CHAPTER 12
Urinary System
221
Urinary Bladder
Figure 12-14A
Urinary Bladder, Bladder Wall
Figure 12-14B
Urothelium, Bladder Wall
Figure 12-14C
Clinical Correlation: Urothelial (Transitional) Carcinoma
Figure 12-15A,B
Transitional Epithelium, Urinary Bladder
Urethra
Figure 12-16A
Prostatic Urethra, Male Urethra
Figure 12-16B
Penile (Spongy) Urethra, Male Urethra
Figure 12-16C
Female Urethra
Introduction and Key Concepts for the
Urinary System
The
urinary system
is composed of two
kidneys
, two
ureters
,
the
bladder
, and the
urethra
. The
kidneys
produce urine, the
ureters
transport urine to the bladder, and the
bladder
tempo-
rarily stores and empties urine through the
urethra
to outside of
the body. The urinary system functions to (1) ± lter blood and
reabsorb nutrients; (2) control the water, ion, and salt balance
of the body; (3) maintain the acid-base balance of the blood;
(4) excrete metabolic wastes (urea and uric acid), toxins, and
drug components; (5) secrete hormones, such as renin and eryth-
ropoietin; and (6) produce calcitriol (an active form of vitamin
D) to help the body absorb dietary calcium into the blood.
Kidneys
The
kidneys
are bean-shaped organs located in the posterior
abdominal region on each side of the vertebral column. The
kidney can be divided into the
renal cortex
, the
renal medulla
,
and the
hilum
. The
renal cortex
is composed of renal corpuscles
and various cortical tubules, which include the proximal con-
voluted tubules, the distal convoluted tubules, and the corti-
cal collecting tubules. The
renal medulla
is located deep to the
cortex, and its tubules extend as
medullary rays
into the cortex
region. The medulla comprises 10 to 18 renal pyramids; each
pyramid contains the
loops of Henle
,
collecting ducts
, and
pap-
illary ducts
. The apical projection of a renal pyramid is called
the
renal papilla
. The papillary ducts empty urine at the tip of a
renal papilla onto its surface, which is called the
area cribrosa
(perforated area). Each renal papilla is surrounded by a space,
the minor calyx; several minor calices unite to form a major
calyx. There are two or three major calyces for each kidney.
The major calices unite to form the renal pelvis, which funnels
urine into the ureter. The
hilum
is the region in the medial por-
tion of the kidney where the renal artery, the renal vein, and
the ureter enter and exit the kidney (Fig. 12-2). Functionally
and structurally, the kidney can be divided into the
nephron
and the
collecting system
(Fig. 12-8B). The
nephron
produces
urine. The
collecting system
adjusts the composition of urine
and transports urine to the calyces.
THE NEPHRON
comprises a
renal corpuscle
, a
proximal
convoluted tubule
, a
loop of Henele
, and a
distal convoluted
tubule
.
A renal corpuscle
is composed of a
glomerulus
and a
Bowman capsule
. (1)
A
glomerulus
consists of a spherical
knot of capillaries, which is fed by an afferent arteriole and
drained by an efferent arteriole at the vascular pole. (2) A
Bow-
man capsule
consists of a
visceral layer
and a
parietal layer
.
The
visceral layer
is composed of podocytes, which cover the
capillaries of a
glomerulus. These cells have long, interdigitating
cellular processes and play an important role in blood ± ltration.
The interstitial tissues surrounding the glomerular capillaries
contain cells called
intraglomerular mesangial cells
. The
pari-
etal layer
of the Bowman capsule is a hollow spherical struc-
ture lined by simple squamous epithelium. The space between
the visceral and the parietal layers of the Bowman capsule is
called the
Bowman space
. Blood fl
ows through the glomerular
capillaries, and its plasma passes through the glomerular ± ltra-
tion barrier (the fused basal laminae of the endothelial cells and
the podocytes); the ± ltrate is collected in the Bowman space
(Fig. 12-6A,B). Thus, the renal corpuscle, as a whole, forms a
blood-± ltering unit, which allows water, metabolic wastes, ions,
and small molecules to pass through the capillary wall but pre-
vents circulating cells and large plasma proteins from leaving
the blood.
Proximal Convoluted Tubules
are long tubes that follow
a serpentine course as they drain the ±
ltrate from the renal
corpuscles into the loop of Henle. Each is lined by a simple
cuboidal epithelium with abundant long microvilli (brush bor-
der) bordering the lumen. Each proximal convoluted tubule
connects to a renal corpuscle at its
urinary pole
. The relatively
large epithelial cells of the proximal convoluted tubule contain
many mitochondria, which render their cytoplasm brightly
acidophilic (pink). The lateral boundaries between the cells
interdigitate, so that the boundaries between adjacent cells are
unclear in light microscopy. Their long microvilli appear to ± ll
much of the space within the lumen (Fig. 12-9A,B). The com-
bined structural features of the proximal convoluted tubules
contribute to their functions of actively transporting ions and
reabsorbing water, glucose, amino acids, proteins, and vitamins
from the ±
ltrate.
The Loop of Henle
is a continuation of the proximal
convoluted tubule. It is a U-shaped structure that includes a
descending limb
and an
ascending limb
(Fig. 12-8B). The
descending limb
consists of a
thick descending limb
(proximal
straight tubule) and a
thin descending limb
(descending thin
segment). The
ascending limb
contains a
thin ascending limb
(ascending thin segment) and a
thick ascending limb
(distal
straight tubule). The loop of Henle plays a crucial role in gen-
erating a high sodium concentration gradient in the interstitium
of the renal medulla. This permits water to move passively
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