Renal excretion: Renal filtration accounts for most Medicine excretion. About one fifth of the plasma reaching the glomerulus is filtered through pores in the glomerular endothelium; nearly all water and most electrolytes are passively and actively reabsorbed from the renal tubules back into the circulation. However, polar compounds, which account for most Medicine metabolites, cannot diffuse back into the circulation and are excreted unless a specific transport mechanism exists for their reabsorption (eg, as for glucose, ascorbic acid, and B vitamins). With aging, renal Medicine excretion decreases ; at age 80, clearance is typically reduced to half of what it was at age 30. The principles of transmembrane passage govern renal handling of Medicines. Medicines bound to plasma proteins remain in the circulation; only unbound Medicine is contained in the glomerular filtrate. Un-ionized forms of Medicines and their metabolites tend to be reabsorbed readily from tubular fluids. Urine pH,