Electrolyte balance exists when the intake of electrolytes from all sources equals the output of electrolytes.

foods, but they may also be found in drinking water and other beverages. In addition, some electrolytes are byproducts of metabolic reactions.

Regulation of Electrolyte Intake

Ordinarily, a person obtains sufficient electrolytes by responding to hunger and thirst. However, a severe electrolyte deficiency may cause salt craving, which is a strong desire to eat salty foods.

Electrolyte Output

The body loses some electrolytes by perspiring (sweat has about half the solute concentration of plasma). The quantities of electrolytes leaving vary with the amount of perspiration. More electrolytes are lost in sweat on warmer days and during strenuous exercise. Varying amounts of electrolytes are lost in the feces. The greatest electrolyte output occurs as a result of kidney function and urine production. The kidneys alter renal electrolyte losses to maintain the proper composition of body fluids.

Diuretics are substances that promote urine production. A number of common substances, such as caffeine in coffee and tea, have diuretic effects, as do a variety of drugs used to reduce the volume of body fluids.

Diuretics produce their effects in different ways. Some, such as alcohol and certain narcotic drugs, promote urine formation by inhibiting ADH release. Certain other substances, such as caffeine, inhibit the reabsorption of sodium ions or other solutes in portions of the renal tubules. As a consequence, the osmotic pressure of the tubular fluid increases, reducing osmotic reabsorption of water and increasing urine volume.

U By what routes does the body lose water?

^9 What is the primary regulator of water loss?

^9 What types of water loss are unavoidable?

Q How does the hypothalamus regulate water balance?

Electrolyte Balance

An electrolyte balance (e-lek'tro-lit bal'ans) exists when the quantities of electrolytes (molecules that release ions in water) the body gains equal those lost (fig. 21.7).

Electrolyte Intake

The electrolytes of greatest importance to cellular functions release sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, bicarbonate, and hydrogen ions. These electrolytes are primarily obtained from

Recall from chapter 2 (p. 45) that water molecules are polar, and molecules that have polar regions within them (such as carbohydrates and proteins) dissolve in water but remain intact, whereas molecules that are held together by ionic bonds (such as the electrolytes) dissociate in water to release ions.

The osmolarity of the body fluids is determined by the total number of dissolved particles. One molecule of glucose yields one dissolved particle and one molecule of sodium chloride yields two, a sodium ion and a chloride ion. Since the osmolarity of body solutions is determined by the total number of dissolved particles, irrespective of the source, the term osmoles is used. Thus, one mole of glucose yields one osmole of dissolved particles, and one mole of sodium chloride yields two osmoles. The total number of osmoles per liter gives the osmolarity of the solution.

U Which electrolytes are most important to cellular functions?

Q Which mechanisms ordinarily regulate electrolyte intake?

H By what routes does the body lose electrolytes?

Regulation of Electrolyte Output

The concentrations of positively charged ions, such as sodium (Na+), potassium (K+), and calcium (Ca+2), are particularly important. For example, certain concentrations of these ions are vital for nerve impulse conduc-

Potassium ion concentration increases

Adrenal cortex is signaled

Sodium ions are conserved and potassium ions are excreted

Renal tubules increase reabsorption of sodium ions and increase secretion of potassium ions

Aldosterone is secreted

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