# Retroperitoneal Anatomy and the Urinary Tract ## Overview This lecture covers the detailed surgical anatomy of the kidney, ureter, bladder, and urethra, with particular emphasis on fascial planes, vascular relationships, and common anatomic variants that are frequently encountered during surgery. Understanding these relationships is crucial for safe and effective operative management. --- ## Retroperitoneal Space ### Boundaries The retroperitoneal space is anatomically bounded anteriorly by the posterior parietal peritoneum. Posteriorly, it is limited by the transversalis fascia and the muscles of the posterior abdominal wall, including the psoas major, quadratus lumborum, and erector spinae. Superiorly, the diaphragm forms the boundary, while inferiorly, the retroperitoneal space continues seamlessly into the pelvic extraperitoneal space. ### Fascial Layers Within the retroperitoneum, several fascial layers compartmentalize the space. The Gerota fascia, also known as the renal fascia, encloses the kidney and the perirenal fat and fuses laterally to form the lateroconal fascia. The anterior renal fascia, or Zuckerkandl fascia, covers the anterior aspect of the kidney, while the posterior renal fascia, also part of Zuckerkandl fascia, fuses with the fascia of the psoas and quadratus lumborum muscles. The perirenal fat lies between the renal capsule and Gerota fascia and is the typical site for perinephric hematomas. Outside the posterior renal fascia lies the pararenal fat, which occupies the posterior pararenal space, while anterior to the anterior renal fascia is the anterior pararenal space. ### Retroperitoneal Compartments The retroperitoneal space is divided into three compartments. The anterior pararenal space contains the pancreas, the second through fourth parts of the duodenum, and the ascending and descending colon. The perirenal space houses the kidney, adrenal gland, renal vessels, proximal ureter, and perirenal fat. The posterior pararenal space contains only fat and no organs. --- ## Kidney ### Gross Anatomy The kidneys are located between the T12 and L3 vertebral levels, with the right kidney positioned slightly lower than the left due to the presence of the liver. In adults, the average kidney measures approximately 11 cm in length, 6 cm in width, and 3 cm in thickness, weighing around 150 grams. The medial hilum of the kidney contains, from anterior to posterior, the renal vein, renal artery, and renal pelvis, a relationship often remembered by the mnemonic VAP. ### Renal Vasculature The renal artery arises from the abdominal aorta at the level of L1-L2. The right renal artery passes posterior to the inferior vena cava (IVC) before dividing into anterior and posterior divisions. The anterior division further divides into four segmental arteries supplying the apical, upper, middle, and lower segments of the kidney, while the posterior division gives rise to a single segmental artery. Accessory renal arteries are present in 25-30% of individuals; these are end arteries, meaning their ligation results in segmental infarction. The renal veins differ between sides: the left renal vein is longer, measuring 6-10 cm, and crosses anterior to the aorta and posterior to the superior mesenteric artery (SMA). It receives tributaries from the left gonadal vein, left adrenal vein, and lumbar veins. The right renal vein is shorter, about 2-4 cm, and typically receives no major tributaries. Importantly, the left renal vein can be ligated near the IVC because collateral drainage occurs via the gonadal, adrenal, and lumbar veins, whereas the right renal vein cannot be safely ligated due to lack of significant collaterals. ### Renal Relations The right kidney is related anteriorly to the liver, occupying the hepatorenal recess (Morison pouch), the second part of the duodenum, the hepatic flexure of the colon, and the right adrenal gland. The left kidney lies adjacent to the spleen, connected by the splenorenal ligament, as well as the stomach, tail of the pancreas, splenic flexure of the colon, and left adrenal gland. ### Renal Collecting System The renal collecting system consists of 8 to 12 minor calyces that drain into 2 to 3 major calyces (upper, middle, and lower). These major calyces converge to form the renal pelvis. The pelvis may be intrarenal or extrarenal, a distinction that has surgical significance, particularly for percutaneous renal access. --- ## Ureter ### Course and Anatomy The ureter is a muscular tube approximately 25-30 cm in length and 2-10 mm in diameter. It courses along the psoas muscle and is crossed anteriorly by the gonadal vessels. ### Three Points of Narrowing There are three physiologic constrictions along the ureter where its diameter narrows: the ureteropelvic junction (UPJ), the point where it crosses the iliac vessels at the common iliac artery bifurcation, and the ureterovesical junction (UVJ), which is the narrowest point. ### Blood Supply The blood supply to the ureter varies along its length. The upper ureter receives branches from the renal artery, the middle ureter is supplied by branches from the gonadal artery, aorta, and common iliac artery, and the lower ureter is supplied by the superior vesical artery, uterine artery in females, and branches of the internal iliac artery. These vessels run within the adventitia, so preservation of the periureteral tissue during mobilization is essential to maintain blood flow. Surgically, it is important to note that the blood supply is medial above the iliac vessels and lateral below them. ### Ureteral Relations The ureter crosses anterior to the psoas muscle and the genitofemoral nerve and passes posterior to the gonadal vessels. On the right side, the ureter lies posterior to the duodenum and lateral to the IVC, while on the left, it is lateral to the aorta. Within the pelvis, the ureter crosses anterior to the internal iliac artery bifurcation. In females, the ureter passes beneath the uterine artery, a relationship often described as "water under the bridge," which places the ureter at risk during hysterectomy. ### Histology The ureter is lined by transitional epithelium, or urothelium, supported by a lamina propria. Its muscular wall consists of three layers of smooth muscle: an inner longitudinal, a middle circular, and an outer longitudinal layer. The ureter is surrounded by adventitia and lacks a serosal covering. --- ## Bladder ### Anatomy The bladder is situated in the extraperitoneal space of the pelvis, posterior to the pubic symphysis. It has a capacity of approximately 400-600 mL. The trigone is a triangular area defined by the two ureteral orifices and the internal urethral meatus. The detrusor muscle of the bladder wall is composed of three layers of smooth muscle—inner longitudinal, middle circular, and outer longitudinal—that function together as a syncytium to facilitate bladder contraction. ### Bladder Relations Anteriorly, the bladder is related to the space of Retzius (retropubic space) and the pubic symphysis. Superiorly, it is covered by peritoneum and lies adjacent to the small bowel. Posteriorly, the relationships differ by sex: in males, the bladder is related to the seminal vesicles, vas deferens, and rectum, separated from the rectum by Denonvilliers fascia; in females, it is related to the vagina and uterus. Inferiorly, the bladder is adjacent to the prostate in males and the pelvic floor in females. ### Blood Supply Arterial supply to the bladder arises from the superior, middle, and inferior vesical arteries, all branches of the internal iliac artery. Venous drainage occurs via the vesical venous plexus into the internal iliac veins. Lymphatic drainage follows to the external iliac, internal iliac, and obturator lymph nodes. ### Anti-reflux Mechanism The ureters enter the bladder wall obliquely through the detrusor hiatus, creating a submucosal tunnel with a length-to-diameter ratio of approximately 5:1. This configuration forms a flap-valve mechanism that prevents vesicoureteral reflux. The Waldeyer sheath anchors the ureter to the detrusor muscle, supporting this anti-reflux function. --- ## Urethra ### Male Urethra (18-22 cm) The male urethra is divided into four segments. The prostatic urethra, measuring 3-4 cm, is the widest and most distensible segment and contains important landmarks such as the verumontanum, prostatic utricle, and ejaculatory duct orifices. The membranous urethra, 1-2 cm in length, passes through the urogenital diaphragm and is surrounded by the external urethral sphincter (rhabdosphincter); this segment is most vulnerable to injury during pelvic fractures. The bulbar urethra, 3-4 cm long, lies within the corpus spongiosum of the bulb and is covered by the bulbospongiosus muscle. Finally, the penile (or pendulous) urethra extends about 15 cm within the corpus spongiosum and terminates at the fossa navicularis and external meatus. ### Female Urethra (3-5 cm) The female urethra runs from the bladder neck to the external meatus, positioned anterior to the vagina. It is surrounded by the rhabdosphincter, the external striated sphincter muscle. Its blood supply arises from the internal pudendal and vaginal arteries. --- ## Common Anatomic Variants Several anatomic variants of the urinary tract are clinically important. Horseshoe kidney results from fusion of the lower poles of the kidneys, with an isthmus crossing anterior to the aorta at the L4 level; this variant is associated with ureteropelvic junction obstruction. A pelvic kidney results from failure of ascent during development and typically receives blood supply from the iliac vessels. Crossed fused ectopia occurs when both kidneys are located on the same side and are usually fused. Duplex collecting systems may be complete or incomplete, involving ureteral duplication. The Weigert-Meyer rule describes the typical insertion pattern in duplex systems: the ureter from the upper pole moiety inserts inferomedially and is often ectopic and obstructed, while the ureter from the lower pole moiety inserts superolaterally and is prone to reflux. Ureteral variants include the retrocaval ureter, where the right ureter passes posterior to the IVC due to abnormal persistence of the right subcardinal vein. Ectopic ureters insert distal to the external sphincter in females, causing incontinence, but are always located above the sphincter in males. Vascular variants include accessory renal arteries, present in 25-30% of individuals, the circumaortic left renal vein found in 1-17%, and the retroaortic left renal vein seen in 2-3%. --- A detailed anatomical cross-section of the retroperitoneal space at the level of L2, showing the kidney within Gerota fascia, the perirenal and pararenal fat compartments, the anterior and posterior renal fascia layers, the lateroconal fascia, and the relationship to the psoas muscle, quadratus lumborum, and posterior peritoneum. The renal artery and vein are labeled at the hilum. Color-coded fascial layers with clear anatomic labels in a medical illustration style. An anterior view of the urinary tract showing both kidneys, ureters, and bladder with the three points of ureteral narrowing highlighted (UPJ, iliac vessel crossing, and UVJ). The blood supply to each segment of the ureter is shown with color-coded arteries (renal artery branches superiorly, gonadal and aortic branches in the mid-ureter, and vesical artery branches inferiorly). Key anatomic landmarks including the psoas muscle, gonadal vessels, and common iliac bifurcation are labeled. A detailed sagittal cross-section of the male pelvis showing the bladder, prostate, and entire length of the male urethra divided into its four segments (prostatic, membranous, bulbar, and penile). The external urethral sphincter surrounding the membranous urethra is highlighted. The verumontanum and ejaculatory ducts within the prostatic urethra are labeled. The space of Retzius and Denonvilliers fascia are clearly identified. --- ## Clinical Pearls The left renal vein can be safely ligated near the IVC because collateral drainage occurs through the gonadal, lumbar, and adrenal veins; in contrast, the right renal vein lacks significant collateral pathways and cannot be safely ligated. The Brodel bloodless line, located along the posterolateral border of the kidney between the anterior and posterior arterial divisions, represents the optimal site for nephrotomy during anatrophic nephrolithotomy. When mobilizing the ureter surgically, it is critical to preserve the periureteral adventitial tissue to maintain its blood supply; mobilization should be performed medially above the iliac vessels and laterally below them. The "water under the bridge" relationship, where the ureter passes beneath the uterine artery, places the ureter at risk during hysterectomy. The membranous urethra is the segment most vulnerable to injury in pelvic fractures due to its fixation at the urogenital diaphragm. Finally, the Weigert-Meyer rule is essential for understanding the pathology of duplex collecting systems: the upper pole moiety typically obstructs, while the lower pole moiety is prone to reflux. --- ## References - Campbell-Walsh-Wein Urology, 12th Edition — Chapters 1-3 (Surgical Anatomy) - Hinman's Atlas of Urologic Surgery, 4th Edition - Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd Edition - AUA Core Curriculum: Anatomy of the Urinary Tract