Content area
Full Text
In the third of a series, Charlotte Nichoilsand Mojgan Sani discuss the action and side effects of the four main groups of diuretics
Diuretics are among the most widely prescribed drugs used in the treatment of hypertension and are the first choice of treatment in older people. They are also used in the relief of peripheral oedema and breathlessness caused by pulmonary oedema in heart failure. They act on the kidney to increase the excretion of salt and water. This reduces the extracellular fluid which results in a decrease in blood pressure and relieves oedema.
There are four main groups of compounds divided according to their modes of action: the thiazides, loop diuretics, potassium-sparing diuretics and aldosterone antagonists.
renal tubular function
The function of the kidney is mainly the excretion of urea, uric acid and creatinine. It is also involved in salt and water regulation of the body. The extracellular fluid containing all constituents of the plasma except for protein is filtered into the kidney tubule. Most of it is reabsorbed and the rest is excreted as urine.
Filtration of extracellular fluid occurs in the nephron, of which about 1.3X10^sup 6^ are located in the cortex (outer shell) of the kidney. The nephron consists of a glomerulus, proximal convoluted tubule, Loop of Henle, distal tubule and collecting duct. Filtrate first reaches the proximal tubule where 75 per cent of it is reabsorbed. The filtrate then enters the descending limb of the Loop of Henle where water is reabsorbed because of high osmotic pressure in the medulla.
Consequently the filtrate that enters the ascending limb of the Loop of Henle is highly concentrated and this is where most of the sodium chloride reabsorption occurs by the Na+/K+/2Clcarrier. The loop diuretics such as frusemide act on this transporter and are the most potent of the diuretics
The filtrate then reaches the distal tubule where more sodium chloride reabsorption occurs, although this is moderate compared to that of the ascending limb. The thiazides such as bendrofluazide act on the Na+/CI- co-transporter to inhibit reabsorption.
Finally the filtrate arrives at the collecting tubules, which have a low permeability to salts and water. Sodium absorption here occurs through sodium channels stimulated by a hormone known as aldosterone, and water...