Blood Cleaning by the Kidneys - Part (3)

(3) Tubular Secretion

The third process by which the kidneys clean blood (regulating its composition and volume) is called tubular secretion and involves substances being added to the tubular fluid. This removes excessive quantities of certain dissolved substances from the body, and also maintains the blood at a normal healthy pH (which is typically in the range pH 7.35 to pH 7.45).

The substances that are secreted into the tubular fluid (for removal from the body) include:

  • Potassium ions (K+),
  • Hydrogen ions (H+),
  • Ammonium ions (NH4+),
  • creatinine,
  • urea,
  • some hormones, and
  • some drugs (e.g. penicillin).

Tubular secretion occurs from the epithelial cells that line the renal tubules and collecting ducts.

It is the tubular secretion of H+ and NH4+ from the blood into the tubular fluid (i.e. urine - which is then excreted from the body via the ureter, bladder, and urethra) that helps to keep blood pH at its normal level. The movement of these ions also helps to conserve sodium bicarbonate (NaHCO3).

The typical pH of urine is about 6.

... and finally

Urine formed via the three processes outlined above trickles into the kidney pelvis. At this final stage it is only approx. 1% of the originally filtered volume but includes high concentrations of urea and creatinine, and variable concentrations of ions.

The typical volume of urine produced by an average adult is around 1.5 - 2.0 dm3 per day.


Quick Summary:

The Processes of Blood Filtration by the Kidneys

  1. Glomerular Filtration
  2. Tubular Reabsorption
  3. Tubular Secretion


Prefer to read about all three stages on the same page ?

Prefer to read about all of the three stages on the same page ?

See How the Kidneys Clean Blood.

More about the Urinary System

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