Chapter 25
The Urinary System
963
25
Tubular secretion.
Tubular secretion
(“selectively adding
to the waste container”) is the process of selectively mov-
ing substances from the blood into the filtrate. Like tubular
reabsorption, it occurs along the length of the tubule and
collecting duct.
Te kidneys process an enormous volume of blood each day.
Of the approximately 1200 ml of blood that passes through the
glomeruli each minute, some 650 ml is plasma, and about one-
fiFh of this (120–125 ml) is forced into the glomerular capsules
as filtrate. Tis is equivalent to filtering your entire plasma vol-
ume more than 60 times each day! Considering the magnitude
of their task, it is not surprising that the kidneys (which account
for only 1% of body weight) consume 20–25% of all oxygen
used by the body at rest.
±iltrate and urine are quite different. ±iltrate contains every-
thing found in blood plasma except proteins.
Urine
contains
unneeded substances such as excess salts and metabolic wastes.
Te kidneys process about 180 L (47 gallons!) of blood-derived
fluid daily. Of this amount, less than 1% (1.5 L) typically leaves
the body as urine; the rest returns to the circulation.
Te JGC includes three populations of cells that help regulate
the rate of filtrate formation and systemic blood pressure.
Te
macula densa
(mak
9
u-lah den
9
sah; “dense spot”) is
a group of tall, closely packed cells in the ascending limb
of the nephron loop that lies adjacent to the granular cells
(±igure 25.8). Te macula densa cells are chemoreceptors
that monitor the NaCl content of the filtrate entering the
distal convoluted tubule.
Granular cells
[also called
juxtaglomerular
(
JG
)
cells
] are in
the arteriolar walls. Tey are enlarged smooth muscle cells
with prominent secretory granules containing the enzyme
renin
(see p. 621). Granular cells act as mechanoreceptors
that sense the blood pressure in the afferent arteriole.
Extraglomerular mesangial cells
lie between the arteriole and
tubule cells, and are interconnected by gap junctions. Tese
cells may pass regulatory signals between macula densa and
granular cells.
We discuss the physiological role of the JGC in the next section.
Check Your Understanding
4.
Name the tubular components of a nephron in the order that
filtrate passes through them.
5.
What are the structural differences between juxtamedullary
and cortical nephrons?
6.
What type of capillaries are the glomerular capillaries? What
is their function?
For answers, see Appendix H.
Kidney Physiology:
Mechanisms of Urine Formation
If you had to design a system to chemically balance and cleanse
the blood, how would you do it? Conceptually, it’s really very
simple. Te body solves this problem in the following way. ±irst,
it “dumps” cell- and protein-free blood into a separate “waste
container.” ±rom this container, it reclaims everything the body
needs to keep (which is almost everything filtered). ±inally,
the kidney selectively adds specific things to the container, fine-
tuning the body’s chemical balance. Anything leF in the con-
tainer becomes urine. Tis is basically how nephrons work.
Urine formation and the adjustment of blood composition
involve three processes
(Figure 25.9)
:
Glomerular filtration.
Glomerular filtration
(“dumping into
the waste container”) takes place in the renal corpuscle and
produces a cell- and protein-free filtrate.
Tubular reabsorption.
Tubular reabsorption
(“reclaiming
what the body needs to keep”) is the process of selectively
moving substances from the filtrate back into the blood. It
takes place in the renal tubules and collecting ducts. ²u-
bular reabsorption reclaims almost everything filtered—all
of the glucose and amino acids, and some 99% of the wa-
ter, salt, and other components. Anything that is
not
reab-
sorbed becomes urine.
Cortical
radiate
artery
Afferent arteriole
Glomerular
capillaries
Efferent arteriole
Glomerular capsule
Renal tubule and
collecting duct
containing filtrate
Peritubular
capillary
To cortical radiate vein
1
2
3
Urine
Glomerular filtration
Tubular reabsorption
Tubular secretion
1
2
3
Three major
renal processes:
Figure 25.9
A schematic, uncoiled nephron showing the
three major renal processes that adjust plasma composition.
Each kidney actually has more than a million nephrons acting in
parallel.
1
2
3
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