Drug Elimination VII

The Kidney as Excretory Organ
Most drugs are eliminated in urine either
chemically unchanged or as metabolites.
The kidney permits elimination
because the vascular wall structure in
the region of the glomerular capillaries
allows unimpeded passage of blood
solutes having molecular weights (MW)
<> 70000. With
few exceptions, therapeutically used
drugs and their metabolites have much
smaller molecular weights and can,
therefore, undergo glomerular filtration,
i.e., pass from blood into primary
urine. Separating the capillary endothelium
from the tubular epithelium, the
basal membrane consists of charged
glycoproteins and acts as a filtration
barrier for high-molecular-weight substances.
The relative density of this barrier
depends on the electrical charge of
molecules that attempt to permeate it.
Apart from glomerular filtration
, drugs present in blood may pass
into urine by active secretion. Certain
cations and anions are secreted by the
epithelium of the proximal tubules into
the tubular fluid via special, energyconsuming
transport systems. These
transport systems have a limited capacity.
When several substrates are present
simultaneously, competition for the
carrier may occur .