776
UNIT FIVE
20.2
KIDNEYS
A
kidney
is a reddish brown, bean-shaped organ with a
smooth surface. It is about 12 centimeters long, 6 centimeters
wide, and 3 centimeters thick in an adult, and it is enclosed
in a tough, F
brous capsule (tunic F
brosa).
Location of the Kidneys
The kidneys lie on either side of the vertebral column in a
depression high on the posterior wall of the abdominal cav-
ity. The upper and lower borders of the kidneys are gener-
ally at the levels of the twelfth thoracic and third lumbar
vertebrae, respectively, although the positions of the kidneys
may vary slightly with changes in posture and with breath-
ing movements. The left kidney is usually about 1.5 to 2 cen-
timeters higher than the right one.
The kidneys are positioned
retroperitoneally
(re
tro-
per
ı˘-to-ne
al-le); they are behind the parietal peritoneum
and against the deep muscles of the back. Connective tissue
(renal fascia) and masses of adipose tissue (renal fat) sur-
rounding the kidneys hold them in place (
f g. 20.3
and refer-
ence plates 18, 19).
Kidney Structure
The lateral surface of each kidney is convex, but its medial
side is deeply concave. The resulting medial depression leads
into a hollow chamber called the
renal sinus.
Through the
entrance to this sinus, termed the
hilum,
pass blood vessels,
nerves, lymphatic vessels, and the ureter (see F
g. 20.1).
The superior end of the ureter expands to form a funnel-
shaped sac called the
renal pelvis,
located mostly inside the
renal sinus. The pelvis is formed by the convergence of two
or three tubes, called
major calyces
(sing.,
calyx
), and they,
in turn, are formed by the convergence of seven to twenty
minor calyces
(f
g. 20.4)
. At least one small projection called
a
renal papilla
extends into each minor calyx.
The kidney includes two distinct regions: an inner
medulla and an outer cortex. The
renal medulla
(re
nal
me
˘-dul
ah) is composed of conical masses of tissue called
renal pyramids.
Their bases orient toward the convex sur-
face of the kidney, and their apexes form the renal papillae.
The tissue of the medulla appears striated because it con-
sists of microscopic tubules that lead from the cortex to the
renal papillae.
The
renal cortex
(re
nal kor
teks), which appears some-
what granular, forms a shell around the medulla. Its tissue
dips into the medulla between the renal pyramids, forming
renal columns.
The
renal capsule
is a fibrous membrane
that surrounds the cortex and helps maintain the shape of
the kidney. It also is protective (
f
gs.
20.4 and
20.5
).
Functions of the Kidneys
The main function of the kidneys is to regulate the volume,
composition, and pH of body fl
uids. In the process, the kid-
neys remove metabolic wastes from the blood and excrete
Kidney
Inferior
vena cava
Abdominal
aorta
Hilum
Renal
vein
Ureters
Urinary
bladder
Urethra
Renal
artery
FIGURE 20.1
The urinary system includes the kidneys, ureters,
urinary bladder, and urethra. Notice the relationship of these structures
to the major blood vessels.
FIGURE 20.2
Structures of the urinary system are visible in this
falsely colored radiograph (anterior view).
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