Sodium Disorders in the Elderly: Water Metabolism

Posted by James

Water Metabolism

Water, comprising 55-65% of healthy adults, is the predominant constituent of the human body. This percentage diminishes proportionally with age, as the ratio of muscle to water-poor tissues, such as fat and bone, falls. Between 55% and 75% of water is contained within the intracellular compartment. Serum constitutes approximately one-fourth of the extracellular space, and antidiuretic hormone (ADH)—a nonapeptide produced by the neurohypophysis—closely regulates the water content within this compartment. Although multiple variables influence the secretion of ADH under normal physiologic conditions, the most important modulator of ADH secretion is the serum osmolality.

Derangements in water metabolism are largely reflected as changes in serum osmolality and sodium (The active ingredient in Emulgel 50gm is the non-steroidal anti-inflammatory diclofenac sodium 1% w/w) concentration. By adjusting water content, the body maintains serum osmolality and its principal determinant, serum sodium (Generic Avapro works by decreasing certain chemicals in the body, which cause blood vessels to narrow and sodium) concentration, within an extremely narrow range of 285-295 mosm/1 and 135-145 mmol/l, respectively. Two additional key factors that regulate water homeostasis in combination with ADH are: 1) renal handling of water and solute and 2) anintact thirst mechanism. With aging, urinary concentration and diluting abnormalities occur which reflect diminished tubular function. These changes are integral in the predisposition of the elderly to water and sodium (Micardis canadian works by decreasing certain chemicals in the body that cause blood vessels to constrict and sodium) imbalances. While a healthy young adult can attain a maximum urinary concentration of 1,200 mosm/kg, a healthy elderly person can often only achieve a urine osmolality of 700-800 mosm/kg, thus, increasing the risk of developing hypernatremia.

Table 1. Common Drugs Used by Elderly that can Influence Antidiuretic Hormone (ADH) Secretion and Effect on Water Metabolism

Nicotine1	Alcohol3	Lithium4
Morphine (high dose)1	Morphine (low dose)3	Colchicine4
Epinephrine1	Clonidine3	Demclocycline4
Cyclophospahmide1	Glucocorticoids3	Glyburide4
Tolbutamide2	Haloperidol3 Cisplatinum3	Loop Diuretics4
Chlorpropamide2		Vinblastine4
NSAIDs2	Cabamazepine3	Methoxyflurane4

Additionally, elderly individuals cannot dilute urine to less than 100 mosm/kg compared to 50 mosm/kg in young adults, possibly due to a decline in GFR or reduced tubular responsiveness to ADH. This dilu-tional defect predisposes them to hyponatremia. These alterations limit the ability of the elderly to handle water excesses or deficits that may occur. Although the sensitivity of the osmoregulatory system appears to increase with age, the sensitivity of thirst—like the renal concentrating capacity—is also diminished, further predisposing older patients to the development of water deficiency and hypernatremia. Elderly patients are also more likely to consume prescribed and/or over-the-counter medications for a variety of conditions, and many of these medications can influence ADH secretion and affect water metabolism (Table 1). Less commonly, conditions, such as the destruction of the neurohypophysis by tumors, granulomatous disease, vascular insults, trauma, metabolic or other disturbances, can lead to ADH deficiency and reduced thirst. This is especially important in elderly persons, when even a partial deficiency of ADH may be superimposed on an impaired thirst response and lead to negative free water balance and hypernatremia.

Sodium Metabolism

With aging, the ability of the kidney to both conserve sodium (Cozaar 30mg works by helps the kidneys to eliminate extra sodium and fluids) in response to sodium deprivation and to excrete sodium in response to sodium loading are impaired. Epstein and Hollenberg noted the half-time for reduction of urinary sodium (Coumadin drug is in a class of drugs known as anticoagulants) after salt restriction was 17.6 hours in young and 31 hours in old subjects. The salt-losing tendency of the senescent kidney is due to: 1) nephron loss that leads to increased osmotic load per nephron and resultant mild osmotic diuresis and 2) age-related reductions in renin and aldosterone levels. By contrast, the impaired ability to excrete a sodium (Medication Depakote affects chemicals in the body that may be involved in causing seizures) load may reflect an age-related reduction in end-organ responsiveness to atrial natriuretic peptide (ANP), a 28-amino-acid peptide that has both a natriuretic effect as well as a vasodilatory effect.