CHAPTER 11 Fluids and Electrolytes^421
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.
IV fluids
Osmolarity Hypo-, Iso-, or
Solution (mOsm/L) Hypertonic
0.9% saline 308 Isotonic
(normal saline, NS)
0.45% saline 154 Hypotonic
(1/2 normal saline)
5% dextrose in water (D 5 W) 272 Isotonic
10% dextrose in water 500 Hypertonic
(D 10 W)
5% dextrose in Ringer’s lactate 525 Hypertonic
5% dextrose in 0.45% saline 406 Hypertonic
5% dextrose in 0.9% saline 560 Hypertonic
Ringer’s lactate 273 Isotonic