How Fluids and Electrolytes Work

Fluids in the body are found in three basic places: within the cells (intracellular),
outside the cells (extracellular), and within the tissue spaces (interstitial space or
third space). A balance should be maintained to keep concentrations of both fluids
and electrolytes in the proper areas for normal function. The cell walls are semi-
permeable to allow for movement (diffusion) of molecules. This helps to maintain
osmotic pressure.
Edema occurs when too much fluid enters the interstitial space. Peripheral
edema usually collects in subcutaneous areas. The higher hydrostatic pressure in
the vessel causes fluids to move into the interstitial areas which have lower pres-
sure, allowing the fluid to build up.
Normal osmolarity of plasma is 270 to 300 mOsm/L. Isotonic or normotonic flu-
ids have similar concentrations. This prevents fluids from shifting into spaces they
do not belong. Hypertonic solutions have a concentration greater than 300 mOsm/L
and exert a greater pressure, which pulls water from the isotonic area to the hyper-
tonic solution in an attempt to equalize the osmolarity. Hypotonic solutions have a concentration of less than 270 mOsm/L and exert less pressure, which allows water
to be pulled from the hypotonic area into the isotonic area.
HORMONAL REGULATION OF FLUIDS AND ELECTROLYTES
Aldosterone is secreted by the adrenal cortex in response to sodium changes. Where
sodium goes, water follows. Aldosterone signals the tubules within the nephrons
in the kidneys to reabsorb sodium and therefore water. This increases blood osmo-
larity. Aldosterone also aids in control of potassium levels.
Renin is secreted by the kidneys in responses to changes in sodium or fluid vol-
ume. In the circulation, renin acts on a plasma protein called renin substrate (also
called angiotensinogen), converting it to angiotensin I. In the pulmonary circula-
tion, angiotensin-converting enzyme converts angiotensin I to angiotensin II. This
causes vascular constriction and aldosterone secretion.
Antidiuretic hormone (ADH) is produced in the brain and stored in the poste-
rior pituitary. It is released when there is a change in the osmolarity of the blood.
ADH acts on the renal tubules, causing them to reabsob more water, which de-
creases blood osmolarity. When the osmolarity gets too low, the release of ADH is
not needed and the water is excreted in the urine.
Natriuretic peptides are secreted in response to increases in blood volume and
blood pressure. When atrial natriuretic peptide (ANP) and brain natriuretic peptide
(BNP) are secreted, kidney reabsorption of sodium is inhibited and the glomerular fil-
tration rate is increased. Blood osmolarity is decreased and urine output is increased.

ACID BASE BALANCE
Maintaining acid-base balance will keep the pH level within the normal range of
7.35 to 7.45. The lungs and the kidneys are integral in maintaining the normal
acid-base balance. The body constantly monitors the pH level and makes adjust-
ments in an attempt to correct any abnormalities. pHCO3 is regulated by the kid-
neys. pCO2 is regulated by the lungs. If the patient develops acidosis there will be
a low pH and either a drop in pHCO3 (metabolic) or a rise in pCO2 (respiratory).
If the patient develops alkalosis there will be an increase in pH and either an
increase in pHCO3 (metabolic) or a drop in pCO2 (respiratory). In an attempt to
maintain as normal an internal environment as possible, the body will attempt
to compensate for the changes that are occurring. The lungs are able to correct
much more rapidly than the kidneys.