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Tubular secretion of PAH
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If we take a cross-section of the kidney, there are two main parts, the outer cortex and the inner medulla.
If we zoom in, there are millions of tiny functional units called nephrons which go from the outer cortex down into the medulla and back out into the cortex again.
Now, when blood gets filtered, some fluid remains in the glomerulus, and some fluid goes into the renal tubule.
The renal tubule is a structure with several segments: the proximal convoluted tubule, the U- shaped loop of Henle with a descending and ascending limb and the distal convoluted tubule, which winds and twists back up again, before emptying into the collecting duct, which collects the final urine.
Now, zooming in on this nephron’s tubule, each one’s lined by brush border cells which have two surfaces. One is the apical surface which faces the tubular lumen and is lined with microvilli, which are tiny little projections that increase the cell’s surface area to help with solute reabsorption.
The other is the basolateral surface, which faces the peritubular capillaries, which run alongside the nephron.
Alright, now, one substance that’s filtered out of the glomerulus and into the tubule is para-aminohippuric acid, or PAH for short.
Actually, PAH is an organic acid, and about 90% is bound to plasma proteins. So, really, only the unbound 10% can pass through the glomerular capillaries.
Essentially, the higher the unbound PAH concentration, the more PAH will get filtered.
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.
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