Page 55 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 3

Disorders of Sodium and Water:

Hypernatremia and Hyponatremia

Stephen P. DiBartola

The volume and tonicity of body fluids are maintained TERMINOLOGY
within a narrow normal range by regulation of sodium
and water balance. The volume of extracellular fluid OSMOLALITY
(ECF) is determined by the total body sodium content,
whereas the osmolality and sodium concentration of The osmolality of a solution refers to the concentration
ECF are determined by water balance. The kidneys play of osmotically active particles in that solution. Osmolality
a crucial role in these processes by balancing the excretion is a function only of the number of particles and is not
of salt and water with their intake and by avidly conserv- related to their molecular weight, size, shape, or charge.
ing them when intake is restricted (Table 3-1). One mole of a nondissociating substance (e.g., glucose or
urea) dissolved in 1 kg of water decreases the freezing
point of the resultant solution by 1.86 C. Such a solution

has an osmolality of 1 Osm/kg or 1000 mOsm/kg.
TABLE 3-1 Renal Regulation of The term osmolarity refers to the number of particles
Sodium and Water of solute per liter of solution, whereas the term osmolality
Balance refers to the number of particles of solute per kilogram of
solvent. When considering the physiology of body fluids,
Osmoregulation Volume Regulation the difference between osmolality and osmolarity is neg-
ligible because body fluids typically are dilute aqueous
What is Plasma osmolality Effective circulating
solutions. In clinical medicine, the term osmolality is
sensed volume
used, and the osmolality of body fluids usually is measured
Sensors Hypothalamic Carotid sinus
by freezing-point depression osmometry. A solution is
osmoreceptors
said to be hyperosmotic if its osmolality is greater than
Aortic arch
that of the reference solution (often plasma) and
Glomerular afferent
arterioles hypoosmotic if its osmolality is less than that of the refer-
Cardiac atria ence solution. An isosmotic solution has an osmolality
Large pulmonary vessels identical to that of the reference solution.
Effectors Vasopressin Renin-angiotensin- The normal plasma osmolality ofdogs and cats is slightly
aldosterone system higher than that of humans and ranges from 290 to 310
Thirst Sympathetic nervous mOsm/kg in dogs and from 290 to 330 mOsm/kg in cats.
system In one study, 20 dogs under resting conditions had plasma
Atrial natriuretic peptide osmolality values of 292 to 308 mOsm/kg with a mean
“Pressure natriuresis” 67 In a study of the effects of
Antidiuretic hormone value of 301 mOsm/kg.
What is Water excretion Urine sodium excretion sodium bicarbonate infusion in cats, baseline serum
22
affected Water intake osmolality ranged from 290 to 330 mOsm/kg. Plasma
osmolality can be estimated from the equation:
Modified from Rose BD. Clinical physiology of acid base and electrolyte
disorders, 4th ed. New York: McGraw-Hill, 1994: 256, with permission of BUN glucose
the McGraw-Hill Companies. Calculated Plasma Osmolality ¼ 2Na þ þ
2:8 18

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