Sodium, Potassium, Chloride, and Water

Sodium and potassium are monovalent cations. Chloride is a monovalent anion. Na and K generally do not function by supporting the activity of specific enzymes, 

Metal Lons that tend to bind to oxygen or sulfur 

Metal tons that tend lo bind to sulfur 

Ln striking contrast to all of the other metal ions, Na and K are unusual in that they function by changing their location, that is, by passing from one side of the plasma membrane to the other. Thi.s passage may be actively driven by ATP-using mechanisms (ion pumps). In other cases, the passage occurs in a passive manner through channels without the use of ATR 

An understanding of ion pumps and channels, especially those of the kidney, may aid in understanding the issues of Na and K nutrition. These issues, discussed along with the material on kidney physiology, include the use of diuretics, sweating, blood pressure, and life-threatening conditions involving abnormal levels of plasma Na and K. 

Metal ions that tend to bind to oxygen Metal ions that tend to bind to oxygen or sulfur Metal ions that tend to bind to sulfur 

Most diets, even poor ones, provide sufficient amounts of Na, K, Cl, and water. Deficiencies in these nutrients tend to occur in a variety of pathological situations. These situations often involve excessive losses of these nutrients, rather than the lack of intake or impaired absorption. Plasma levels of Na or K can be low even with sufficient intakes of these nutrients and with a normal body content. Low plasma Na and K conditions are called hyponatremia and h)rpokalemia, respectively. 

Na and K are sometimes called electrolytes. This term arose because these ions are used for generating an electric charge difference across the plasma membrane of most or all cells. These ions, however, have many functions unrelated to this use. Na, for example, is used to maintain blood pressure and K is used as a cofactor by several enz3nnes. 

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The necessary components of oral rehydration therapy (ORT) solutions include glucose, sodium, potassium, chloride, and water (Table 39-2). The American Academy of Pediatrics recommends rehydration with an electrolyte-concentrated rehydration phase followed by a maintenance phase using dilute electrolyte solutions and larger volumes. 

Further details on renal physiology and water regulation appear in the Sodium, Potassium, Chloride, and Water section. 

Chronic renal failure is treated by dialysis, kidney transplants, and drugs, as well as by low-protein diets. For this reason, an outline of chronic renal failure occurs in this chapter. The normal glomerular filtration rate (GFR) is 80 to 120 ml/min. In severe renal disease, the GFR can be reduced to 10 ml/min or less. This represents a 90% loss of renal function. Diabetes mellitus and hypertension (high blood pressure) are the main causes of chronic renal failure. Sustained and chronic injury to the kidneys leads to the destruction of the nephrons, where this destruction is usually not reversible. The nephron, which is the smallest unit of kidney function, is detailed in the section on Sodium, Potassium, Chloride, and Water. The severe loss of nephrons results in alterations of functions of many other organs of the body. The collection of abnormalities that results is called uremia. 

During the passage through the tubules, the glomerular filtrate is not only concentrated, but its composition is considerably modified by reabsorption and secretion in the tubular cells. Several forms of tubular reabsorption are discussed in the paragraphs on sodium, potassium, chloride, and water metabolism. It appears that the mechanisms of electrolyte and water absorption vary greatly. Absorption can be passive, as is the case for chloride, or active and influenced by hormones, as is the case for sodium and water (in at least some portions of the nephron) [6]. 

Source Adapted from Davenport HW, Alzamora F, Sodium, potassium, chloride, and water in frog gastric mucosa. Am J Physiol 202 711-715, 1962. 

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