glomerular filtration

The glomerulus is a tiny tuft of leaky blood vessels at the start of each nephron, acting like a coffee filter under pressure. Blood arrives, and the fluid part along with small dissolved molecules is squeezed through the filter's tiny pores into the start of the kidney tubule, while blood cells and large proteins are held back.

For drugs, glomerular filtration is a passive, non-selective process driven only by blood pressure in the capillary. Only drug that is free in the plasma can pass; drug bound to plasma proteins such as albumin is too large and stays in the blood. So the more highly protein-bound a drug is, the less is filtered each minute.

The total volume filtered per minute is the glomerular filtration rate, about 120 mL/min in a healthy adult, and it sets a baseline for renal handling. Filtration alone cannot concentrate or selectively remove a drug; it simply samples whatever free molecules are present. Secretion and reabsorption then modify that filtered load downstream.