Overview of the urinary tract

The urinary system is composed of the kidneys, ureters, urinary bladder, and the urethra. This group of organs functions to maintain the fluid balance of the body and to filter toxic substances from the bloodstream. Urine is generated by the kidneys and carried to the bladder through the ureters. From the bladder, it is released through the urethra. The ureters have smooth muscle fibers that contract in a peristaltic fashion to propel urine to the bladder. They have a narrow lumen at the ureteropelvic junction, the pelvic inlet, and the ureterovesical junction, which renders them susceptible to stone impaction. The bladder is located in the extraperitoneal space, behind the pubic symphysis, within the pelvis, and has a detrusor muscle that contracts during micturition. It is divided into apex, body, fundus, and neck. The bladder can rupture from blunt abdominal trauma, resulting in extravasation of urine and, potentially, peritonitis. The internal urethral sphincter is a circular smooth muscle that surrounds the neck of the bladder and prevents urine leakage. The urethra is a tubular structure that transports urine from the bladder to the external urethral meatus. The male urethra, which also transports semen, is divided into three parts: prostatic urethra, membranous urethra, and penile urethra (spongy urethra), while the female urethra has only one part. The membranous urethra, penile urethra, and female urethra are lined by stratified squamous epithelium. The ureters, bladder, and prostatic urethra are lined by transitional epithelium., which is derived from the endoderm.

• Glomeruli (renal cortex) → Bowman capsule and space → proximal convoluted tubule (PCT) → loop of Henle → distal convoluted tubule (DCT) → collecting ducts → drain into the minor calyces → major calyces → renal pelvis → ureter → bladder → urethra
• See "Kidneys" and "Physiology of the kidney."

Overview

• Retroperitoneal, hollow tubes that extend from the renal medulla (renal pelvis) to the urinary bladder
• Contain smooth muscles that contract and relax in a wave-like manner (peristalsis) to propel urine forward towards the bladder
• Course along the anterior surface of transverse processes of the lumbar spine and the psoas major muscles

Gross anatomy ^[1]

• Renal pelvis
  • The dilated, funnel-shaped, proximal part of the ureter
  • The point of convergence of 2 or 3 major calyces
  • Common site for struvite stone precipitation (staghorn calculi)
  • Close proximity to both the iliohypogastric nerve and ilioinguinal nerve: renal pathologies (e.g., kidney stones) can cause referred pain in the groin
• Constrictions of the ureters
  • Ureteropelvic junction (in the kidneys)
  • Pelvic inlet (where the ureter crosses the common iliac or the external iliac artery)
  • Ureterovesical junction (where the ureters enter the posterior wall of the urinary bladder)
• Entry into the bladder
  • The ureters travel down the posterolateral surface of the urinary bladder towards its inferior region, where they insert into the trigone of the bladder.
  • The intramural parts of the ureters are compressed during bladder contraction, preventing vesicoureteral reflux.
• Relationship to neighboring structures: The ureters travel from the kidneys (cranial) down to the urinary bladder (caudal).
  • They are located:
    • Posterior to the gonadal arteries
    • Anterior to the common iliac arteries at the level of the bifurcation or anterior to the external iliac arteries
    • Posterior to the vas deferens (in males) and the uterine artery (in females)
• Vasculature

• Innervation
  • Sympathetic: lumbar splanchnic nerves
  • Parasympathetic: pelvic splanchnic nerves, vagus nerve (CN X)

Common sites of ureteral obstruction are the three constrictions of the ureters: the ureteropelvic junction, the pelvic inlet, and the ureterovesical junction.

The ureter travels posterior to the gonadal artery, ventral to the common iliac artery, and posterior to the vas deferens/uterine artery.

Due to their close anatomical association with the female reproductive organs, the ureters are at risk of injury during gynecological procedures (e.g., ligation, dissection of the uterine or ovarian vessels). Ureteral injury is a serious complication and may lead to ureterovaginal fistula formation as well as ureteral obstruction and discontinuity.

Microscopic anatomy ^[2]

• Epithelium: transitional epithelium
• Muscular layers: contract and relax in a peristaltic pattern
  • Inner longitudinal layer
  • Outer circular layer
  • Outer longitudinal layer (mainly present in the distal third of the ureter)

Overview ^[1]

• Hollow, triangular-shaped organ
• Located extraperitoneally, behind the pubic symphysis, within the pelvis, and beneath the peritoneum
• Can hold ∼ 500–1,000 ml of urine
• Sensation of bladder fullness is felt at ∼ 300–500 mL.
• Contains smooth muscle (the detrusor muscle of the bladder) that contracts during micturition.

Gross anatomy ^[1]

Muscles

• Detrusor muscle
  • Primary smooth muscle of the bladder
  • Contracts during micturition to enhance bladder emptying
  • Controlled by the parasympathetic nervous system
• Internal urethral sphincter
  • Circular smooth muscle surrounding the neck of the bladder, where the urethra begins
  • Constriction prevents urinary leakage
  • Controlled by the sympathetic nervous system

Vasculature

• Arteries
  • Branches of the internal iliac arteries
    • Superior vesical artery (supplies the superior bladder, seminal vesicles, ductus deferens, and portions of the ureter)
    • Inferior vesical artery
    • Vaginal artery (in females)
  • Branches of the obturator artery and inferior gluteal artery
• Veins
  • Branches that mirror the arterial supply
  • Drain into the internal iliac veins
• Lymphatics
  • Internal iliac nodes
  • Superior part: external iliac nodes

Innervation

• Sympathetic: causes relaxation of the detrusor muscle and constriction of the internal urethral sphincter → retention of urine
• Parasympathetic: pelvic splanchnic nerves (S2–S4) → stimulate contraction of the detrusor muscle and relaxation of the internal urethral sphincter → emptying of the bladder

Rupture of the bladder dome (e.g., blunt abdominal trauma), especially when the bladder is full, can cause peritonitis due to extravasation of urine into the peritoneal cavity

Microscopic anatomy ^[2]

• Epithelium: transitional epithelium
  • Empty bladder
    • Composed of 5–6 layers of cells
    • Cells are round and thick
  • Full bladder
    • Composed of 3–4 layers of cells
    • Cells are flat and thin
• Muscular layers
  • Inner longitudinal layer
  • Middle circular layer
  • Outer longitudinal layer

Function

Micturition ^[3][4]

• Definition: the action of urinating, triggered by a spinal reflex that is subject to voluntary control
• Process
  1. Filling of the urinary bladder
  2. Increase in intravesical pressure
  3. Stretching of the bladder wall
  4. Activation of mechanoreceptors
  5. Sensory information is sent to the spinal cord through pelvic splanchnic nerves
  6. Information is relayed to the pontine micturition center (PMC), a collection of neuronal cell bodies located in the pons that coordinate the process of micturition
  7. Information is integrated in higher (cortical) brain centers
  8. Perception of bladder fullness
     • Urination if feasible: activation of the PMC → stimulation of parasympathetic fibers and inhibition of somatic pudendal nerve fibers → contraction of the detrusor muscle and relaxation of the external urethral sphincter → micturition
     • Urination is unfeasible: cortical inhibition of the PMC → no stimulation of parasympathetic fibers or inhibition of somatic pudendal nerve fibers → no contraction of the detrusor muscle or relaxation of the external urethral sphincter → no micturition

The involuntary function of the bladder is regulated through the coordination of the sympathetic and parasympathetic nervous systems by the micturition center in the pons.

Overview

• A hollow and tubular structure that begins in the neck of the bladder, continues through the urogenital sinus, and ends in the external urethral meatus
• Transports urine from the urinary bladder to the exterior of the body under control of the urethral sphincter (see “Urogenital diaphragm”)

Gross anatomy ^[1]

Urinary catheterization (e.g., Foley catheter) should be avoided in patients with suspected urethral injury!

• Germ layer derivatives
  • Mesoderm: the kidneys and ureters
  • Endoderm: epithelium of the urinary bladder and the urethra
• Kidney and ureters
  • The kidneys and ureters develop from a succession of events that involve the appearance of three renal precursors: pronephros, mesonephros, and metanephros.
  • See “Embryology” in “Kidneys.”
• Urinary bladder and urethra
  • Derived from the cloaca
  • Communicates with the surface ectoderm and the allantois
  • Divided by the urorectal septum
    • Ventral portion develops into the bladder and urogenital sinus
    • Dorsal portion develops into the rectum and anal canal
  • Trigone: derived from mesonephric ducts (mesoderm)
• Allantois
  • Endodermal origin (hindgut)
  • Formed from the yolk sac in the 3^rd week
  • Extends into the urogenital sinus and communicates with the umbilicus
  • Gives rise to the umbilical arteries and umbilical vein
  • Becomes the urachus
• Urachus
  • Connects the fetal bladder to the umbilicus
  • Removes fetal nitrogenous waste
  • Becomes the median umbilical ligament when obliterated, which is:
    • Attached to the apex and extends to the umbilicus
    • Covered by the median umbilical fold, a layer of peritoneum that is a remnant of the obliterated urachus that extends from the apex of the urinary bladder to the umbilicus
  • Failure of the urachus to obliterate causes the following abnormalities:
    • Patent urachus (urachal fistula): complete failure to obliterate → urine discharge from the umbilicus
    • Urachal cyst
      • Partial failure to obliterate results in a fluid-filled cavity covered by urothelium between the bladder and the umbilicus.
      • Suspect urachal cyst infection in a patient with a painful mass under the umbilicus.
    • Urachal sinus: partial failure of the cranial end to obliterate → cloudy, serous (or even bloody) fluid from the umbilicus
    • Vesicourachal diverticulum: partial failure of the caudal end to obliterate → outpouching of the bladder

References:^[6]

• Bladder exstrophy, abnormalities of the male urethra, and webbed penis
• Congenital anomalies of the kidneys
• Disorders of the glans penis and foreskin
• Glomerular diseases
• Nephritic syndrome
• Nephrotic syndrome
• Penile fracture
• Polycystic kidney disease
• Posterior urethral valves
• Renal artery stenosis
• Renal cell carcinoma
• Stress incontinence
• Scrotal abnormalities
• Traumatic injuries of the kidney and bladder
• Urethritis
• Urge incontinence
• Urinary incontinence
• Urinary retention
• Urinary tract infections
• Urolithiasis
• Urinary tract cancer
• Vasculitides
• Vesicoureteral reflux
• Nephroblastoma