Osmolarity, or tonicity, is a property derived from the number of particles in a solution. Sodium and chloride are the primary electrolytes contributing to the osmolarity of the extracellular compartment. Potassium and, to a lesser extent, magnesium and phosphate are the major intracellular electrolytes. Water ﬂows freely between the intracellular and the extracellular spaces to maintain osmotic neutrality throughout the body. Any shifts in osmolarity in any ﬂuid spaces within the body are accompanied by corresponding shifts in free water from spaces of lower to higher osmolarity, thus maintaining equilibrium.
The average adult daily ﬂuid maintenance requirement is approximately 30 mL/kg/day, or 2,000 to 3,000 mL per day (26). This level is offset partially by insensible losses of 1,200 mL per day, which include losses from the lungs (600 mL), skin (400 mL), and gastrointestinal tract (200 mL). Urinary output from the kidney accounts for the remainder of the ﬂuid loss, and this output will vary depending on total body intake of water and sodium. Approximately 600 to 800 mOsm of solute are excreted by the kidneys daily. Healthy kidneys can concentrate urine up to approximately 1,200 mOsm and, therefore, the minimum output can range between 500 and 700 mL per day. The maximal urine output of the kidney can be as high as 20 L per day, as seen in patients with diabetes insipidus. In healthy individuals, the kidney adjusts output commensurate with daily ﬂuid intake.
The major extracellular buffer used in the acid-base balance is the bicarbonate-carbonic acid system: CO2 + H2O ↔ H2CO3 ↔ H++ HCO3 + (27). Typically, the body will maintain a bicarbonate-to-carbonic acid ratio of 20:1 to maintain an extracellular pH of 7.4. Both the lung and the kidney play integral roles in the maintenance of normal extracellular pH via retention or excretion of carbon dioxide and bicarbonate. Under conditions of alkalosis, minute ventilation decreases and renal excretion of bicarbonate increases to restore the normal ratio of bicarbonate to carbonic acid; the opposite occurs with acidosis.
Ultimately, the kidney plays the most important role in ﬂuid and electrolyte balance through excretion and retention of water and solute. Circulating antidiuretic hormone and aldosterone help modulate the process. Serum osmolarity affects hypothalamic release of antidiuretic hormone and aldosterone secretion in response to renal perfusion. Under states of dehydration or hypovolemia, serum antidiuretic hormone levels increase, leading to increased resorption of water in the distal tubule of the kidney. Increased aldosterone release promotes increased sodium and water retention. The opposite occurs in states of ﬂuid excess. Individuals with normal renal function and circulating antidiuretic hormone and aldosterone levels maintain normal serum osmolarity and electrolyte composition, despite daily ﬂuctuations of ﬂuid and electrolyte intake.