6.4 : Renal Drug Excretion: Glomerular Filtration

The kidney serves as the primary organ responsible for eliminating drugs and their metabolites from the body. This process, known as renal elimination, starts with glomerular filtration and results in urine formation. Each kidney houses millions of functional units called nephrons, where urine production occurs. A nephron has two main components: a renal corpuscle and a renal tubule.

Drugs gain access to the kidney via the renal artery, which progressively branches off into afferent arterioles. The drug is then transported to the glomerulus, a network of capillaries enclosed by Bowman's capsule, located within the renal corpuscle section of the nephron. Due to the porous nature of the glomerular capillaries' endothelium and the elevated hydrostatic blood pressure inside the capillaries, unbound drug molecules, plasma, and other small solutes are pushed out of the glomerulus and into Bowman's capsule through a mechanism known as glomerular filtration. The resulting fluid, or renal filtrate, moves into the renal tubule portion of the nephron, where it ultimately forms urine.

Factors such as the glomerular filtration rate and the level of drug binding to plasma proteins determine the number of drugs that enter the tubule. Drugs not included in the filtrate leave the glomerulus via efferent arterioles.