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A.FrostAnatomy ofthe Kidneys and Genitourinar y Tract
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22
■■■
Anatomy of the Kidneys and
Genitourinary Tract
ANDREI FROST
The urinary system comprises the upper urinary tracts
(kidneys and ureters), the urinary bladder, and the urethra. The genital system consists of the testes, epididymides, deferent ducts, seminal vesicles, ejaculatory ducts,
the prostate, and the penis in males; and the ovaries,
uterine tubes, uterus, and vagina in females. A complete
review of the embryology and anatomy is beyond the
scope of this book; instead, an overview is given from the
perspective of the vascular and interventional radiologist
(VIR). The emphasis, therefore, is on the urinary system
and the uterine tubes, which provide a fertile ground for
interventional procedures. Anatomy of the penis is more
appropriately discussed in Chapter 2. Embryology is presented for a better understanding of the anatomy and of
the more frequently encountered variants and anomalies.
■ Embryology
The urinary tract develops in three overlapping stages:
the pronephros, the mesonephros, and the metanephros
(Fig. 22-1).
aspect toward the cloaca. As the more caudal segments
develop, the earlier, cranial ones involute. The pronephros does not have an excretory function in the human
and involutes completely. The mesonephros is constituted by a pair of medially located mesonephric ducts
(wolffian ducts) and laterally located nephrogenic cords.
The nephrogenic cords form a collection of glomeruli
and primitive tubules connected to the mesonephric
ducts. The mesonephric duct eventually extends caudally
and establishes a communication with the cloaca, allowing excretion of urine. Unlike the pronephros, some of
1
Development progresses from the cranial
the mesonephric remnants participate in the formation
of the adult male genital tract (efferent ductules, epididymis, paradidymis, vas deferent, seminal vesicles, and
ejaculatory ducts). The metanephros develops from two
separate components: the metanephric duct (ureteral
bud) and the metanephric blastema. The ureteral bud is
a small outpouching that arises from the caudal aspect of
the mesonephric (wolffian) duct in approximately the
5-week-old embryo. This bud grows ventrally and superiorly, forming a long tube with a dilated cranial blind
end, called the ampulla. Surrounding the ampulla of the
metanephric duct on each side is a cellular mass called
the metanephric blastema. As the metanephric ducts elongate, they also divide and penetrate the blastema, inducing differentiation.
As the ureteral bud lengthens, the lumbosacral region
of the embryo undergoes accelerated growth. The combination of these processes results in an apparent ascension of the kidneys from the initial caudal origin toward
their final position in the midabdomen. Concomitant
with this apparent cranial migration, the kidneys also
rotate medially, which results in the final position at
birth, with the excretory portion of the kidney being
lateral and the renal pelvis located medially. Failure of
these processes results in a variety of abnormalities of
size, shape, position, and number of the kidneys. Frequently, these anomalies coexist. For example, in the
horseshoe kidney (the most frequent fusion anomaly, where
the lower poles of the kidneys are fused across the
midline), the renal pelves are ventral because of the
absence of normal rotation, the kidney is lower than
normal (ectopic), and the arterial supply is commonly
aberrant (multiple renal arteries originating from the
255
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A
FIGURE 22-1. A: Pronephros (2nd through 6th somites) and early
mesonephros; approximately 3 weeks. B: Mesonephros (9th through
16th somites) and early metanephros; 3⫹ weeks. C: Metanephros and
apparent renal migration; 5 weeks to term. Note the complete degeneration of the pronephros and partial degeneration of the mesonephros.
B
distal aorta and the iliac arteries). During ascension, the
kidneys are supplied with blood by paired mesonephric
branches of the aorta. As kidneys ascend, branches arising at lower levels involute and more cranial branches
substitute for them. Failure of this involution results in
accessory or aberrant renal arteries (25% of normal);
these arteries are usually lower and of smaller caliber
than the main renal arteries and supply the polar regions.
Renal ontogenesis concludes with the process of matu-
ration, which entails the overall growth of the organ and
variable degree of fusion of the approximately 14 initial
C
lobes (seven ventral, seven dorsal)(Fig. 22-2A).
2
A result
of this lobar fusion is the smoothing of the renal surface
with effacement of interlobar septations (Fig. 22-2B).
This process is more pronounced at the poles than at
the interpolar region (Fig. 22-3). Another consequence
of the lobar fusion is the formation of compound calyces. This process is also more pronounced in the polar
regions; therefore, the upper and lower pole calyces tend
to be compound, whereas the interpolar calyces usually
maintain their original ventral and dorsal distribution.
The implications of lobar fusion for the VIR will be further discussed in the subsection on renal anatomy.
Anatomy of the Kidneys and Genitourinary Tract 257
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A B
FIGURE 22-2. A: Diagram of the fetal kidney at approximately 28 weeks (coronal section). On average, there are 14 lobes (7
ventral and 7 dorsal), each irrigated by at least two interlobar arteries. The lobes are separated by connective tissue planes and
demarcated by deep grooves on the surface of the kidney. B: Fetal kidney at term (coronal section).Variable degree of lobar
fusion results in compound calices and effacement of the surface grooves.
■ Anatomy
Kidneys
Good knowledge of renal anatomy is a prerequisite for
percutaneous access to the renal collecting system, a wellestablished procedure performed for relief of obstruction or as a preamble for other diagnostic or therapeutic
procedures (biopsy, diversion of the urinary stream, stricture treatment, ureteral occlusion, or percutaneous stone
extraction).
The mature kidneys are paired organs. The left is usually slightly larger than the right, and there is significant
individual variation in size; however, as an overall rule,
kidney size is relatively symmetric (Fig. 22-4). The kidneys
are bean shaped with a medial indentation called hilum.
The contours are variably smooth, depending on the
degree of lobar fusion. Contours can be deformed by
indentation by adjacent organs, most frequently in the
FIGURE 22-3. Marked bilateral fetal lobulation of the kidneys
(left greater than right); abdominal aortogram, late venous
phase. Note the predominance in the interpolar area (right).
left kidney, secondary to splenic impression (Fig. 22-5).
Normally, the kidneys are located at the level of the
twelfth thoracic to the third lumbar vertebrae. The left
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FIGURE 22-4. Normal urogram. Note the symmetry of the
shape and size of the kidneys, the higher position of the left
kidney, and the compound calices in the polar areas. Some
asymmetry can be seen in the morphology of the pelves.
kidney is usually slightly higher than the right. Both kidneys are mobile and can change position significantly,
both with the phase of respiration and with change in
position of the patient.
3
This mobility can be used to the
advantage of the VIR because a needle inserted percutaneously into the kidney will swing characteristically with
respiratory motion, whereas an extrarenal needle will not
change position appreciably with respiratory motion.
Both kidneys are retroperitoneal organs. The retroperitoneal space is delineated anteriorly by the parietal
peritoneum, posteriorly by the fascia transversalis, superiorly by the diaphragm, and inferiorly by the pelvic brim.
The retroperitoneal space is subdivided in three important compartments (Fig. 22-6).
4
The anterior pararenal
space is delineated by the parietal peritoneum and posteriorly by the anterior renal fascia (Gerota’s fascia). This
space contains the ascending and descending colon, the
FIGURE 22-5. Splenic impression on the left kidney (
abdominal aortogram, late venous phase.
arrow
pancreas, duodenum, and a small amount of retroperitoneal fat. The two sides of the anterior pararenal space
are continuous across the midline, and there is potential
communication with the posterior pararenal space inferiorly.
The perirenal space is delineated anteriorly by the
anterior renal fascia and posteriorly by the posterior renal fascia (fascia of Zuckerkandl). It contains the kidneys,
adrenal glands, ureters, abundant perirenal fat, and the
great vessels (aorta and the inferior vena cava). The perirenal fat is continuous with the renal sinuses. The
perirenal space does not communicate across the midline
and therefore has two separate compartments (right and
left). A strong, fibrous, nonadherent capsule envelopes
each kidney, separating it from the perirenal fat. This
capsule and the thoracolumbar fascia (aponeurosis of
transverse abdominal muscle) are the points where a
needle meets resistance during percutaneous puncture
of the renal collecting system.
2
The posterior pararenal space is bordered anteriorly by
the posterior renal fascia and posteriorly by the fascia
transversalis. This space contains only fat. It communicates potentially with the anterior pararenal space at the
pelvic rim and with the mediastinum superiorly through
the paravertebral space. The importance of the retroperi-
);
Anatomy of the Kidneys and Genitourinary Tract
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FIGURE 22-6. Axial (A) and right (B) and left (C)
parasagittal diagramatic sections through the abdomen, illustrating the retroperitoneal compartments. The anterior pararenal space contains the
pancreas, ascending and descending colon and
the duodenum. The perirenal space contains the
kidneys, ureters and adrenal glands. The posterior pararenal space contains fat. Note rotation of
the kidneys about the longitudinal and transverse
axes. From Castaneda-Zuniga, WR, Tadavarty,
SM, eds. Interventional radiology, 2nd ed. Balti-
A
more: William & Wilkins, 1992.
259
B C
toneal space divisions lies in the characteristic radiological findings of fluid or gas collections within each of these
compartments.
Posterior relationships of the kidneys within the renal
fossa are important for the understanding and planning
of percutaneous procedures.5Superficially, a muscle layer
formed by the quadratus lumborum muscle laterally and
the psoas muscle medially extends from the costal margin
to the iliac crest. Superiorly, the posterior aspect of the
diaphragm comes in relationship with the upper poles of
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A. Frost
the kidneys. The aponeurosis of the transverse abdominal
muscle completes the posterior wall of the abdomen. Because of the slightly higher position of the left kidney, it is
crossed posteriorly by the eleventh and the twelfth ribs,
whereas usually the right kidney comes in posterior contract with only the twelfth rib. As mentioned, the position
of the kidneys and their mobility with respiratory motion
will vary significantly. As a result, the posterior pleural
space may be traversed during an attempted upper pole
(intercostal) percutaneous puncture. Because such punctures sometimes are required by the clinical situation, care
must be taken to ensure an extrapleural tract by careful
fluoroscopic examination with steep oblique or lateral
projections or by cross-sectional studies [computed tomography (CT) scan].
2
Transpleural punctures can result in
significant complications such as pneumothorax, hydrothorax, or hemothorax. Also noteworthy is the need to
avoid the intercostal vessels, which course along the inferior groove of the ribs. Therefore, intercostal punctures
should be performed with the needle hugging the superior aspect of the rib.
The lateral relationships of the kidneys are also relevant to the VIR because of the potential injury to adjacent organs during performance of percutaneous nephrostomy. On the right side, the most important
relationships are to the liver and to the hepatic flexure
of the colon; on the left side, the important relationships
are with the spleen and the splenic flexure of the colon.
Both the spleen and the liver are bulkier in their cranial
aspects, which makes for a more medial relationship with
the upper poles of the kidneys. More inferiorly, as the
spleen and liver decrease in bulk, their relationship with
the kidneys becomes more lateral. As a consequence, the
lower the percutaneous puncture (subcostal, lower pole
caliceal puncture versus supracostal, upper pole puncture), the more lateral the skin puncture site can be
while still avoiding injury to the adjacent organs. A more
lateral puncture will allow for a straighter tract into the
collecting system, which in turn ensures easier manipulations during subsequent procedures, such as percutaneous stone extraction or antegrade ureteral stenting.
This concept will be better understood after discussion
of the orientation of the kidneys and collecting systems.
The kidneys are obliqued in reference to the sagittal,
coronal, and axial planes of the body.
2
The upper poles
are most medial and posterior; lower poles are most lateral and anterior. In the axial plane, the lateral borders
of the kidneys are rotated approximately 30 degrees posterior to the coronal plane (Fig. 22-6). The hilum points
anteromedially and the lateral border posterolaterally.
Because of the embryological development of anterior
and posterior renal lobes, the mature collecting system
has two rows of calyces, one anterior and one posterior.
Lobar fusion leads to the formation of compound calyces, particularly at the poles. The nonfused calyces, pre-
dominantly in the interpolar segment, are oriented at a
90-degree angle to each other. The disposition of the
calyces follows one of the two extreme configurations
(Hodson’s or Brodel’s) or an in-between variation (Fig.
22-7). The Hodson type is more common and has the
ventral calyces positioned at a shallow 20-degree angle
anterior to the coronal plane of the kidney, whereas the
dorsal calyces are positioned at a steeper 70-degree angle
posterior to this plane. In the Brodel type, these angles
are reversed.
1,2
The perpendicular orientation of the anterior and posterior calyces, together with the overall rotation of the
kidneys, has important consequences for both diagnostic
and therapeutic procedures. Opacification of the collecting system depends on the type of contrast used and the
position of the patient. Iodinated contrast, which has a
higher specific gravity than urine, accumulates in the dependent regions. In a prone patient, iodinated contrast
preferentially will opacify the anterior and lower pole calyces. Air or carbon dioxide, which are gases, on the other
hand will preferentially opacify the posterior and upper
pole calyces in a prone patient. Carbon dioxide occasionally is used as a contrast agent in patients who are allergic
to iodinated contrast. Drainage of calyces into the renal
pelvis is also influenced by gravity and patient position.
For example, with the patient prone, the upper pole and
posterior calyces will empty of iodinated contrast first
while the remainder of the collecting system may retain
some contrast. Knowledge of the anatomy and understanding of preferential filling and emptying with the contrast medium used will help to identify and select the most
appropriate calyx for puncture. The straightest possible
tract is desirable for a successful percutaneous intervention (e.g., nephrostomy, antegrade ureteral stenting,
stone extraction). The trajectory of the needle must be
planned in such a way as to minimize the angle with the
calyx, infundibulum, and pelvis while also avoiding injury
to the adjacent organs (liver, spleen, colon, intercostal
arteries, and lungs). Fluoroscopy at various oblique angulations, using a C-arm or turning the patient, and occasionally cross-sectional studies are used for this purpose.
The arterial blood supply and the venous drainage are
also important considerations in performing percutaneous renal interventions. Usually, there are single right
and left renal arteries, which are direct branches of the
abdominal aorta. These branches originate at the upper
level of the second lumbar vertebra. The left renal artery
is straighter and shorter than the right renal artery and
has a posterolateral course, whereas the right renal artery
is longer and has an anterolateral proximal course. On
an angiogram, therefore, the origins of the renal arteries
are best visualized on the left anterior oblique projection.
The renal arteries have a variable branching pattern. The
most frequent pattern shows a ventral division, which
usually has four segmental branches and supplies the
Anatomy of the Kidneys and Genitourinary Tract
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A
261
B
FIGURE 22-7. Diagramatic cross-sectional representation of the two basic types of the renal collecting system: A: Brodel type
(dorsal calyces are positioned at a 20-degree angle posterior to the coronal plane and ventral calyces at a 70-degree angle
anterior to the coronal plane of the kidney) and B: Hodson type (dorsal clayces are at 70-degree angle posterior to the coronal
plane and ventral calyces at a 20-degree angle anterior to the coronal plane). From Castaneda-Zuniga WR, Tadavarty SM, eds.
Interventional radiology, 2nd ed. Baltimore: Williams & Wilkins, 1992.
upper pole and the ventral aspect of the kidney and a
dorsal division, which usually has a single segmental
branch and supplies the lower pole and dorsal aspect of
the kidney (Fig. 22-8). The five segmental arteries mentioned already are “end” arteries. They branch into interlobar arteries, which further branch into arcuate arteries
at the level of the corticomedullary junction. Interlobular
arteries branch from the arcuate arteries into the renal
cortex. Renal blood flow is distributed about 80% to the
cortex and 20% to the medulla. A relatively hypovascular
area is located along the lateral-posterior aspect of the
kidney, between the territories of distribution of the ventral and dorsal divisions of the renal artery. This hypovas-
cular area is the ideal site for percutaneous punctures
because it minimizes chances of injury to a large sized
vessel (Fig. 22-9).
2
The intrarenal venous drainage of the kidney is constituted by stellate, arcuate, and interlobar veins, which
parallel the arteries; unlike the arterial circulation, however, the venous drainage is highly anastomosed (Fig.
22-10). Eventually, single right and left renal veins usually
form and drain directly into the inferior vena cava at
approximately the level of the second lumbar vertebra.
The left renal vein courses anteriorly to the aorta and
posteriorly to the superior mesenteric artery and is
longer than the right renal vein. The left adrenal vein
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FIGURE 22-8. Normal renal artery: selective right renal angiogram, arterial phase. The ventral division (
the upper pole and the ventral aspect of the kidney; the dorsal
division (
(Courtesy of Andrew Kerr, M.D.)
arrow
) supplies the lower pole and the dorsal aspect.
arrowhead
) supplies
and gonadal vein are its tributaries. On the right side, the
gonadal and adrenal veins drain directly into the inferior
vena cava. This anatomic distribution is of obvious interest when sampling venous blood for evidence of adrenal
adenomas and explains the higher incidence and increased severity of left-sided varicocele caused by valvular
incompetence of the testicular vein. This subject is discussed further in Chapter 26.
Ureters
The ureters are paired muscular tubes. They connect the
right and left renal pelves to the urinary bladder. At the
level of the bladder, each ureter penetrates the thick wall
of the bladder in an oblique anteromedial course, which
constitutes the intramural segment. Ureters are approximately 25 to 30 cm long, are distensible, and have a
largest diameter of approximately 8 mm. At the entrance
into the true pelvis, both ureters cross anteriorly to the
common iliac vessels, which frequently cause extrinsic
impression upon the ureters. The ureteropelvic junction,
the ureterovesical junction, and the crossing of the common iliac vessels are the three most common sites for
stone impaction. The most important relationship of ure-
ters in their abdominal portion, in the perirenal space, is
to retroperitoneal lymph nodes: Because of this relationship, ureters can be involved and obstructed by metastatic adenopathy. Relationships in the pelvic course are
different in males and females. In males, the pelvic ureter
is related to the ipsilateral deferent duct, seminal vesicle,
internal iliac vessels, and iliac lymph nodes. The distalmost segment of the ureter is lateral and cranial to the
prostate gland, as it approaches the trigone. In females,
the pelvic ureter crosses posteriorly to the ovary, in the
base of the broad ligament of the uterus. At this level, it
is crossed posteriorly by the uterine artery. More caudally,
the pelvic ureter is crossed anteriorly by the uterine artery, as it lies lateral and anterior to the cervix and superior to the fornix of the vagina. In this location, the ureter
can be injured during obstetric or gynecologic surgery.
The vascular supply of the ureter is variable and highly
anastomosed. The arterial supply usually consists of a supe-
rior ureteric artery, which is a branch of the renal artery, and
an inferior ureteric artery, which is a branch of the middle
vesical artery. The middle vesical artery is a branch of the
umbilical arter y, which is one of the ventral branches of
the hypogastric artery. Another name for the middle vesical artery in males is the vesiculodeferential arter y. Less
common and more variable arterial supply for the ureter
derives from peritoneal arteries, directly from the abdominal aorta, from the external and common iliac artery branches, and from branches of the gonadal arteries.
The venous drainage of the ureter follows the arterial
supply and eventually drains into the renal veins, inferior
vena cava, and its tributaries.
Urinary bladder
The urinary bladder is a muscular sac located in the true
pelvis, posterior to the pubic symphysis. Inferiorly, the
male urinary bladder lies atop the prostate, which separates it from the floor of the perineum. In females, the
bladder lies directly on the muscular perineum.
The arterial supply of the urinary bladder is variable. It
is supplied by branches of the anterior division of the
internal iliac arteries. The three most constant such
branches are the superior, middle, and inferior vesical
arteries. The superior vesical artery is a terminal branch of
the umbilical artery. The middle vesical artery (also called
vesiculodeferential in males) can be a branch of the umbilical artery or of the uterine artery in females. The
inferior vesical artery is a branch of the internal iliac artery.
There are extensive anastomoses between the right and
left internal iliac arteries and between the ventral and
posterior division branches of the ipsilateral internal iliac
artery, resulting in a perivesical plexus. This arrangement
explains the need to perform bilateral embolization of
the hypogastric arteries to control hemorrhage of the
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FIGURE 22-9. Cross-section through the interpolar aspect of the kidney, in prone position, showing ventral and dorsal distribution of the renal vessels. The hypovascular, peripheral area of the kidney (diagram) is the preferred site for percutaneous
punctures, since it is least likely to contain large caliber vessels. From Castaneda-Zuniga, WR, Tadavarty, SM (eds): Interventional radiology, 2nd ed. Baltimore: Williams & Wilkins, 1992.
urinary bladder. The high degree of anastomosis also
prevents bladder ischemia following such embolization.
The venous drainage of the urinary bladder is through
the perivesical plexus, which is highly anastomosed with
the dorsal vein of the penis/clitoris, parietal pelvic veins,
and veins of the thigh and buttock. The vesical plexus
drains into the right and left internal iliac veins.
tends from the bladder neck to the level of the external
sphincter (the floor of the perineum). This segment is
enveloped completely by the prostate gland. On its posterior wall, a contour defect is formed by an elongated
cranial portion termed the urethral crest and a rounded
caudal portion called the verumontanum. Multiple prostatic ducts open in the region of the urethral crest,
whereas the utricle and the two ejaculatory ducts open
Urethra
The urethra, the lowest component of the lower urinar y
tract, differs in females and males. The female urethra
is a straight conduit, approximately 4 cm long, which
stretches from the bladder neck to the external meatus
(Fig. 22-11). It is susceptible to traumatic injuries, particularly related to childbirth. The male urethra is longer
and more complex. It is usually divided into three segments (Fig. 22-12): the most cranial is the prostatic urethra, which measures approximately 3 cm long, and ex-
at the level of the verumontanum. This segment of the
urethra can be stenosed by tumors or benign prostatic
hyperplasia, conditions that are targets of recent developments in inter ventional radiology, such as balloon
prostatic urethroplasty, hyperthermic ablation, and prostatic stents.
6,7
The second segment of the male urethra is the membranous urethra, which is approximately 1cm long, and is the
segment of the urethra that pierces the floor of the perineum. It is circled by muscle fibers that form the external
sphincter. This segment of the urethra is believed toplay a
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