The Urinary Tract

• The ureters are bilateral muscular tubes which conduct urine from the kidneys to the bladder. They're around 25cm in length, and they're retroperitoneal organs with both abdominal and pelvic parts, known as the abdominal ureter and pelvic ureter respectively.
• The ureter is a continuation of the renal pelvis in the hilum of the kidney. The renal pelvis narrows down to form the ureter proper. The pelviureteric junction is the narrowest part of the ureter. The urine is propelled along the ureter via peristaltic waves, triggered by an intrinsic pathway and also by parasympathetic innervation.
• The ureter has a number of relations:

  • at it's origin, it lies posterior to the renal artery and renal vein
  • the proximal part of the right the ureter is in close contact with the duodenum
  • both ureters lie on the posterior abdominal wall, anterior to psoas major
  • the genitofemoral artery is a close relation to the ureters because it pierces psoas major.
• The right ureter lies behind the duodenum. It's crossed by the right gonadal artery (which head laterally and inferiorly), arteries within the mesentery such as the right colic artery and the ileocolic artery. The right ureter also passes behind the root of the mesentery and the terminal ileum.
• The left ureter is crossed by the left gonadal artery, the left colic artery, and the sigmoidal arteries. The left ureter passes behind the root of the sigmoid mesocolon.
• The pelvic part of the ureter in males has three key relations:

  • the ureter crosses over the external iliac artery at its origin from the common iliac artery at the pelvic inlet. The ureter is usually compressed by the external iliac artery, resulting in narrowing of the lumen of the tube.
  • the ureter crosses over the obliterated umbilical artery
  • the ureter crosses under the ductus (vas) deferens.
• The pelvic course of the ureter in females is:

  • the ureter crosses over the external iliac artery
  • the ureter crosses over the obliterated umbilical artery
  • the ureter crosses under the uterine artery
• As the ureter heads towards the bladder, it passes close to the lateral fornix of the vagina. The lateral fornix of the vagina is lateral to the cervix of the uterus, and below both the broad ligament and the uterine vessels.

  As a result of this relation, there can be cases of cervical cancer where the ureter may be compressed by a cancerous growth.
• The ureter is lined by transitional epithelium or urothelium.

  There is no submucosa.

  It has a thick, fibroelastic, lamina propria which lies underneath the epithelium. This mucosa is protective. There are no mucosal or submucosal glands.

  There are layers of smooth muscle outside the mucosa; the upper two-thirds have two layers, inner longitudinal and outer circular layers. The lower third has 3 layers: inner longitudinal, middle circular, and outer longitudinal.

  The outer adventitial layer has fibroelastic connective tissue, with blood vessels, lymphatics and nerves.
• There are three sites of natural constrictions of the ureter:

  • pelvo-ureteric junction. This is the narrowest part of the ureter, and hence, is the site where renal stones may become lodged.
  • site where the ureter crosses over the external iliac artery.
  • uretero-vesical junction (UVJ). Where the ureter passes into the bladder.
• The intramural part is the part of the ureter that passes through the wall of the bladder. This distal end of the ureter has 3 layers of smooth muscle: inner longitudinal, middle circular, and outer longitudinal. The intramural part passes through the detrusor muscle of the bladder and enters the bladder via the ureteric orifice in the trigone area of the bladder wall.
• The intramural ureter passed through the wall of the bladder obliquely, and in doing so, acts as its own sphincter.

  When the bladder is empty, the ureteric orifice is open. When the bladder is pull, it pushes the wall of the bladder towards the ureter and closes the opening and prevents urine from refluxing back up the ureter.
• The ureter doesn't have a dedicated artery, but instead receives a segmental blood supply. Arterial branches to the abdominal ureter approach medially, whilst the arterial branches to the pelvic ureter approach laterally.
• The arterial blood supply to the abdominal ureter, superior to inferior, is the renal artery, gonadal artery, abdominal artery, and the common iliac artery.

  The arterial blood supply to the pelvic ureter receives its blood supply from the internal iliac artery, or its branches (the vesical and uterine arteries), the middle rectal artery, and vaginal artery.

  All of these arteries anastomose with each other freely. The venous and lymphatic drainage of the ureter parallels the arterial supply.
• The nerve supply to the ureter is derived from the adjacent autonomic plexuses. Those are the aortico-renal, superior and inferior hypogastric plexuses.

  The abdominal ureter receives parasympathetic fibres from the vagus nerve, and the pelvic ureter receives fibres from the S2-S4 pelvic splanchnic nerves.

  Afferent pain fibres from the ureter follow sympathetic fibres to reach posterior root ganglia and the spinal cord at the levels of T11- L2. Ureteric pain moves according to site of obstruction. It is classically described as a loin to groin pain.
• The urinary bladder is a muscular bag. Its walls are composed of smooth muscle, known as the detrusor muscle. The base is known as the “trigone” and is positioned postero-inferiorly, whilst the apex is positioned antero-superiorly. The sides of the bladder are angled downwards and form an inferolateral surface.

  The actual shape of the bladder is dictated by how full it is. As it fills the superior surface extends upwards, whilst the base remains relatively fixed.
• When the bladder is empty, the walls of the bladder are rugged, but the trigone is smooth. As the bladder fills with urine, its rugged walls smooth out, and its epithelium (urothelium) is stretched.
• The bladder receives the ureters at the superior angles of the trigone (the ureteric orifices entering the wall of the bladder obliquely), and at its inferior angle the urethra leaves it.
• The male bladder is covered on its superior surface with peritoneum. Lateral to the bladder on each side are the ductus deferentes. Inferior to the male bladder is the prostate gland. Enlargement of the prostate gland may cause the inferior part of the bladder to be pushed upwards. This can result in stagnant pools of urine, and incomplete emptying of the bladder.
• Behind the male bladder are a pair of glands known as the seminal vesicles. Above the seminal lands are enlargements at the terminal end of the ductus deferens, known as the ampullae. The ducts of the seminal vesicles and ampullae join to form the ejaculatory duct. The ejaculatory duct enters the posterior wall of the prostate gland and then into the prostatic ureter.

  Behind the seminal vesicles is the rectovesical pouch, which separates the bladder from the rectum.
• The female bladder sits behind the pubic symphysis, and its surface is covered by peritoneum. The bladder sits directly above the urogenital hiatus of the pelvic floor and deep perineal pouch. Posterior to the bladder is the vagina, and connected to this, tilted forwards onto the superior surface of the bladder, is the uterus.

  Between the bladder and uterus, there is a pouch of peritoneum known as the vesicouterine pouch, and between the uterus and rectum, there is the rectouterine pouch.
• When empty the urinary bladder is a pelvic organ, whilst when full it becomes an abdominal organ.

  Normal voiding volumes are 210-300mL with the first morning voiding usually being the largest at 400-500mL. Usually the urge to micturate is felt when there is about 250 mL of urine in the bladder. Voiding is via the urethra.

  If there is a constriction of the urethra, which cannot be remedied easily (due to a tumour of the prostate gland), then it is possible to drain the bladder by a suprapubic catheter.
• To perform a suprapubic catheterisation, urinary retention must first be confirmed via clinical examination and a bladder scan.

  A point is identified two finger-breadths above symphysis pubis in the midline and this is infiltrated with local anaesthetic. The anaesthetic is also infiltrated below the skin to include the fascia. Using the same needle, one can test that urine can be released from the bladder by having the needle perpendicular to skin. If that is positive the catheter can be inserted, and a balloon inflated near its end once it is in place. This prevents it from being dislodged.
• The prostate gland lies inferior to the bladder. The prostate contributes large amounts of citric acid to seminal fluid. Prostatic secretions are rich in proteolytic enzymes such as acid phosphatases which are important in phospholipid metabolism. This aids the maintenance of the sperm within the ejaculate. The secretions are slightly acidic. The ducts of the prostate gland enter into the prostatic urethra.
• The prostate encloses the urethra, and its secretions drain directly into the lumen of the urethra via several prostatic ducts.

  On the posterior wall of the prostatic urethra is a swelling, known as the seminal colliculus. Superiorly, the seminal colliculus narrows to form the urethral crest, and this in turn is connected to the trigone of the bladder.

  There are three openings into the seminal colliculus: the prostatic utricle, and 2 ejaculatory ducts.
• The bladder has a supply from the superior and inferior vesical arteries. The prostate is supplied by the inferior vesical arteries. The superior and inferior vesical arteries are branches of the internal iliac artery.
• The venous drainage is via equivalent veins which drain the bladder via a vesical plexus. The prostatic venous plexus sits inferior to the bladder. Bladder and prostate cancers can travel via this plexus.

  Carcinoma of the prostate has a poor prognosis because the prostate is surrounded by a rich venous plexus, and metastases spread through these veins to the vertebral venous plexus (of Batson) and then to spinal cord, brain, and beyond. The vertebral venous plexus is valveless, and this enhances the spread of cancers through this system.
• Lymphatic spread is to lymph vessels and nodes alongside the internal iliac arteries. From there, lymph joins the para-aortic lymph trunks to the cysterna chyli.

  The superior surface of the bladder though, drains mostly to external iliac lymph nodes.
• The motor supply to the detrusor muscle of the bladder is via parasympathetics from S2-S4, also known as pelvic splanchnic nerves. Parasympathetic afferents respond to stretch of the bladder wall. There is parasympathetic reflex where stretch of the bladder wall results in its contraction. This reflex is seen in babies and infants, who are unable to control bladder emptying. Later, parts of the brain (such as the inferior frontal gyrus and pontine reticular formation) take control of the spinal reflex.
• Young children develop control over micturition by suppressing the parasympathetic reflex and gaining control of the external urethral sphincter. The external urethral sphincter is a skeletal muscle and is stimulated by the pudendal nerve (S2 principally). In this part of the spinal cord is Olaf’s nucleus, and this controls the pudendal nerve, which tonically stimulates the sphincter to maintain urinary continence. Olaf's nucleus is controlled by a lateral part of the pontine reticular formation.
• To urinate in adulthood, the external urethral sphincter must relax. Hence, the micturition centres in the brain co-ordinate the stimulation of the bladder contraction and relaxation of the external urethral sphincter. When a patient has a lesion of the spinal cord above S2, all of the central processing is lost. Hence, the simple reflex returns.
• In the adult, emptying of the bladder is prevented by tonic contraction of the external urethral sphincter. Relaxation of this muscle is voluntary although if the bladder is over-full it may be difficult to restrain its release. The parasympathetic efferents arise from S2-S4 and these are excitatory to the bladder. They also inhibit the internal urethral sphincter in males. Females lack an internal sphincter.
• The sympathetic preganglionic nerves of the bladder arise from T11-L2. These give rise to the hypogastric nerves which travel from the abdomen to the pelvis. Here they reach the inferior hypogastric plexus to be distributed to the bladder. These are inhibitory to the parasympathetic nerves and the detrusor muscle of the bladder. They are excitatory to the internal urethral sphincter at the neck of the bladder. All of these sympathetic pathways will be silent during micturition, to ensure that there is no obstruction to urine flow and the detrusor muscle can contract fully.
• The urethra takes urine from the bladder and expels it from the body. The opening of the urethra in the female is located in the vestibule of the vulva, between the two labia minora. It lies anterior to the opening for the vagina which is also located within this vestibule.

  The urethral opening in the male is located at the tip of the glans penis, as a small vertical slit.

  These openings are referred to as the external urethral meatus.
• The urethra in the male passes from the neck of the bladder, through the penis to the external urethral meatus. The urethra can be divided for descriptive purposes into four parts:

  • pre-prostatic urethra
  • prostatic urethra
  • membranous urethra
  • penile urethra
• The membranous urethra (in males) is defined as such when the urethra leaves the prostate gland and passes through the deep perineal pouch. Attached to the under-surface of the deep perineal pouch is the bulb of the penis, and the urethra enter it. This bulb and its continuation into the body of the penis is composed of tissue called the corpus spongiosum. The urethra contained here is known as the penile urethra.
• The female urethra is shorter than the male one, only about 4cm in length. It only passes through the deep perineal pouch.
• The urethra is similar in structure to the ureter, though it is shorter. The lumen is kept closed unless urine is being passed, and this helps to reduce infections.

  The epithelial lining of the female urethra changes from transitional epithelium at the bladder to nonkeratinizing stratified squamous epithelium distally.

  The epithelial lining of the male urethra changes from transitional epithelium at the bladder to pseudostratified epithelium through the prostatic, membranous and most of the penile urethra, eventually changing to nonkeratinizing stratified squamous epithelium distally.
• The lamina propria of both male and female urethra are vascular, but the male is much more vascular with its extensive corpus spongiosum. There are a few mucous glands in the submucosa.
• Just prior to reaching the external urethral meatus, the urethra in the male expands to form a cavity known as the navicular fossa, or fossa terminalis. When inserting a catheter into the male urethra, one needs to take care as there is usually a fold of mucosa which hangs down inside the external urethral orifice called the valvula fossae navicularis. Don't hit the valvula fossae navicularis.

  As the catheter is fed through the male urethra, there are two bends that need to be navigated. The 2nd bend between the bulb of the penis and the deep perineal pouch needs to be navigated carefully.