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434
C
HAPTER
T
WENTY
T
WO
Excretion
nating the products of red blood cell destruction and in
breaking down certain drugs and toxins.
The urinary system is also called the
excretory system
be-
cause one of its main functions is
excretion
, removal and
elimination of metabolic waste products from the blood.
It has many other functions as well, including regulation
of the volume, acid–base balance (pH), and electrolyte
composition of body fluids.
Although the focus of this chapter is the urinary system,
certain aspects of other systems are also discussed, because
body systems work interdependently to maintain home-
ostasis (internal balance). The systems active in excretion
and some of the substances they eliminate are the following:
The
respiratory system
eliminates carbon dioxide and
water. The latter appears as vapor, as can be demon-
strated by breathing on a windowpane.
The skin, or
integumentary system
,
excretes water,
salts, and very small quantities of nitrogenous wastes.
These all appear in perspiration, although water also
evaporates continuously from the skin without our
being conscious of it.
Checkpoint 22-1:
The main function of the urinary system is to
eliminate waste. What are some other systems that eliminate
waste?
The
urinary system
excretes water, nitrogen-contain-
ing waste products, and salts. These are all constituents
of the urine.
Organs of the Urinary System
The
digestive system
eliminates water, some salts, and
bile in addition to digestive residue, all of which are
contained in the feces. The liver is important in elimi-
The main parts of the urinary system, shown in
Figure
22-1
, are as follows:
Diaphragm
Hepatic veins
Inferior
vena cava
Adrenal
gland
Abdominal
aorta
Right
kidney
Renal
artery
Renal
vein
Right
ureter
Common
iliac vein
Common
iliac artery
Internal
iliac vein
Urinary
bladder
Internal
iliac artery
External
iliac vein
Prostate
gland
External
iliac artery
Urethra
Figure 22-1
Male urinary system, showing blood vessels.
ZOOMING IN
✦
What vessel supplies blood to the kidney? What ves-
sel drains the kidney?
T
HE
U
RINARY
S
YSTEM
435
Two
kidneys
.
These organs extract wastes from the
blood, balance body fluids, and form urine.
Blood Supply to the Kidney
The kidney’s blood supply is illustrated in
Figure 22-2.
Blood is brought to the kidney by a short branch of the
abdominal aorta called the
renal artery
.
After entering the kidney, the renal artery subdivides
into smaller and smaller branches, which eventually
make contact with the functional units of the kidney, the
nephrons
(NEF-ronz). Blood leaves the kidney by ves-
sels that finally merge to form the
renal
vein, which car-
ries blood into the inferior vena cava for return to the
heart.
Two
ureters
(U-re-ters). These tubes conduct urine
from the kidneys to the urinary bladder.
A single
urinary bladder
. This reservoir receives and
stores the urine brought to it by the two ureters.
A single
urethra
(u-RE-thrah). This tube conducts
urine from the bladder to the outside of the body for
elimination.
Checkpoint 22-2:
What are the organs of the urinary system?
The Kidneys
Checkpoint 22-3:
The kidneys are located in the retroperi-
toneal space. Where is this space?
The kidneys lie against the back muscles in the upper ab-
domen at about the level of the last thoracic and first
three lumbar vertebrae. The right kidney is slightly lower
than the left to accommodate the liver. Each kidney is
firmly enclosed in a membranous
renal capsule
made of
fibrous connective tissue. In addition, there is a protective
layer of fat called the
adipose capsule
around the organ.
An outermost layer of fascia (connective tissue) anchors
the kidney to the peritoneum and abdominal wall. The
kidneys, as well as the ureters, lie posterior to the peri-
toneum. Thus, they are not in the peritoneal cavity but
rather in an area known as the
retroperitoneal
(ret-ro-
per-ih-to-NE-al)
space
.
Checkpoint 22-4:
What vessel supplies blood to the kidney and
what vessel drains blood from the kidney?
Structure of the Kidney
The kidney is a somewhat flattened organ about 10 cm (4
inches) long, 5 cm (2 inches) wide, and 2.5 cm (1 inch)
thick
(Fig. 22-3)
. On the medial border there is a notch
called the
hilum
, where the renal artery, the renal vein,
and the ureter connect with the kidney. The lateral bor-
der is convex (curved outward), giving the entire organ a
bean-shaped appearance.
The kidney is divided into two regions: the renal cor-
tex and the renal medulla
(Fig. 22-3)
. The
renal cortex
is
the kidney’s outer portion. The
renal medulla
contains
the tubes in which urine is formed and collected. These
tubes form a number of cone-shaped structures called
renal pyramids
. The tips of the pyramids point toward
the
renal pelvis
, a funnel-shaped basin that forms the
upper end of the ureter. Cuplike extensions of the renal
pelvis surround the tips of the pyramids and collect urine;
these extensions are called
calyces
(KA-lih-seze; singular,
calyx
, KA-liks). The urine that collects in the pelvis then
passes down the ureters to the bladder.
22
Checkpoint 22-5:
What are the outer and inner regions of the
kidney called?
Figure 22-2
Blood supply and circulation of the kidney.
ZOOMING IN
✦
What vessel supplies blood to the renal artery?
What vessel receives blood from the renal vein?
The Nephron
As is the case with most organs, the
most fascinating aspect of the kidney is too small to be
seen with the naked eye. This basic unit, which actually
does the kidney’s work, is the
nephron
(Fig. 22-4)
. The
nephron is essentially a tiny coiled tube with a bulb at one
end. This bulb, known as the
glomerular
(Bowman)
cap-
sule
,
surrounds a cluster of capillaries called the
glomeru-
lus
(glo-MER-u-lus) (pl., glomeruli [glo-MER-u-li]).
Each kidney contains about 1 million nephrons; if all
these coiled tubes were separated, straightened out, and
laid end to end, they would span some 120 kilometers (75
miles)!
Figure 22-5
is a microscopic view of kidney tissue
showing several glomeruli, each surrounded by a
436
C
HAPTER
T
WENTY
T
WO
Nephrons
Calyx
Renal
cortex
Hilum
Renal
pelvis
Ureter
Renal
medulla
Renal capsule
Pyramids
of medulla
Figure 22-3
Longitudinal section through the kidney showing its internal structure (
left
) and an enlarged diagram of
nephrons (
right
).
Each kidney contains more than 1 million nephrons.
ZOOMING IN
✦
What is the outer region of the kidney called?
What is the inner region of the kidney called?
glomerular capsule. This figure also shows sections
through the tubular portions of the nephrons.
A small blood vessel, the
afferent arteriole
,
supplies
the glomerulus with blood; another small vessel, called
the
efferent arteriole
,
carries blood from the glomerulus.
When blood leaves the glomerulus, it does not head im-
mediately back toward the heart. Instead, it flows into a
capillary network that surrounds the nephron’s tubular
portion. These
peritubular capillaries
,
are named for
their location.
The tubular portion of the nephron consists of several
parts. The coiled part leading from the glomerular cap-
sule is called the
proximal convoluted
(KON-vo-lu-ted)
tubule
(
PCT
, or just proximal tubule). The tubule then
uncoils to form a hairpin-shaped segment called the
loop
of Henle
. The first part of the loop, which carries fluid to-
ward the medulla, is the
descending limb
(see Fig. 22-4)
.
The part that continues from the loop’s turn and carries
fluid away from the medulla, is the
ascending limb
. Con-
tinuing from the ascending limb, the tubule coils once
again into the
distal convoluted tubule
(
DCT
, or just dis-
tal tubule), so called because it is farther along the tubule
from the glomerular capsule than is the PCT. The distal
end of each tubule empties into a collecting duct, which
then continues through the medulla toward the renal
pelvis.
The glomerulus, glomerular capsule, and the proxi-
mal and distal convoluted tubules of the nephron are
within the renal cortex. The loop of Henle and collecting
duct extend into the medulla
(see Fig. 22-3)
.
Checkpoint 22-6:
What is the functional unit of the kidney
called?
Checkpoint 22-7:
What name is given to the coil of capillaries
in the glomerular (Bowman) capsule?
The Juxtaglomerular (JG) Apparatus
The first
portion of the DCT curves back toward the glomerulus to
pass between the afferent and efferent arterioles
(Fig. 22-
6)
. At the point where the DCT makes contact with the
afferent arteriole, there are specialized cells in each that
together make up the
juxtaglomerular
(juks-tah-glo-
MER-u-lar)
(JG) apparatus.
The JG apparatus helps to
regulate kidney function. When blood pressure falls too
low for the kidneys to function effectively, cells in the
wall of the afferent arteriole secrete the enzyme
renin
(RE-nin), which raises blood pressure by a mechanism
described later.
Functions of the Kidney
The kidneys are involved in the following processes:
Excretion of unwanted substances, such as cellular
metabolic waste, excess salts, and toxins. One product
of amino acid metabolism is nitrogen-containing waste
material, a chief form of which is
urea
(u-RE-ah). After
synthesis in the liver, urea is transported in the blood to
the kidneys for elimination. The kidneys have a spe-
cialized mechanism for the elimination of urea and
other nitrogenous (ni-TROJ-en-us) wastes.
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