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
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+),
- 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.
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
concentrations of ions.
The typical volume of urine produced by an average adult
is around 1.5 - 2.0 dm3 per