The Kidneys and Regulation of Water and Inorganic Ions
491
To emphasize general principles,
Figure 14–7
illus-
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.
Renal
pelvis
Ureter
To urinary
bladder
Nephron
(enlarged)
Renal cortex
Renal medulla
Sympathetic
nerve fiber
Podocytes
Juxtaglomerular
cells
Afferent arteriole
Efferent arteriole
Distal tubule
Macula densa
Smooth muscle
cells
Mesangial cells
Figure 14–4
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.
Figure 14–5
The juxtaglomerular apparatus.
Artery
Afferent
arteriole
Glomerular
capillary
Efferent
arteriole
Bowman’s
space
Tubule
Peritubular
capillary
Vein
Urinary
excretion
3
2
1
1. Glomerular
filtration
2. Tubular
secretion
3. Tubular
reabsorption
Figure 14–6
The three basic components of renal function. This fi
gure is to
illustrate only the
directions
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.
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