Page 104 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Abnormalities of Renal Excretion
The elimination of a given substance is im- amount and the transport maximum are the
paired if filtration and tubular secretion are re- same is called the renal threshold (→ B1, red
duced; conversely, it is increased when tubular portion of the blue curve).
reabsorption is decreased and/or tubular se- In transport processes with low affinity
cretion is increased. This can change the plas- (e.g., uric acid, glycine) not everything is reab-
ma concentration of the substance, although sorbed even at low plasma concentration, so
the latter depends on extrarenal factors (→ A), that both the reabsorption rate and the renal
such as production or breakdown, enteric ab- excretion increase with increasing plasma
Kidney, Salt and Water Balance time from the sum of extrarenal processes is ed substance is excreted (→ B1, violet curve).
sorption or extrarenal excretion (e.g., via gut
concentration (→ B1, yellow curve).
or sweat), deposition or mobilization. The
In secretion (e.g., of p-aminohippuric acid
[PAH]) not only the filtered by also the secret-
amount of substance that results per unit
In high affinity of the transport system and
the so-called prerenal load.
The right interpretation of changed plasma
low plasma concentration, the entire amount
reaching the kidney will be excreted. Renal
concentrations presupposes a knowledge of
the quantitative correlation between plasma
clearance thus corresponds to renal plasma
flow, i.e., the amount of plasma flowing
concentration and renal excretion (→ B).
This correlation is simple with substances
amount of substance that is presented exceeds
or reabsorbed (e.g., creatinine). The excreted
the maximal transport rate, excretion can be
raised only by an increase in the amount fil-
amount (M e ) is identical to the filtered amount
5 that are filtered but not significantly secreted through the kidney per unit of time. If the
(M f ) and thus equal to the product of plasma tered, and renal clearance is reduced (→ B2).
concentration (P) and the GFR: M e = M f = P · An abnormality of prerenal factors can, de-
GFR (→ B1, green line). The clearance (M e /P) is spite unimpaired tubular transport, raise the
identical to the GFR and thus independent of excretion of the affected substance via an in-
the plasma concentration (→ B2, green line). crease in its plasma concentration and the
If the production of creatinine is constant, a re- amount filtered. Thus, glycosuria may occur
duction in GFR transiently leads to a reduction even when renal transport of glucose is nor-
in creatinine excretion (→ B3a). The amount mal, if the plasma concentration of glucose is
produced is thus greater than that excreted, higher than its renal threshold, as is the case
so that the plasma concentration and also the in diabetes mellitus (overflow glycosuria).
excreted amount of creatinine per unit time Similarly, impaired breakdown of amino acids
rises (→ B3b) until as much creatinine is ex- leads to overflow aminoaciduria. Conversely, a
creted as is produced by the body. In equilib- change in plasma concentration in the pres-
rium, the renal excretion mirrors the prerenal ence of an abnormal renal transport can be
load. With substances which are filtered but prevented by extrarenal regulatory mecha-
neither reabsorbed nor secreted there is a line- nisms (→ A). Thus, hypocalcemia due to im-
ar correlation between plasma concentration paired renal reabsorption of Ca 2+ is prevented
and renal excretion and thus between prerenal by the release of PTH which mobilizes Ca 2+
load and plasma concentration (→ B4, green from bone and increases enteric absorption of
line). Ca 2+ via the release of calcitriol (→ p.128). The
In reabsorption by transport processes with result is hypercalciuria but not hypocalcemia.
high affinity (e.g., glucose, most amino acids,
phosphate, sulfate) practically the entire fil-
tered amount is reabsorbed and nothing elimi-
nated, as long as the plasma concentration is
low (→ B1, blue curve). If the filtered amount
exceeds the maximal transport rate, the whole
94 of the excess filtered amount is excreted. The
plasma concentration at which the filtered
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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