The Kidneys and Regulation of Water and Inorganic Ions
489
now mentioned two sets of capillaries in the kidneys—the
glomerular capillaries (glomeruli) and the peritubular capil-
laries. Within each nephron, the two sets of capillaries are
connected to each other by an efferent arteriole, the vessel
by which blood leaves the glomerulus (see Figures 14–2 and
14–3a). Thus, the renal circulation is very unusual in that
it includes
two
sets of arterioles and
two
sets of capillaries.
After supplying the tubules with blood, the peritubular cap-
illaries then join to form the veins by which blood leaves the
kidney.
There are two general types of nephrons (see Figure
14–2a). About 15 percent of the nephrons are
juxtamedullary,
which means that the renal corpuscle lies in the part of the cor-
tex closest to the cortical-medullary junction. The Henle’s loops
of these nephrons plunge deep into the medulla and, as we will
see, are responsible for generating an osmotic gradient in the
medulla responsible for the reabsorption of water. In close prox-
imity to the juxtamedullary nephrons are long capillaries known
as the
vasa recta,
which also loop deeply into the medulla and
then return to the cortical-medullary junction. The majority
of nephrons are
cortical,
meaning their renal corpuscles are
located in the outer cortex and their Henle’s loops do not pen-
etrate deep into the medulla. In fact, some cortical nephrons
do not have a Henle’s loop at all; they are involved in reab-
sorption and secretion but do not contribute to the hypertonic
medullary interstitium described later in the chapter.
One additional anatomical detail involving both the
tubule and the arterioles is important. Near its end, the ascend-
ing limb of each loop of Henle passes between the afferent
and efferent arterioles of that loop’s own nephron (see Figure
14–2). At this point there is a patch of cells in the wall of the
ascending limb as it becomes the distal convoluted tubule
called the
macula densa,
and the wall of the afferent arteri-
ole contains secretory cells known as
juxtaglomerular (JG)
cells.
The combination of macula densa and juxtaglomeru-
lar cells is known as the
juxtaglomerular apparatus (JGA)
(
Figures 14–3a
and
14–5
). The juxtaglomerular cells secrete
renin into the blood.
Basic Renal Processes
As we have said, urine formation begins with the fi ltration of
plasma from the glomerular capillaries into Bowman’s space.
This process is termed
glomerular fi ltration,
and the fi ltrate
is called the
glomerular fi ltrate.
It is cell-free and, except
for proteins, contains all the substances in virtually the same
concentrations as in plasma. This type of fi ltrate is also termed
an ultrafi ltrate.
During its passage through the tubules, the fi ltrate’s com-
position is altered by movements of substances from the tubules
to the peritubular capillaries and vice versa (
Figure 14–6
).
When the direction of movement is from tubular lumen to peri-
tubular capillary plasma, the process is called
tubular reab-
sorption,
or simply reabsorption. Movement in the opposite
direction—that is, from peritubular plasma to tubular lumen—
is called
tubular secretion,
or simply secretion. Tubular secre-
tion is also used to denote the movement of a solute from the
cell interior to the lumen in the cases in which the kidney tubu-
lar cells themselves generate the substance.
To summarize: A substance can gain entry to the tubule
and be excreted in the urine by glomerular fi ltration or tubu-
lar secretion or both. Once in the tubule, however, the sub-
stance need not be excreted but can be reabsorbed. Thus, the
amount of any substance excreted in the urine is equal to the
amount fi ltered plus the amount secreted, minus the amount
reabsorbed.
Amount
=
Amount
+
Amount
Amount
excreted
fi ltered
secreted
reabsorbed
We must stress that not all these processes—fi ltration,
secretion, and reabsorption—apply to all substances. For
example, important solutes like glucose are completely reab-
sorbed, whereas toxins are secreted and not reabsorbed.
Glomerulus
(glomerular capillaries)
Bowman’s space in
Bowman’s capsule
Proximal convoluted tubule
Proximal straight tubule
Descending limb of loop of Henle
Thin segment of ascending limb
of loop of Henle
Thick segment of ascending limb
of loop of Henle
Distal convoluted tubule
Cortical collecting duct
Medullary collecting duct
Renal pelvis
Renal tubule
Loop of Henle
Collecting duct system
Renal corpuscle
Distal convoluted tubule
Proximal tubule
Figure 14–2
continued
Basic structure of a nephron. (b) Consecutive segments of the
nephron. All segments in the yellow area are parts of the renal
tubule; the terms to the right of the brackets are commonly used for
several consecutive segments.
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