In this Chapter:
16.1. Diuretics directly affecting the function of the renal tubular epithelium
16.2. Aldosterone antagonists
16.3. Osmotic diuretics
Diuretics are used mostly for excreting excess water from the body and eliminating edemas of different origin. They are also used for lowering blood pressure and to treat chemical poisoning to accelerate the elimination of poisons from the body.
Increased diuresis caused by diuretics results from their effect on urine formation. The latter is known to include 3 processes: glomerular filtration, tubular reabsorption and secretion.
Urine formation begins with filtration of blood plasma through membranes of a capillary glomerulus and its capsule. Almost all components of plasma pass through the pores in these membranes with the exception of high relative molecular weight proteins, substances bound to these proteins, and lipids. The daily volume of glomerular filtrate in a normal adult is about 170 litres of which only 1.5 litres is excreted as urine. Glomerular filtration depends on hydrostatic arterial pressure in renal capillaries, oncotic pressure of indiffusible part of plasma and the number of functioning glomeruli.
The process of reabsorption and secretion occurs throughout all segments of the nephron: proximal tubules, Henle's loop, distal tubules and collecting tubules/ducts.
Reabsorbtion of sodium, chlorine ions and water begins in the proximal convoluted tubules. Reabsorbtion of bicarbonate ions (HCO3-) is assisted by carbonic anhydrase enzyme. Potassium ions are reabsorbed almost completely. In total, about 70-80% of filtrate is reabsorbed in the proximal tubules. As a result of all these processes the intratubular fluid remains isoosmotic (compared to plasma and interstitial fluid).