Water and Sodium Regulation in Health and Disease

Abstract

The maintenance of normal circulatory volume is the single most important characteristic of body fluid homeostasis. The constituents of body fluids in turn are regulated within narrow limits. The kidney plays a critical role in these regulatory processes.

Keywords: fluid compartments; circulation; salt and water; antidiuretic hormone; aldosterone

Figure 1.

Nephron segments with the major transport processes identified across each segment. The proximal convoluted tubule reabsorbs approximately two‐thirds of the glomerular filtrate secondary to active transport of sodium bicarbonate (NaHCO3), glucose, amino acids and other transport processes. The pars recta has a lower capacity for net fluid reabsorption, but secretes organic acids and various of the toxins produced in metabolism. The descending limb of Henle is highly permeable and absorbs water because it descends through the medulla, which is progressively hypertonic. The ascending limb of Henle allows passive permeation of sodium chloride out of it. The segments of the thick ascending limb of Henle absorb salt by an active transport mechanism. The distal convoluted tubule transports salt out as an electrochloride‐neutral sodium chloride transport mechanism. The collecting duct segments reabsorb sodium from the lumen by an aldosterone‐sensitive mechanism, which is also responsible in part for potassium and hydrogen secretion. The collecting duct also reabsorbs water in response to antidiuretic hormone (ADH). In addition urea is reabsorbed by both active and passive mechanisms across the terminal segments of the collecting duct. The red line, starting at the bend of the loop and running through the ascending limb, convoluted tubule and collecting duct segments, denotes water impermeability in the absence of ADH.

Figure 2.

Schematics of the principal cells of the collecting duct. Antidiuretic hormone (ADH) increase the movement of water from the urinary to the blood side of the cell by binding to the receptor (V2, a subtype of ADH receptor that is specific for increasing water movement) on the blood side and stimulating a number of intermediaries, including cyclic adenosine monophosphate (cAMP). cAMP ultimately causes the insertion of water‐permeable proteins called aquaporins on the urinary (apical) side. Water (H2O) then moves through these channels passively down an osmotic gradient between the urine and the hypertonic medulla. Also depicted is the mechanism of sodium (Na+) reabsorption and potassium (K+) secretion, which is aldosterone sensitive and which derives most of its energy from the sodium–potassium adenosine triphosphatase (ATPase) pump ( exchange pump). The Na–K ATPase decreases cell concentrations of sodium, allowing Na+ to move down its electrochemical gradient from urine to the intracellular space, whereas K+ moves down its electrochemical gradient from cell to urine. The negative signs on the urinary side depict the transepithelial potential difference: the urinary side is significantly more electronegative than the blood side.

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Further Reading

Kokko JP (1996) Disorder of fluid volume, electrolyte, and acid–base balance. In: Bennett JC and Plum F (eds) Cecil Textbook of Medicine, 20th edn, chap. 75, pp. 521–551. Philadelphia, PA: WB Saunders.

Kokko JP and Tannen RL (eds) (1996) Fluids and Electrolytes, 3rd edn. Philadelphia, PA: WB Saunders.

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Kokko, Juha P(Apr 2001) Water and Sodium Regulation in Health and Disease. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0002312]