The Kidneys and Regulation of Water and Inorganic Ions
To emphasize general principles,
trates the renal handling of three hypothetical substances.
Approximately 20 percent of the plasma that enters the glo-
merular capillaries is ﬁ ltered into Bowman’s space. This ﬁ l-
trate, which contains X, Y, and Z in the same concentrations
as in the capillary plasma, enters the proximal tubule and
begins to ﬂ ow through the rest of the tubule. Simultaneously,
the remaining 80 percent of the plasma, containing X, Y, and
Z, leaves the glomerular capillaries via the efferent arteriole
and enters the peritubular capillaries.
Assume that the tubule can secrete 100 percent of the
peritubular capillary substance X into the tubular lumen, but
cannot reabsorb X. Therefore, by the combination of ﬁ ltration
and tubular secretion, the plasma that originally entered the
renal artery is cleared of all of its substance X, which leaves the
body via the urine.
By contrast, assume that the tubule can reabsorb, but
not secrete, Y and Z. The amount of Y reabsorption is small,
so that much of the ﬁ
ltered material is not reabsorbed and
escapes from the body. For Z, however, the reabsorptive
mechanism is so powerful that all the ﬁ ltered Z is reabsorbed
back into the plasma. Therefore, no Z is lost from the body.
Section of a human kidney. For clarity, the juxtamedullary nephron
illustrated to show nephron orientation is not to scale—its outline
would not be clearly visible without a microscope. The outer kidney,
which contains all the renal corpuscles, is the cortex, and the inner
kidney is the medulla. Note that in the medulla, the loops of Henle
and the collecting ducts run parallel to each other. The medullary
collecting ducts drain into the renal pelvis.
The juxtaglomerular apparatus.
The three basic components of renal function. This ﬁ
gure is to
illustrate only the
of reabsorption and secretion, not
speciﬁ c sites or order of occurrence. Depending on the particular
substance, reabsorption and secretion can occur at various sites
along the tubule.