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.
In kidneys, drugs enter the renal artery, pass through the afferent arterioles, and drain into the glomerular capillaries surrounded by the Bowman's capsule.
The difference in diameters between afferent and efferent arterioles, increases vascular resistance and glomerular blood pressure.
This increased pressure forces water, drugs weighing below 20 kDa, and other solutes out of the glomeruli via the porous endothelium into the Bowman's capsule. This unidirectional glomerular filtration forms renal filtrate, which flows through renal tubules to form urine.
The glomerular filtration rate or GFR is the volume of renal filtrate collectively formed per minute by all the kidneys' glomeruli.
Clinically, GFR estimation depends on drugs like inulin, which is excreted by filtration only. As a result, inulin clearance is approximately equal to GFR.
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