The rate of renal clearance is expressed as the sum of the rate of glomerular filtration and the rate of tubular secretion minus the rate of tubular reabsorption. Arterial blood passes through the glomerulus — the part of the nephron that filters plasma water and some of its contents. The pores within the capillary endothelium and the ultrafiltration membrane of the glomerulus allow only small molecules (less than 400 to 600 A in diameter or about 5 kDa molecular weight) to be filtered into the tubular fluid. Therefore, large macromolecules, such as most proteins, and hence the drugs bound to them, cannot pass through the filter. The filtration process is passive and only small, unbound drugs can be filtered.
Although the glomerular filtration rate is about 120 mL/min in adults, reabsorption along the proximal, distal and collecting tubules leads to only 1 to 2 mL/min of the filtered water being eliminated as urine. The tubular epithelium is the site of reabsorption of many substances, with the net effect of these molecules passing through the renal interstitial fluid and back to the plasma. Glucose, for example, is reabsorbed by an active transport process while a large number of other compounds are passively reabsorbed. buy flovent inhaler
Lipid soluble or nonionized substances are able to diffuse across cell membranes, while charged molecules (including most drugs) are usually not and are subsequently excreted in the urine. The pH of tubular fluid is an important factor influencing the reabsorption of drugs (because it may affect the ratio of drug in nonionized form to ionized form) and clinically this is useful. For example, alkalinization of urine may be used to promote drug excretion in cases of acidic drug overdose, such as salicylate.