Posterior Musculature and Lumbodorsal Fascia

See Figures 1-3 to 1-6 and Table 1–1. The lumbodorsal fascia surrounds the sacrospinalis and quadratus lumborum, which together comprise the posterior abdominal wall. The lumbodorsal fascia originates from the spinous processes of the lumbar vertebrae and extends anteriorly and cranially. As it progresses upward, it separates into three layers: posterior, middle, and anterior.

Figure 1–3 Posterior abdominal wall musculature, superficial dissection. A section of the latissimus dorsi muscle has been removed. The location of the right kidney within the retroperitoneum is shown by dashed outline.

Figure 1–4 Posterior abdominal wall musculature, intermediate dissection. The sacrospinalis muscle and three anterolateral flank muscle layers are seen in cut section, and the three layers of the lumbodorsal fascia posteriorly can be appreciated.

Figure 1–5 Posterior abdominal wall musculature, deep dissection. The lumbodorsal fascia and costovertebral ligament are visualized, arising from the transverse processes of the lumbar vertebrae. The relation of the kidney and pleura is also shown.

Figure 1–6 Muscles of the posterior abdominal wall.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 316.)

Table 1–1 Musculature of the Posterior and Lateral Abdominal Wall

The posterior layer provides the posterior covering for the sacrospinalis muscle and is the origin of the latissimus dorsi muscle. The middle layer forms the fascial layer separating the anterior aspect of the sacrospinalis muscle from the posterior aspect of the quadratus lumborum. The anterior layer of the lumbodorsal fascia provides the anterior covering to the quadratus lumborum muscle and forms the posterior margin of the retroperitoneum. As one moves laterally away from the sacrospinalis and quadratus lumborum muscles, the lumbodorsal fascial layers fuse together and then connect with the transversus abdominis muscle.

The quadratus lumborum and sacrospinalis muscles (see Figs. 1-6 and 1-7) form the muscular portion of the posterior abdominal wall, filling the space among the 12th rib, spine, and iliac crest. The quadratus lumborum serves a number of functions. It supports the 12th rib, thus improving diaphragmatic contraction and inspiration, as well as aiding intercostal muscle function during forced expiration. Finally, it controls lateral bending of the trunk. The sacrospinalis also controls movement of the trunk by promoting extension of the spine. These muscular and fascial relationships become important clinically when performing a dorsal lumbotomy incision. As seen in Figure 1–7, this is a vertical incision lateral to the border of the sacrospinalis and quadratus lumborum. This approach allows entrance to the retroperitoneum without violation of the musculature.

Figure 1–7 Transverse section through the kidney and posterior abdominal wall showing the lumbodorsal fascia incised. Note that through such a lumbodorsal incision, the kidney can be reached without incising muscle.

(After Kelly and Burnam, from McVay C. Anson & McVay surgical anatomy. 6th ed. Philadelphia: WB Saunders; 1984.)

Lateral Flank Musculature

See Figure 1–8 and Table 1–1. Three muscular layers comprise the lateral flank musculature. From superficial to internal, these are the external oblique, internal oblique, and transversus abdominis muscles. The most superficial structure is the external oblique muscle. This muscle arises from the lower ribs and moves from lateral to medial as it progresses caudally. Final attachment is to the iliac crest caudally and the rectus sheath anteriorly. The posterior border remains free as it terminates before reaching the lumbodorsal fascia. Next is the internal oblique muscle. Again, this muscle arises from the lower rib cage, but the orientation of the fibers is from medial to lateral as they move caudally. Final attachment is to the iliac crest and lumbodorsal fascia. The final structures are the transversus abdominis muscle and transversalis fascia. The transversus abdominis muscle arises from the lumbodorsal fascia with fibers running directly transversely until it attaches anteriorly and medially onto the rectus sheath. Immediately deep to the transversus abdominis muscle is the transversalis fascia and then the retroperitoneal space. The function of the lateral flank musculature is to compress and stabilize the abdomen and trunk. This provides controlled movement and protection for the abdominal organs.

Figure 1–8 Transverse section showing layers of the lateral flank musculature.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 252.)

Psoas and Iliacus Muscles

The psoas major muscle originates on the 12th thoracic through the 5th lumbar vertebrae (see Fig. 1–6). A smaller psoas minor is identifiable in about one half of the population and resides medial to the psoas major. The psoas muscle(s) is covered by the psoas fascia. In close proximity to the psoas muscle is the iliacus muscle, which attaches to the inner aspect of the iliac pelvic wing. As the iliacus progresses caudally it joins with the psoas muscle to form the iliopsoas muscle. This combined muscle then joins to the lesser trochanter of the femur and controls flexion of the hip.

Lower Rib Cage

See Figure 1–9. In addition to the protection provided by the muscular layers of the posterior and lateral abdominal wall, the 10th, 11th, and 12th ribs safeguard the upper retroperitoneal space and are intimately related to the adrenal glands and kidneys. Given the close proximity, injury to these ribs can be associated with significant retroperitoneal injury. While providing protection, the lower ribs and the accompanying pleura and lung limit surgical exposure to the upper retroperitoneum. The limits of the pleura are the 8th rib anteriorly, the 10th rib in the midaxillary line, and the 12th rib posteriorly. Given this location of the pleura, flank incisions at or above the 11th or 12th ribs risk pleural violation.

Figure 1–9 Structures related to the posterior surface of the kidney.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 322.)

Abdominal Aorta

The aorta enters the abdomen via the aortic hiatus found between the diaphragmatic crura in the posterior diaphragm at the level of the 12th thoracic vertebrae (see Fig. 1–2). It continues caudally to the 4th lumbar vertebrae, where it bifurcates into the common iliac arteries. During its course through the abdomen the aorta gives off a number of large branches (Table 1–2). The paired inferior phrenic arteries are first. They supply the inferior diaphragm and the superior portion of the adrenal gland (see Fig. 1–2). Next is the celiac trunk, which is the origin for the common hepatic, left gastric, and splenic arteries that supply the liver, stomach, and spleen, respectively. The paired adrenal arteries follow with an artery going to each adrenal gland. The superior mesenteric artery leaves the aorta on the anterior side and supplies the entire small intestine and majority of the large intestine. Also of note, this artery communicates with the celiac trunk vasculature via the pancreaticoduodenal artery. Overlying the 2nd lumbar vertebrae, the paired renal arteries are the next branching point of the aorta. To the urologist, renal artery anatomy is obviously of great importance and is discussed in detail in the kidney section.

Table 1–2 Branches of the Abdominal Aorta

Moving distally on the aorta, the paired gonadal arteries are encountered. In the male this artery is also called the testicular artery and in the female it is the ovarian artery. The initial course in both the male and female is similar, with the artery moving caudally and laterally from the aorta, with the right gonadal artery crossing anterior to the inferior vena cava. In men, the gonadal artery then crosses over the ureter and exits the retroperitoneum at the internal inguinal ring. In women the course is different: Instead of exiting the pelvis, the artery crosses medially back over the external iliac vessels and enters the pelvis. It then proceeds via the suspensory ligament to the ovary. The destination of the gonadal artery (the testis in the male and the ovary in the female) has significant collateral sources of arterial blood, from the deferential and cremasteric arteries in the male and the uterine artery in the female. Thus the gonadal artery can generally be ligated during retroperitoneal surgery without detrimental effect.

After the gonadal arteries, the inferior mesenteric artery is found on the anterior side of the aorta before its bifurcation into the common iliac vessels. This vessel provides vascular supply to the left third of the transverse colon, descending colon, sigmoid colon, and rectum. In patients without significant vascular disease, this artery can be sacrificed without ill effect because there is collateral circulation to these bowel segments from the superior mesenteric, middle hemorrhoidal, and inferior hemorrhoidal arteries.

In addition to the listed arteries that exit the aorta from its anterior or lateral aspect, there are a number of small branches from the posterior side of the aorta. Lumbar arterial branches are found at regular intervals along the length of the aorta, with generally four pairs located within the retroperitoneum. These branches supply the posterior body wall and spine. Again these arteries can generally be ligated without detrimental effects, although spinal ischemia and paralysis has occurred after ligation at multiple levels. The final posterior branch from the aorta is the middle sacral artery, which exits the aorta immediately before the branching of the common iliac arteries and then sends branches to the rectum and anterior sacrum. The common iliac arteries then proceed into the pelvis, thus completing the arterial course through the retroperitoneum.

Inferior Vena Cava

The inferior vena cava (IVC) arises from the confluence of the common iliac veins at the level of the fifth lumbar vertebra (see Fig. 1–10). Because the common iliac veins lie medial and posterior to the iliac arteries, the confluence of the iliac veins is posterior and to the right of the aortic bifurcation. As the IVC progresses cranially through the abdomen, tributaries include the gonadal, renal, adrenal, and hepatic veins. In addition, the middle sacral vein enters the inferior vena cava posteriorly and the lumbar veins enter throughout the length of the abdominal vena cava.

The first tributary encountered along the IVC is the middle sacral vein, which enters at the junction of the common iliac veins. Also entering along the posterior aspect of the IVC throughout its course are lumbar veins. These veins course anterior to the spinal transverse processes and generally parallel the lumber arteries. In addition to providing vascular drainage, the lumbar veins connect the IVC to the azygous venous system on the right side and hemiazygos venous system on the left side of the thorax. This provides alternate routes of venous drainage within the retroperitoneum (Fig. 1–12).

Figure 1–12 Lumbar, azygos, and hemiazygos veins.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 332.)

The next tributaries to the IVC are the gonadal veins, whose course is analogous to the gonadal arteries until approaching the IVC. During the cranial portion of their course these veins are more lateral and closer to the ipsilateral ureter. Of surgical importance is their terminal drainage because the right gonadal vein drains directly into the IVC and the left empties into the inferior aspect of the left renal vein (see Fig. 1–10).

After the gonadal veins, the renal veins are encountered. The renal veins are generally directly anterior to the accompanying renal artery, but it is not unusual for them to be separated by 1 to 2 cm in the craniocaudal direction. The right renal vein typically is short and without branches, but in a small minority of patients the right gonadal vein can enter the right renal vein as opposed to the IVC. In a second anatomic variation, a lumbar vein will enter on the posterior aspect of the right renal vein as opposed to entering the IVC directly. The left renal vein is significantly longer than the right and receives additional branches before entering the IVC. Typically after exiting the renal hilum, the left renal vein receives a lumbar vein posteriorly, the left gonadal vein inferiorly, and the adrenal vein superiorly. Next, the left renal vein crosses anterior to the aorta and under the caudal edge of the superior mesenteric artery before draining into the IVC. Rarely the left renal vein crosses the aorta to the IVC in a retroaortic or circumaortic path.

Proceeding cranially, the posterior aspect of the IVC receives the right adrenal vein. This short vein is located posteriorly on the IVC, making it challenging to expose during right adrenal or renal surgery. As noted already, the left adrenal vein drains into the left renal vein as opposed to the IVC. The inferior phrenic vein on the right side enters along the posterior or posterior lateral aspect of the IVC, with the left inferior phrenic vein typically entering the left renal vein. The final tributaries to the IVC before it leaves the retroperitoneum are the short hepatic veins draining the liver. Inferiorly these veins are small, but superiorly three large hepatic trunks are encountered.

Autonomic System

The autonomic system is further divided into sympathetic and parasympathetic fibers. The origin of these two nerve types is quite different, with the sympathetic preganglionic fibers originating from the thoracic and lumbar portions of the spinal column and the parasympathetic preganglionic fibers beginning in the cranial and sacral spinal column segments. Preganglionic sympathetic fibers enter the retroperitoneum through both the paired sympathetic chains and input from the lumbar spinal nerves (Fig. 1–14). The lumbar portion of this sympathetic chain then sends preganglionic fibers to autonomic plexuses associated with the major branches of the abdominal aorta. Within these aortic plexuses the preganglionic fibers synapse and postganglionic fibers are then distributed to the various abdominal viscera and organs. Parasympathetic input from the vagus nerve also supplies these ganglia.

Figure 1–14 Sympathetic chain and splanchnic nerves.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 309.)

In more detail, the thoracic and lumbar portions of the sympathetic chain originate from preganglionic sympathetic fibers arising from the first thoracic through the third lumbar spinal nerves (see Fig. 1–14). This chain then courses vertically along the anterolateral aspect of the spine just medial to the psoas muscle. Within the retroperitoneum, lumbar arteries and veins are closely associated with the lumbar sympathetic chain, in some instances even splitting the fibers as they cross the chain perpendicularly. From this sympathetic chain preganglionic fibers follow one of three courses. First, preganglionic fibers are sent to the various autonomic plexuses (splanchnic nerves). Once in the plexus, the preganglionic fibers synapse within a ganglion to postganglionic fibers, which in turn proceed to the abdominal viscera. Second, preganglionic fibers can synapse within the sympathetic chain ganglia and send postganglionic fibers to the body wall and lower extremities. Finally, preganglionic sympathetic fibers can proceed directly to the adrenal gland without synapsing. Within the adrenal medulla, these preganglionic fibers control release of catecholamines.

The major autonomic nerve plexuses are associated with the primary branches of the aorta. These plexuses include the celiac, superior hypogastric, and inferior hypogastric plexuses (Fig. 1–15). These plexuses receive sympathetic input from the sympathetic chains via the greater, lesser, and least thoracic splanchnic nerves originating from the 5th through 12th thoracic spinal nerves. They also receive input from the lumbar portion of the sympathetic chain via the lumbar splanchnic nerves, as well as parasympathetic input via the vagus nerve.

Figure 1–15 Autonomic plexuses associated with branches of the aorta.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 337.)

The largest is the celiac plexus and is located on either side of the celiac arterial trunk as a paired structure. It is through this plexus that much or all of the autonomic input to the kidney, adrenal, renal pelvis, and ureter passes. In addition, some of the sympathetic innervation to the testes passes through this ganglion before continuing in parallel with the testicular artery to the testis. A separate aorticorenal ganglion usually exists as an inferior extension of the celiac ganglion, forming part of the renal autonomic plexus. The latter plexus surrounds the renal artery and its branches and is contiguous with the celiac plexus. At the lower extent of the abdominal aorta, much of the autonomic input to the pelvic urinary organs and genital tract travels through the superior hypogastric plexus. This plexus lies on the aorta anterior to its bifurcation and extends inferiorly on the anterior surface of the fifth lumbar vertebra. This plexus is contiguous bilaterally with inferior hypogastric plexuses, which extend into the pelvis. Disruption of the sympathetic nerve fibers that travel through these plexuses during retroperitoneal dissection can cause loss of seminal vesicle emission and/or failure of bladder neck closure, resulting in retrograde ejaculation.

Somatic

The somatic sensory and motor innervation to the abdomen and lower extremities arises in the retroperitoneum and is called the lumbosacral plexus. The lumbosacral plexus is formed from branches of all lumbar and sacral spinal nerves, with some contribution from the 12th thoracic spinal nerve as well (Fig. 1–16). Superiorly, nerves of this plexus form within the body of the psoas muscle and pierce this muscle, with more inferior branches passing medial to the psoas as the pelvis is entered (Fig. 1–17). The origins and functions of these lumbosacral somatic nerves are summarized in Table 1–3.

Figure 1–16 Diagrammatic representation of the lumbosacral nervous plexus.

Figure 1–17 Lumbar plexus in the posterior abdominal region.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 341.)

Table 1–3 Branches of the Lumbosacral Plexus

The subcostal nerve is the anterior extension of the 12th thoracic nerve and extends laterally beneath the 12th rib. As one proceeds inferiorly, the iliohypogastric nerve and the ilioinguinal nerve originate together as an extension from the first lumbar spinal nerve. These three nerves cross the anterior or inner surface of the quadratus lumborum muscle before piercing the transversus abdominis muscle and continuing their course between this and the internal oblique muscle. Together they provide multiple motor branches to the muscles of the abdominal wall, as well as sensory innervation to the skin of the lower abdomen and genitalia. The lateral femoral cutaneous nerve and the genitofemoral nerve arise from the 1st through 3rd lumbar nerves and are primarily sensory nerves to the skin of the upper thigh and genitalia; however, the genital branch of the genitofemoral nerve also supplies the cremaster and dartos muscles in the scrotum. The genitofemoral nerve lies directly atop and parallels the psoas muscle throughout most of its retroperitoneal course and is easily identified in this position.

The femoral nerve is a larger structure arising from the second through fourth lumbar spinal nerves and is largely hidden by the body of the psoas muscle before exiting the abdomen just lateral to the femoral artery. This important nervous structure supplies the psoas and iliacus muscles, as well as the large muscle groups of the anterior thigh. It also provides sensory innervation of the anteromedial portions of the lower extremity. Intraoperatively, it may be compressed by retractor blades placed inferolaterally against the inguinal ligament in lower abdominal incisions, producing a significant motor palsy that prevents active extension of the knee.

The final lumbosacral plexus nerves include the obturator and sciatic nerves. The obturator nerve, an important pelvic landmark, arises behind the psoas muscle in the retroperitoneum from the third and fourth lumbar spinal nerves. It then courses inferiorly, where its major function is to supply the adductor muscles of the thigh. The sciatic nerve receives input from the fourth lumbar through third sacral spinal nerves, taking final form in the deep posterior pelvis as the body’s single largest nerve, supplying the bulk of both sensory and motor innervation to the lower extremity.

Duodenum, Pancreas, Colon

See Figure 1–18 on the Expert Consult website. The duodenum is divided into four anatomic components. The first (ascending) portion is short (5 cm) and intimately related to the gallbladder. The second (descending portion) is of most importance to the urologist because it lies immediately anterior to the right renal hilum and pelvis. This portion of the duodenum is frequently mobilized (referred to as a Kocher maneuver) to expose the right kidney, right renal pelvis, and additional upper abdominal structures. The second portion of the duodenum also receives the common bile duct and surrounds the head of the pancreas. The third (horizontal) and fourth (ascending) portions of the duodenum cross from right to left over the IVC and aorta before transitioning into the jejunum.

Figure 1–18 Colon, duodenum, and pancreas within the retroperitoneum.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 274.)

As noted earlier, the head of the pancreas is on the medial border of the descending duodenum. The body and tail of the pancreas continue across the IVC and aorta to the left side of the abdomen, where the pancreas is closely related to the left adrenal gland and the upper pole of the left kidney. The splenic artery and vein travel laterally along the posterior aspect of the pancreas, with the artery just superior to the vein. In this position these vascular structures are also closely related to the upper pole of the left kidney.

The final retroperitoneal gastrointestinal structure is the colon, with the ascending and descending portions being retroperitoneal. Both the ascending colon at the hepatic flexure and descending colon at the splenic flexure overlie the ipsilateral kidney. In addition, the hepatocolic ligament and splenocolic ligament tether the liver and spleen to the respective portions of the colon. Given the close anatomic relationship to the kidneys, mobilization of the colon and its mesentery is important for transperitoneal exposure of the kidneys and ureters.

Anatomic Relationships

The position of the kidney within the retroperitoneum varies greatly by side, degree of inspiration, body position, and presence of anatomic anomalies (Fig. 1–24 on the Expert Consult website). The right kidney sits 1 to 2 cm lower than the left in most individuals owing to displacement by the liver. Generally, the right kidney resides in the space between the top of the first lumbar vertebra to the bottom of the third lumbar vertebra. The left kidney occupies a more superior space from the body of the 12th thoracic vertebral body to the 3rd lumbar vertebra.

Figure 1–24 Variation among individuals in level of the kidneys relative to the spinal column. Lighter lateral lines represent upper poles of kidneys; darker lateral lines represent lower poles.

(From Anson BJ, Daseler EH. Common variations in renal anatomy, affecting blood supply, form, and topography. Surg Gynecol Obstet 1961;112:439–49.)

Of surgical importance are the structures surrounding the kidney (see Figs. 1-9 and 1-25). Both kidneys have similar muscular surroundings. Posteriorly, the diaphragm covers the upper third of each kidney, with the 12th rib crossing at the lower extent of the diaphragm. Also important to note for percutaneous renal procedures and flank incisions is that the pleura extends to the level of the 12th rib posteriorly. Medially the lower two thirds of the kidney lie against the psoas muscle, and laterally the quadratus lumborum and aponeurosis of the transversus abdominis muscle are encountered. The effect of the muscular relations on the kidneys is severalfold (Fig. 1–26). First, the lower pole of the kidney lies laterally and anteriorly relative to the upper pole. Second, the medial aspect of each kidney is rotated anteriorly at an angle of approximately 30 degrees. An understanding of this renal orientation is again of particular interest for percutaneous renal procedures in which kidney orientation influences access site selection.

Figure 1–25 Structures related to the anterior surfaces of each kidney.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 321.)

Figure 1–26 Normal rotational axes of the kidney. A, Transverse view showing approximate 30-degree anterior rotation of the left kidney from the coronal plane, relative positions of the anterior and posterior rows of calyces, and location of the relatively avascular plane separating the anterior and posterior renal circulation. B, Coronal section demonstrating slight inward tilt of the upper poles of the kidneys. C, Sagittal view showing anterior displacement of the lower pole of the right kidney.

Anteriorly, the right kidney is bordered by a number of structures (see Fig. 1–25). Cranially, the upper pole lies against the liver and is separated from the liver by the peritoneum except for the liver’s posterior bare spot. The hepatorenal ligament further attaches the right kidney to the liver because this extension of parietal peritoneum bridges the upper pole of the right kidney to the posterior liver. Also at the upper pole, the right adrenal gland is encountered. On the medial aspect, the descending duodenum is intimately related to the medial aspect of the kidney and hilar structures. Finally, on the anterior aspect of the lower pole lies the hepatic flexure of the colon.

The left kidney is bordered superiorly by the tail of the pancreas with the splenic vessels adjacent to the hilum and upper pole of the left kidney. Also cranial to the upper pole is the left adrenal gland and further superolaterally, the spleen. The splenorenal ligament attaches the left kidney to the spleen. This attachment can lead to splenic capsular tears if excessive downward pressure is applied to the left kidney. Superior to the pancreatic tail, the posterior gastric wall can overlie the kidney. Caudally, the kidney is covered by the splenic flexure of the colon.

Gerota Fascia

Interposed between the kidney and its surrounding structures is the perirenal or Gerota fascia (Figs. 1-27 through 1-29). This fascial layer encompasses the perirenal fat and kidney and encloses the kidney on three sides: superiorly, medially, and laterally. Superiorly and laterally Gerota fascia is closed, but medially it extends across the midline to fuse with the contralateral side. Inferiorly, Gerota fascia is not closed and remains an open potential space. Gerota fascia serves as an anatomic barrier to the spread of malignancy and a means of containing perinephric fluid collections. Thus perinephric fluid collections can track inferiorly into the pelvis without violating Gerota fascia.

Figure 1–27 Organization of the fat and fascia surrounding the kidney.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 322.)

Figure 1–28 Anterior view of Gerota fascia on the right side, split over the right kidney (which it contains), and showing inferior extension enveloping the ureter and gonadal vessels. The ascending colon and overlying peritoneum have been reflected medially.

(From Tobin CE. The renal fascia and its relation to the transversalis fascia. Anat Rec 1944;89:295–311.)

Figure 1–29 Posterior view of Gerota fascia on the right side, rotated medially with the contained kidney, ureter, and gonadal vessels, exposing the muscular posterior body wall covered by the transversalis fascia.

(From Tobin CE. The renal fascia and its relation to the transversalis fascia. Anat Rec 1944;89:295–311.)

Renal Artery

Specifically, the right renal artery leaves the aorta and progresses with a caudal slope under the IVC toward the right kidney. The left renal artery courses almost directly laterally to the left kidney. Given the rotational axis of the kidney (see Fig. 1–26), both renal arteries move posteriorly as they enter the kidney. Also, both arteries have branches to the respective adrenal gland, renal pelvis, and ureter.

Upon approaching the kidney, the renal artery splits into four or more branches, with five being the most common. These are the renal segmental arteries (Fig. 1–30). Each segmental artery supplies a distinct portion of the kidney with no collateral circulation between them (Fig. 1–31). Thus occlusion or injury to a segmental branch will cause segmental renal infarction. Generally, the first and most constant branch is the posterior segmental branch, which separates from the renal artery before it enters the renal hilum. Typically there are four anterior branches, which from superior to inferior are apical, upper, middle, and lower. The relationship of these segmental arteries is important because the posterior segmental branch passes posterior to the renal pelvis while the others pass anterior to the renal pelvis. Ureteropelvic junction obstruction caused by a crossing vessel can occur when the posterior segmental branch passes anterior to the ureter causing occlusion. This division between the posterior and anterior segmental arteries has an additional surgical importance in that between these circulations is an avascular plane (see Figs. 1-26 and 1-31). This longitudinal plane lies just posterior to the lateral aspect of the kidney. Incision within this plane results in significantly less blood loss than outside this plane. However, there is significant variation in the location of this plane, requiring delineation before incision. This can be done with either preoperative angiography or intraoperative segmental arterial injection of a dye such as methylene blue.

Figure 1–30 A and B, Segmental branches of the right renal artery demonstrated by renal angiogram.

Figure 1–31 Typical segmental circulation of the right kidney, shown diagrammatically. Note that the posterior segmental artery is usually the first branch of the main renal artery and it extends behind the renal pelvis.

Once in the renal sinus, the segmental arteries branch into lobar arteries, which further subdivide in the renal parenchyma to form interlobar arteries (Fig. 1–32). These interlobar arteries progress peripherally within the cortical columns of Bertin, thus avoiding the renal pyramids but maintaining a close association with the minor calyceal infundibula. At the base (peripheral edge) of the renal pyramids, the interlobar arteries branch into arcuate arteries. Instead of moving peripherally, the arcuate arteries parallel the edge of the corticomedullary junction. Interlobular arteries branch off the arcuate arteries and move radially, where they eventually divide to form the afferent arteries to the glomeruli.

Figure 1–32 Intrarenal arterial anatomy.

The 2 million glomeruli within each kidney represent the core of the renal filtration process. Each glomerulus is fed by an afferent arteriole. As blood flows through the glomerular capillaries, the urinary filtrate leaves the arterial system and is collected in the glomerular (Bowman) capsule. Blood flow leaves the glomerular capillary via the efferent arteriole and continues to one of two locations: secondary capillary networks around the urinary tubules in the cortex or descending into the renal medulla as the vasa recta.

Renal Veins

The renal venous drainage correlates closely with the arterial supply. The interlobular veins drain the postglomerular capillaries. These veins also communicate freely via a subcapsular venous plexus of stellate veins with veins in the perinephric fat. After the interlobular veins, the venous drainage progresses through the arcuate, interlobar, lobar, and segmental branches, with the course of each of these branches paralleling the respective artery. After the segmental branches, the venous drainage coalesces into three to five venous trunks that eventually combine to form the renal vein. Unlike the arterial supply, the venous drainage communicates freely through venous collars around the infundibula, providing for extensive collateral circulation in the venous drainage of the kidney (Fig. 1–33). Surgically, this is important because unlike the arterial supply, occlusion of a segmental venous branch has little effect on venous outflow.

Figure 1–33 Venous drainage of the left kidney showing potentially extensive venous collateral circulation.

The renal vein is located directly anterior to the renal artery, although this position can vary up to 1 to 2 cm cranially or caudally relative to the artery. The right renal vein is generally 2 to 4 cm in length and enters the right lateral to posterolateral edge of the IVC. The left renal vein is typically 6 to 10 cm in length and enters the left lateral aspect of the IVC after passing posterior to the superior mesenteric artery and anterior to the aorta (Fig. 1–34 on the Expert Consult website). Compared with the right renal vein, the left renal vein enters the IVC at a slightly more cranial level and a more anterolateral location. Additionally, the left renal vein receives the left adrenal vein superiorly, lumbar vein posteriorly, and left gonadal vein inferiorly (see Fig. 1–33). The right renal vein typically does not receive any branches.

Figure 1–34 Renal vasculature. Note path of left renal vein under the superior mesenteric artery.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 324.)

Common Anatomic Variants

Anatomic variations in the renal vasculature are common, occurring in 25% to 40% of kidneys. The most common variation is supernumerary renal arteries, with up to five arteries reported. This occurs more often on the left. These additional arteries can enter through the hilum or directly into the parenchyma. Lower pole arteries on the right tend to cross anterior to the IVC, whereas lower pole arteries on either side can cross anterior to the collecting system, causing a ureteropelvic junction obstruction. When the kidney is ectopic, supernumerary arteries are even more common and their origin even more varied, with the celiac trunk, superior mesenteric artery, or iliac arteries all possible sources of ectopic renal arteries. Supernumerary veins occur as well, but this is a less common entity. The most common example is duplicate renal veins draining the right kidney via the right renal hilum. Polar veins are quite rare. Finally, the left renal vein may course behind the aorta or divide and send one limb anterior and one limb posterior to the aorta, resulting in a collar-type circumaortic formation.

Microscopic Anatomy from Glomerulus to Collecting System

Microscopically, the renal collecting system originates in the renal cortex at the glomerulus as filtrate enters into Bowman capsule (Fig. 1–37 on the Expert Consult website). Together the glomerular capillary network and Bowman capsule form the renal corpuscle (malpighian corpuscle) (Fig. 1–38). The glomerular capillary network is covered by specialized epithelial cells called podocytes that, along with the capillary epithelium, form a selective barrier across which the urinary filtrate must pass. The filtrate is initially collected in Bowman capsule and then moves to the proximal convoluted tubule. The proximal tubule is composed of a thick cuboidal epithelium covered by dense microvilli. These microvilli greatly increase the surface area of the proximal tubule, allowing a large portion of the urinary filtrate to be reabsorbed in this section of the nephron.

Figure 1–38 Renal nephron and collecting tubule.

(From Netter FH. Atlas of human anatomy. 4th ed. Philadelphia: Elsevier; 2006. p. 336.)

Figure 1–37 Electron micrograph of the renal corpuscle. The glomerular capillary network is enveloped by podocytes and contained within Bowman capsule. CS, capsular space of Bowman; PL, parietal layer of Bowman capsule; Po, podocyte. Arrows indicate cytoplasmic extensions from podocytes that enwrap the glomerular capillaries.

(From Kessel RG, Kardon RH. Tissues and organs: a text-atlas of scanning electron microscopy.

The proximal tubule continues deeper into the cortical tissue where it becomes the loop of Henle. The loop of Henle extends variable distances into the renal medulla. Within the renal medulla, the loop of Henle reverses course and moves back toward the periphery of the kidney. As it ascends out of the medulla the loop thickens and becomes the distal convoluted tubule. This tubule eventually returns to a position adjacent to the originating glomerulus and proximal convoluted tubule. Here the distal convoluted tubule turns once again for the interior of the kidney and becomes a collecting tubule. Collecting tubules from multiple nephrons combine into a collecting duct that extends inward through the renal medulla and eventually empties into the apex of the medullary pyramid, the renal papilla.