The retroperitoneum is the area that lies between the posterior abdominal wall and the peritoneal cavity. Its superior boundary is the diaphragm; the inferior boundary is the pelvic floor. Anteriorly is the peritoneum, posteriorly the vertebral column and transversus abdominus muscles. The retroperitoneum contains the kidneys, adrenals, ureters, bladder, and prostate gland. The aorta and the vena cava with their branches lie on the vertebrae, as do the lymphatic channels, nodes, and the cisterna chyli (Fig. 1) 1515. The lumbar nerve plexus lies within the psoas muscle and the sacral plexus lies on the pyriformis muscle. The sympathetic nerves arise from the coeliac plexus and sympathetic chain on the bodies of the lumbar vertebrae while the parasympathetic nerves arise from the vagus nerve and the sacral outflow.

The adrenals are paired endocrine glands found at the superomedial aspect of both kidneys. The right gland is triangular, and the left is crescentic. The anatomical relationships og the glands are the same as for the upper poles of their respective kidneys. Each adrenal consists of a cortex, which secretes glucosteroids, mineralosteroids, and sex steroids, and a medulla, which secretes adrenaline and noradrenaline. The adrenals receive an arterial supply from the aorta, and from the renal and inferior phrenic arteries. The left adrenal vein drains to the ipsilateral renal vein. The right adrenal has very short veins that go directly to the vena cava. Sympathetic fibres reach the adrenal medulla from the coeliac plexus.

The kidneys lie on the psoas and quadratus lumborum muscles. The left kidney (hilum opposite L1) is slightly higher than the right (hilum opposite L1–2), which is displaced inferiorly by the liver. Each kidney, and its adrenal gland, is surrounded by a layer of fat encircled by Gerota's fascia. Anterior to the right kidney is the bare area of the liver, the duodenum, and the ascending colon. Anterior to the left kidney lies the spleen, the posterior surface of the stomach, the pancreas, the descending colon, and the jejunum.

The renal artery is the first major lateral branch of the abdominal aorta. On the right this artery is approximately 3 cm long; it passes behind the inferior vena cava before branching in the renal hilum. The left renal artery passes directly from the aorta to the hilum. Thirty per cent of individuals have more than one renal artery, the most common variant being an aberrant upper pole vessel. The renal veins lie anterior to the arteries. The left renal vein receives the adrenal vein, the gonadal vein, and one lumbar vein prior to crossing the aorta and draining into the inferior vena cava. The right renal vein is shorter and usually has no tributaries. The lymphatics drain to aortocaval nodes. Sympathetic and parasympathetic nerves pass from the coeliac plexus to both kidneys and probably have a role in regulating renal blood flow.

The kidney consists of a cortex (site of the glomeruli) and a medulla (site of the collecting ducts). The collecting ducts drain via papillae into minor calices, which combine in groups of two or three to form one of six to seven major calices, which together make up the renal pelvis, the most posterior structure of the hilum (Fig. 1) 1515. Common congenital abnormalities of the kidney include unilateral agenesis, ectopia, horseshoe kidney, and a fused pelvic kidney (Fig. 2) 1516.

The ureter is a narrow muscular tube that transmits urine from the renal pelvis to the bladder, a distance of about 25 cm. It runs through the retroperitoneum along the psoas muscle, at the tips of the lumbar transverse processes (an important radiological landmark). It crosses the pelvic brim at the bifurcation of the common iliac artery and lies on the levator ani muscle before swinging medially into the base of the bladder at the level of the ischial spine. Anteriorly, the ureter is adherent to the peritoneum for much of its length and it can easily be injured during pelvic and abdominal surgery.

The ureter receives blood from the aorta, and from the renal, gonadal, common iliac, vesical, and uterine arteries. The lymphatics drain with each of these vessels. The somatic sensory nerves innervate the ureter in a segmental fashion which explains why ureteric obstruction can give rise to referred ipsilateral testicular pain.

There are three anatomical points of narrowing within the ureter at the ureteropelvic junction, the site where it crosses the iliac vessels, and at the ureterovesical junction. Renal stones commonly impact at these sites.

A common congenital abnormality is duplication of the ureter, which may be complete or partial. When duplication is partial, the ureter from the upper pole moiety joins the lower pole ureter at any point from the renal pelvis to the bladder. When duplication is complete, the ureters cross, with the lower pole ureter entering the bladder more laterally and having a tendency to reflux urine up to the kidney (Fig. 2) 1516.

In men, the bladder sits on top of the prostate gland, in front of the rectum, behind the lower abdominal wall and pubic symphysis. In women the bladder lies on the perineal membrane and anterior to the uterus. The bladder is covered superiorly by loosely adherent peritoneum that allows it to expand upwards on filling. The ureters enter the bladder base posterolaterally and, together with the urethra, form a triangular area, the trigone. The bladder wall contains the smooth muscle known as the detrusor (Fig. 3) 1517.

The blood supply of the bladder is derived from the internal iliac artery via its superior and inferior vesical branches. The venous drainage is through the prostatovesical plexus to the internal iliac veins. The lymphatic drainage is to the obturator, internal, and external iliac nodes. The sympathetic nervous supply is from the hypogastric plexus (T5–L4) and is largely inhibitory to the detrusor and stimulatory to the bladder neck: this allows the bladder to expand and store urine. The parasympathetic supply originates from the sacral outflow (S1–5) and has the opposite effect on the bladder muscle, initiating and maintaining voiding.

The prostate gland is a chestnut-sized organ, present only in the male, that lies below the bladder, nestled underneath the pubic symphysis. Immediately posterior to it lies the seminal vesicles and the ampullae of the ductus deferens, the combined excretory duct of which (the ejaculatory duct) passes through the prostate into the urethra. Separating these structures from the rectum is a fibrous sheath, derived from a fused extension of the peritoneum, known as Denonvillier's fascia. The urethra passes through the centre of the prostate and is joined in its distal two-thirds by the ejaculatory ducts, which enter the urethra posterolaterally. These landmarks serve to divide the prostate into four anatomic zones. The peripheral zone, which is about 1 cm thick and extends around the posterior surface of the prostate, contains the prostatic capsule. This is the area in which most prostatic malignancies arise. Lying between the peripheral zone posteriorly and the urethra and ejaculatory ducts anteriorly, is the central zone. The transitional zone lies in the superior part of the gland, around the urethra above the level of the ejaculatory ducts. This is the area that enlarges in benign prostatic hyperplasia. The area anterior to the urethra, below the level of the ejaculatory ducts is known as the fibromuscular stroma (Fig. 4) 1518.

The arterial supply is from the inferior vesical artery. The prostate is surrounded by a plexus of veins that drain to the internal iliac system. The lymphatics drain to the obturator, external, and internal iliac nodes. The nervi ergenti (parasympathetic), which run to the penis and control erectile function, travel at the base of the prostate, with vessels, at the five and seven o'clock positions. These can be spared in a radical prostatectomy if one is trying to preserve potency (Fig. 4) 1518.

The urethra conducts urine from the bladder to the outside. The female urethra is about 3 cm long and lies in the anterior vaginal wall. It is surrounded at its midpoint by the somatic external sphincter (supplied by the pudendal nerve S234), which, in combination with the internal sphincter at the level of the bladder neck, maintains continence.

The male urethra is divided into four segments. The prostatic urethra, extending from the bladder neck to the apex of the prostate, is the widest part of the urethra and is notable for a small protuberance on its posterior surface near its distal end, known as the verumontanum. This serves as an important endoscopic landmark indicating the most proximal aspect of the somatic external sphincter. The ejaculatory ducts and the prostatic glands drain into the floor of the urethra at the level of the verumontanum.

The membranous urethra, running between the apex of the prostate and the external perineal membrane, is 2 cm long and is surrounded by the somatic external sphincter (innervated by the pudendal nerve, S234). The prostatomembranous junction is the most common site of urethral disruption associated with pelvic trauma.

The bulbar urethra extends from the perineal membrane to the beginning of the pendulous urethra. It is surrounded by the bulbocavernos muscle and is the area most often injured when a person falls astride an object.

The pendulous urethra is contained within the penis. Just proximal to the meatus (the narrowest part of the urethra) there is a slight dilatation: this area is known as the navicular fossa.

The penis consists of three expansile fibrovascular tubes: two corpora cavernosa and the corpus spongiosum. The two corpora cavernosa lie side by side along the dorsal aspect of the penis. The corpus spongiosum runs along the ventral surface of the penis and contains the urethra. At the head of the penis the corpus spongiosum enlarges to form the glans. Both corpora cavernosa are surrounded by a tough fiborus sheet, the tunica albuginea, within which the fibrovascular core can expanded to produce an erection. The corpora are attached posteriorly to the ischial rami where they are surrounded by the ischiocavernosa muscle that helps support and firm an erection. The two corpora cavernosa unite at the pubic symphysis and together with the corpora spongiosium form the penis (Fig. 5) 1519.

The penis receives its blood supply from the pudendal artery, which is the terminal branch of the internal iliac artery. The superficial and deep dorsal veins of the penis pass through the suspensory ligament to the periprostatic plexus. The lymphatics pass to the inguinal nodes. The nerve supply is from the pudendal and pelvic plexus.

The testicles are paired gonads that arise on the posterior abdominal wall at the level of the kidneys during the 7th week of intrauterine life. They descend during gestation, preceded by a fibrous structure, the gubernaculum; the testes should be located in the scrotum at birth. As they pass through the anterior abdominal wall they take a covering from each of its three muscle layers, the most prominent being the internal oblique which becomes the cremesteric muscle. A sleeve of peritoneum, the processus vaginalis, accompanies the testis on its journey through the inguinal canal. This sleeve is normally obliterated shortly after birth; if it persists an indirect inguinal hernia may form in its tract. The remnant of peritoneum covering the anterior surface of the testicle is called the tunica vaginalis; this may fill with fluid to form a hydrocele (Fig. 6) 1520.

The testicle is surrounded by a thick fibrous sheath, the tunica albuginea. Inside each testicle there are approximately 400 seminiferous tubules, which produce sperm. These tubules drain to the epididymis, vas deferens, and, ultimately, the seminal vesicle and ejaculatory ducts.

The testicular artery arises from the aorta at the level of the second lumbar vertebrae and travels through the retroperitoneum and the entire length of the cord to the testicle. The veins form a plexus in the scrotum (the pampiniform plexus) which usually unites at the level of the internal inguinal ring and drains into the vena cava on the right and the renal vein on the left. Dilation of the testicular veins of the pampiniform plexus is known as a varicocele (Fig. 6) 1520. The lymphatics drain to the para-aortic nodes at the level of the renal hilum.

The scrotum is a sack of thickened skin with muscle fibres in Colles' fascia, forming the rugose dartos muscle. It maintains the testicles at 1 to 2° below body temperature, allowing optimal spermatogenesis.

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