regulation of sodium and potassium

normal sodium concentration in interstitial fluid and potassium concentration in intracellular fluid depend on many factors, espe-cially the amount of ADH and aldosterone secreted. As shown in Figure 43-15, ADH regulates extracellular fluid electrolyte concen-tration and colloid osmotic pressure by regulating the amount of water reabsorbed into blood by renal tubules. Aldosterone, on the other hand, regulates extracellular fluid volume by regulating the amount of sodium reabsorbed into blood by renal tubules (see Figure 43-6).

If for any reason conservation of body sodium is required, the normal kidney is capable of excreting an essentially sodium-free urine and is therefore considered the chief regulator of sodium levels in body fluids. Sodium lost in sweat can become appreciable with elevated environmental temperatures or fever. However, the thirst that results may lead to replacement of water but not the lost sodium, and because of the increased fluid intake, the remaining sodium pool may be diluted even more. Sodium loss in sweat is not therefore considered a normal means of regulation. In addition to the well-regulated movement of sodium into and out of the body and among the three primary fluid compartments, there is a continuous movement or circulation of this important electrolyte between a number of internal secretions. More than 8 liters of various internal secretions, such as saliva, gastric and intes-tinal secretions, bile, and pancreatic fluid, are produced every day (Figure 43-16). The total daily secretion of sodium into these ali-mentary tract fluids alone will average between 1200 and 1400 mEq. A 70-kilogram (154-pound) adult has a total body sodium pool of only 2800 to 3000 mEq. Precise regulatory and conservation mecha-nisms for sodium are required for survival. Chloride is the most important extracellular anion and is almost always linked to sodium. Generally ingested together, they provide in large part for the isotonic behavior of extracellular fluid. Chloride ions are usually excreted in the urine as a potassium salt, and there-fore chloride defi ciency—hypochloremia—is often found in cases of potassium loss. The total body potassium content in the average-sized adult is approximately 4000 mEq. Because the majority of body potassium is intracellular, plasma (serum) determinations, which normally fall between 4.0 and 5.0 mEq/L, may not be the best index to reflect imbalances. The body may lose a third to a half of its intracellular potassium reserves before the loss is reflected in lowered plasma potassium levels. Potassium deficit, or hypokalemia, occurs whenever there is cell

breakdown, as in starvation, burns, trauma, or dehydration. As indi-vidual cells disintegrate, potassium enters the extracellular fluid and is rapidly excreted because it is not reabsorbed efficiently by the kidney