It is important to recognize that disorders of water balance are manifested by alterations in body fluid osmolality, which are usually measured by changes in plasma osmolality (POSM). Because the major determinant of plasma osmolality is Na+ aFluid intake varies widely for both social and cultural reasons.
of the most common fluid and electrolyte disorders seen in clinical practice is an alteration in serum [Na+]. When an abnormal serum [Na+] is found in an individual, it is tempting to suspect a problem in Na+ balance. However, the problem most often relates to water balance, not Na+ balance. As described later, changes in Na+ balance result in alterations in the volume of ECF, not its osmolality.
The following sections discuss the mechanisms by which the kidneys excrete either hypoosmotic (dilute) or hyperosmotic (concentrated) urine. Control of arginine vasopressin secretion and its important role in regulating excretion of water by the kidneys are also explained (see also • Fig. 35.1 Relationships between plasma AVP levels, and urine osmolality, urine flow rate, and total solute excretion. Max, maximum; Min, minimum. (From Koeppen BM, Stanton BA. Renal Physiology. 5th ed. Philadelphia: Elsevier; 2013.) aIn hot weather and during prolonged heavy exercise, water balance is maintained by increased water ingestion. Decreased excretion of water by the kidneys alone is insufficient to maintain water balance.
Kidneys excrete 1 L of water Kidneys excrete hyperosmotic in hypoosmotic urine, urine as the individual drinks returning volume to 14 L and water, returning volume to 14 L restoring [Na˜] and osmolality and restoring [Na˜] and osmolality to normal. to normal.
• Fig. 35.2 Response to changes in water balance. Illustrated are the effects of adding or removing 1 L of water from the ECF of a 70-kg individual. Positive Water Balance: (1) Addition of 1 L of water increases the ECFV and reduces its osmolality. The [Na+] is also decreased (hyponatremia). (2) The normal renal response is to excrete 1 L of water as hypoosmotic urine. (3) As a result of the renal excretion of water, the ECFV, osmolality, and [Na+] are returned to normal. Negative Water Balance: (4) The loss of 1 L of water from the ECF decreases its volume and increases its osmolality. The [Na+] is also increased (hypernatremia). (5) The renal response is to conserve water by excreting a small volume of hyperosmotic urine. (6) With ingestion of water, stimulated by thirst, and conservation of water by the kidneys, the ECFV, osmolality, and [Na+] are returned to normal. Size of the boxes indicates relative ECFV. (From Koeppen BM, Stanton BA. Renal Physiology. 5th ed. Philadelphia: Elsevier; 2013.)
In the clinical setting, hypoosmolality (a reduction in plasma osmolality) shifts water into cells, and this process results in cell swelling (see ). Symptoms associated with hypoosmolality are related primarily to swelling of brain cells. For example, a rapid fall in Posm can alter neurological function and thereby cause nausea, malaise, headache, confusion, lethargy, seizures, and coma. When Posm is increased (i.e., hyperosmolality), water is lost from cells. Symptoms of an increase in Posm are also primarily neurological and include lethargy, weakness, seizures, coma, and even death.