Osmosis video - Tubular secretion of PAH

Para-aminohippuric acid (PAH) is a substance that is freely filtered by the glomeruli in the kidneys and secreted by the tubular cells into the tubular fluid. The secretion of PAH occurs primarily in the proximal tubule of the nephron and is facilitated by transporters located on the luminal surface of the tubular cells.

The amount of PAH that is secreted by the kidneys is used to estimate the renal blood flow, which is an important parameter in the assessment of kidney function. Since nearly all of the PAH that is filtered by the glomeruli is secreted by the tubular cells, the amount of PAH that is excreted in the urine is directly proportional to the amount of blood that is flowing through the kidneys.