Key Clinical Questions
What is the difference between sodium and water balance and the regulation of each?
How is volume depletion assessed in patients with normal heart function and in those with underlying cardiac failure?
Which fluids are best for volume repletion?
How do diuretics work, and what are their side effects? How can resistance to their action be overcome?
How should hyponatremia and hypernatremia be assessed and safely corrected?
Sodium and water disturbances are among the most commonly encountered disorders in hospitalized and critically ill patients. Sodium and water balance are independently regulated by mechanisms that are designed to maintain circulatory integrity and plasma osmolality, respectively. Sodium balance is regulated by changes in sodium intake and excretion, whereas plasma osmolality is regulated by changes in water intake and water excretion.
Water is the predominant constituent of the human body. In healthy individuals, it makes up 60% of male body weight and 50% of female body weight. Body water is distributed between two compartments: the intracellular fluid compartment, containing 55% to 65%, and the extracellular fluid compartment, containing the remaining 35% to 45%. The extracellular fluid compartment is further subdivided into the interstitial space and the intravascular space. The interstitial space comprises approximately 75% of the extracellular fluid compartment, whereas the intravascular space contains 25%. Total body water diffuses freely between the intracellular space and the extracellular space in response to solute concentration gradients. Therefore, the amount of water in each compartment depends entirely on the quantity of solute in that compartment. The major extracellular solute is sodium, while potassium is the major intracellular solute. This solute distribution is maintained by active transport, via the Na+/K+-ATP-dependent pumps found on cell membranes.
PATHOPHYSIOLOGY OF SODIUM BALANCE
The extracellular fluid compartment depends on the total body sodium content as well as the integrity of the mechanisms responsible for its maintenance. Sodium content is normally tightly regulated by modulating renal retention and excretion when there is deficiency or excess of extracellular fluid. The operative homeostatic mechanisms include an afferent sensing limb and an efferent effector limb. Disorders of either sensing or effector mechanisms can lead to failure to adjust renal sodium handling, resulting in hypertension and edema with positive sodium balance, or hypotension and hypovolemia with negative sodium balance.
EFFECTIVE ARTERIAL BLOOD VOLUME
Effective arterial blood volume refers to the blood volume detected by baroreceptors in the arterial circulation. The effective arterial blood volume can change independently of the total extracellular fluid volume, leading to sodium and water retention in different clinical situations.
Afferent limb sensing sites include low-pressure cardiopulmonary receptors (atrial, ventricular, and pulmonary stretch receptors), high-pressure arterial baroreceptors (carotid, aortic arch, and renal sensors), and central nervous system and hepatic receptors. Activation of these afferent sites engages several effector mechanisms. Activation of ...