492
Chapter 14
Hence, for Z, the processes of fi ltration and reabsorption have
canceled each other out, and the net result is as though Z had
never entered the kidney.
A specifi c combination of fi ltration, tubular reabsorp-
tion, and tubular secretion applies to each substance in the
plasma. The critical point is that, for many substances, the
rates at which the processes proceed are subject to physiolog-
ical control. By triggering changes in the rates of fi ltration,
reabsorption, or secretion whenever the amount of a sub-
stance in the body is higher or lower than the normal limits,
homeostatic mechanisms can regulate the substance’s bodily
balance. For example, consider what happens when a normally
hydrated person drinks a lot of water. Within 1–2 h, all the
excess water has been excreted in the urine, partly as a result
of an increase in fi
ltration but mainly as a result of decreased
tubular reabsorption of water. In this example, the kidneys are
the effector organs of a homeostatic process that maintains
total body water within very narrow limits.
Although glomerular fi ltration, tubular reabsorption,
and tubular secretion are the three basic renal processes, a
fourth process—metabolism by the tubular cells—is also
important for some substances. In some cases, the renal tubu-
lar cells remove substances from blood or glomerular fi ltrate
and metabolize them, resulting in their disappearance from
the body. In other cases, the cells
produce
substances and add
them either to the blood or tubular fl
uid; the most important
of these, as we will see, are ammonium ions, hydrogen ions,
and bicarbonate ions.
In summary, one can study the normal renal processing
of any given substance by asking a series of questions:
1.
To what degree is the substance fi lterable at the renal
corpuscle?
2.
Is it reabsorbed?
3.
Is it secreted?
4.
What factors regulate the quantities fi ltered,
reabsorbed, or secreted?
5.
What are the pathways for altering renal excretion of
the substance to maintain stable body balance?
Glomerular Filtration
As stated previously, the glomerular fi ltrate—that is, the fl
uid
in Bowman’s space—normally contains no cells but contains
all plasma substances except proteins in virtually the same con-
centrations as in plasma. This is because glomerular fi ltration is
a bulk-fl ow process in which water and all low-molecular-weight
substances (including smaller peptides) move together. Most
plasma proteins—the albumins and globulins—are excluded
almost entirely from the fi ltrate. One reason for their exclu-
sion is that the renal corpuscles restrict the movement of such
high-molecular-weight substances. A second reason is that the
fi ltration pathways in the corpuscular membranes are nega-
tively charged, so they oppose the movement of these plasma
proteins, most of which are negatively charged.
The only exceptions to the generalization that all non-
protein plasma substances have the same concentrations in the
glomerular fi ltrate as in the plasma are certain low-molecular-
weight substances that would otherwise be fi lterable but are
bound to plasma proteins and therefore not fi ltered. For exam-
ple, half the plasma calcium and virtually all of the plasma fatty
acids are bound to plasma protein and so are not fi ltered.
Forces Involved in Filtration
Filtration across capillaries is determined by opposing Starling
forces (described in Chapter 12). To review, they are the hydro-
static pressure difference across the capillary wall that favors
fi ltration, and the protein concentration difference across the
wall that creates an osmotic force that opposes fi ltration.
This also applies to the glomerular capillaries, as sum-
marized in
Figure 14–8
. The blood pressure in the glomeru-
lar capillaries—the glomerular capillary hydrostatic pressure
(
P
GC
)—is a force favoring fi ltration. The fl
uid in Bowman’s
space exerts a hydrostatic pressure (
P
BS
) that opposes this fi l-
tration. Another opposing force is the osmotic force (
π
GC
)
that results from the presence of protein in the glomerular
capillary plasma. Recall that there is virtually no protein in the
fi ltrate in Bowman’s space because of the unique structure of
the areas of fi ltration in the glomerulus, so the osmotic force
in Bowman’s space (
π
BS
) is zero. The unequal distribution of
Figure 14–7
Renal handling of three hypothetical fi ltered
substances X, Y, and Z. X is fi ltered and secreted
but not reabsorbed. Y is fi ltered, and a fraction
is then reabsorbed. Z is fi ltered and completely
reabsorbed. The thickness of each line in this
hypothetical example suggests the magnitude of
the process.
Substance X
Bowman’s
space
Glomerular
capillary
Substance Y
Substance Z
Urine
Urine
Urine
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