the extracel-
Intracellular–extracellularfluidbalance:Thecellmem-

lular volume. This process is under the control of both
brane acts as semipermeable to sodium and potas-
local sensing mechanisms and more distant neurohor-
+
+
sium because the Na -K -ATPase pump keeps mov-
monal mechanisms. These drive thirst and water intake
ing sodium out of the cell into the interstitial fluid and
on the one hand and renal excretion or conservation of
moving potassium into the cell. Sodium is the main
sodium and water on the other. In disease states or due to
determinant of extracellular fluid volume.
1