Library · Physiology
What the kidney pushes out into the urine
Make it clearer
Urine is not made by filtering alone. The glomerulus, the kidney's filter, lets through what is small and unbound, but some substances travel stuck to proteins in the plasma, the liquid part of the blood, so they are barely filtered at all. For those there is a second door: tubular secretion, in which the kidney tubule moves them straight from the blood into the urine. That is how many drugs leave the body.
That second door also works as a correction. Unwanted substances that may have been passively taken back up earlier, such as urea and uric acid, get pushed out again. Creatinine, ammonia, and many organic acids and bases leave by that route as well.
It is also a lever the body adjusts. Aldosterone, a hormone, controls how much potassium is secreted in the distal tubule and the collecting duct, the final stretches. And if blood pH falls below the reference range, meaning the blood turns more acidic than normal, more hydrogen ions, the particles that make something acidic, are secreted.
In brief
Tubular secretion moves drugs and metabolites bound to plasma proteins from the blood into the urine, since they filter poorly at the glomerulus.
A second door for what did not get through.
It also clears unwanted substances that may have been passively reabsorbed, such as urea and uric acid, and secretes creatinine, ammonia, and many organic acids and bases.
Aldosterone regulates potassium secretion in the distal tubule and collecting duct, and if blood pH falls below the reference range, more hydrogen ions are secreted.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Renal, StatPearlsPublished
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