| THE IMPORTANCE OF PLASMA POTASSIUM CONCENTRATION
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| Potassium affects the contractility of the heart, and both high (hyperkalemia) and low concentration (hypokalemia) can be life threatening
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| The measurement of serum potassium concentration is a very important laboratory test (Fig. 22.13). The serum concentration of potassium is 3.5-5 mmol/L. Because the intracellular concentration of potassium is much higher than in the ECF and plasma, a relatively minor movement of potassium between ECF and ICF may result in large changes in the serum potassium concentration. A potassium value of less than 2.5 mmol/L or greater than 6.0 mmol/L is dangerous, and therefore maintenance of the potassium concentration close to normal is fundamentally important.
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| Figure 22.13 Potassium balance. Plasma potassium concentration is maintained within narrow limits. Both low (hypokalemia) and high (hyperkalemia) concentrations may be dangerous, because of the effect of potassium on the contractility of heart muscle. The upper panel shows the main sources of potassium loss from the body. |
Diabetes insipidus and the syndrome of inappropriate secretion of vasopressin |
| Deficiency of vasopressin causes the condition known as diabetes insipidus, in which large amounts of dilute urine are lost. In contrast, an uncontrolled, excessive secretion of vasopressin may take place following major trauma or surgery. This is known as the syndrome of inappropriate antidiuretic hormone secretion (SIADH), and leads to water retention. |
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| DIURETICS ARE DRUGS USED FOR TREATMENT OF EDEMA, CARDIAC FAILURE AND HYPERTENSION |
| Diuretics are drugs that stimulate water and sodium excretion. Diuretics act by inhibiting the reabsorption of sodium and chloride in the renal tubules. Thiazide diuretics, e.g., bendrofluazide, decrease sodium reabsorption in the distal tubules by blocking sodium and chloride cotransport. Loop diuretics, such as frusemide, inhibit sodium reabsorption in the ascending loop of Henle. Spironolactone, a potassium-sparing diuretic, is a competitive inhibitor of aldosterone, inhibits sodium-potassium exchange in the distal tubules, and also decreases potassium excretion. Finally, an osmotic diuresis may be induced by the administration of the sugar alcohol, mannitol. The net effect of diuretics is to increase urine volume and induce the loss of water and sodium. They are important in the treatment of edema associated with circulatory problems such as heart failure, in which impaired cardiac function may lead to a severe breathlessness caused by pulmonary edema. They are also essential in the treatment of hypertension. |
| The most common cause of severe hyperkalemia is renal failure: in this condition potassium cannot be excreted in the urine. On the other hand, low serum potassium usually results from its excessive losses, either in urine or through the gastrointestinal tract. Importantly, changes in serum
potassium concentration are also associated with the acid-base disorders (see Chapter 23).
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