Bladder anatomy

Overview

• Empty bladder situated entirely within pelvic cavity
• Empty bladder:
  • Flattened 3-sided pyramid
  • Sharp apex pointing forwards to the top of the pubic symphysis
  • Triangular base facing backwards in front of the rectum or vagina
• Two inferolateral surfaces cradled by the anterior parts of the Levator Ani
• Neck – where Urethra opens
• Superior surface – on which small intestine and sigmoid colon or uterus lies

• Apex
  • Attached to the remains of the Urachus, forming the Median Umbilical Ligament
    • Runs up the midline of the anterior abdominal wall in the Median Umbilical Fold of the Peritoneum
• Base (Posterior Surface)
  • Most lies below the level of the Rectovesical Pouch
  • Only the uppermost portion is covered by peritoneum between the Vas Deferens on each side
    • NB: Thus, can say the base is NOT covered by peritoneum
  • Seminal Vesicles are applied to this surface
  • Ureter enter in the upper outer corners
  • In the female: The base has a firm connective tissue union with the anterior vaginal wall and upper part of the Uterine Cervix with no peritoneum intervening
• Inferolateral Surface
  • Slopes downwards and medially to meet its opposite
  • Lies against the front part of the Pelvic Diaphragm and Obturator Internus
  • Retropubic Space of Retzius – the retroperitoneal space behind the pubic bones and pubic symphysis where the surfaces meet below the Apex
    • Contains loose fatty tissue and the fibromuscular Pubovesical Ligaments that extend from the Bladder Neck to the Inferior Aspect of the Pubic Bones
• Neck
  • Lowest part of the Bladder
  • Where the base and inferolateral surfaces meet
  • Pierced by the Urethra at the Internal Urethral Orifice
  • In the male: Lies against the upper surface or base of the Prostate
  • In the female: Above the Urethra in the connective tissue of the anterior Vaginal Wall
• Superior Surface
  • Covered by peritoneum (which sweeps upwards onto the anterior abdominal wall)
  • Distending bladder strips peritoneum from behind the Rectus Abdominis, leaving the Transversalis Fascia on the back of the muscle
    • Thus, distended bladder can be approached in the midline above the pubic symphysis by a cannula or scalpel without entering the peritoneal cavity
  • In the male: At the posterior margin of this surface, the peritoneum continues onto the uppermost part of the base and is then continued backwards as the floor of the Rectovesical Pouch
  • In the female: Reflected from a little in front of the posterior margin of this surface onto the under-surface of the Uterus
• Interior Appearance:
  • Varies upon state of distension
  • When collapse:
    • Mucous membrane thick and thrown into folds
  • When distended:
    • Mucous membrane thin and smooth
  • Trabeculae of muscle fibres can be seen through the mucous membrane
  • These changes do not apply to the Trigone – varies little with state of distension

Trigone of Bladder

• Triangular area at the base of the bladder lying between the two Ureteral Orifices (above and laterally) and the Internal Urethral Orifice (centrally and below)
  • In the empty bladder – these 3 openings are 2.5cm apart from each other
  • When distended – ureteral orifices may be 5cm apart
  • Ureteric Orifices connected by a transverse ridge – the Interureteric Bar
    • Produced by the continuity of the longitudinal muscle of the two ureters across the bladder wall
  • Ureters pierce the muscle and mucosal walls very obliquely
    • Prevents reflux of urine
  • Orifices closed by rising intravesical pressure
    • Opens rhythmically in response to ureteric peristalsis each time a jet of urine is injected into the bladder (4-5 times per minute)
• Smooth walled
• Mucous membrane is relatively firmly adherent to the underlying muscle
• Fixed on-top of the prostate by the Urethra
• Trigone is the least mobile part of the Bladder
• In the male: Overlies the median part of the central zone of the Prostate
  • After middle-age, the central zone may project above the internal urethral orifice as a rounded elevation, the Uvula of the Bladder
• In the female: Stabilized by the connective tissue surrounding the upper urethra at the front of the Vagina

Blood Supply

• Superior and Inferior Vesical Arteries
  • Provide most of the arterial blood
• Small contributions to the lower part of the bladder from:
  • Obturator Artery
  • Inferior Gluteal Artery
  • Uterine Artery
  • Vaginal Arteries
• Veins do not follow arteries
  • Form a plexus that converges on the Vesicoprostatic Plexus in the groove between Bladder and Prostate
  • Drains backwards across the Pelvic Floor to the Internal Iliac Veins
  • Similar plexus in females – communicates with veins in the base of the Broad Ligament

Lymph Drainage

• Mainly drain to External Iliac Nodes
• Some drain to Internal Iliac Nodes, including nodes in the Obturator Fossa

Nerve Supply

• Parasympathetic Fibres:
  • Provide the main motor innervation of the Bladder
  • Reach it via Pelvic Splanchnic Nerves
  • Carries fibres for the sensation of normal bladder distension
    • Conveyed in the spinal cord in the Gracile Tract
  • NB: Carries sensory fibres from the Trigone of the Bladder
• Sympathetic Fibres:
  • Come from L1 and L2 segments of the cord via the Superior and Inferior Hypogastric Plexuses
  • Vasomotor for most of the Bladder
  • Probably inhibitory to the Detrusor Muscle
  • Motor to the Superficial Trigonal muscle and (in the male) the muscle of the bladder neck
• Bladder pain (e.g. from a stone) reaches the spinal cord (Lateral Spinothalamic Tract) by both Parasympathetic and Sympathetic pathways

Control of Micturition

• In general
  • PNS → emptying
  • SNS → storage
• Normal bladder emptying:
  • Contraction of the Detrusor muscle and reciprocal relaxation of the external sphincter and pelvic floor (Levator Ani)
• Accumulation of urine distends Bladder wall with adjustment of tone (Accommodation) so that tension does not at first increase
• Later – increased tension stimulates stretch receptors
  • Afferent impulses travel along Pelvic Splanchnic Nerves to sacral segments of the spinal cord
  • Parasympathetic cell bodies stimulated and efferent impulses travel down the Pelvic Splanchnic Nerves to synapse with post-ganglionic cells within the bladder wall and cause contraction
• Autonomic stretch reflex with bladder control at the spinal level typical of infant
  • Control by higher centres becomes superimposed on the spinal activity
  • Bladder evacuation is assisted by voluntary contraction of abdominal muscles
  • Cortical inhibitory centres in the Inferior Frontal Gyrus (on the medial surface of the cerebral hemisphere, some distance in front of the motor ‘Perineal’ area) with fibres passing to a Detrusor Motor Centre in the medial part of the Pontine Reticular Formation
  • From there – Reticulospinal fibres run down the cord mixed with those of the Lateral Corticospinal Tract to the Sacral segments
• Skeletal Muscle of the Sphincter Urethrae (External Urethral Sphincter) – controlled by the Perineal Branch of the Pudendal Nerve
  • Carries fibres predominantly from the Anterior Horn Cells of S2 Segment (Onuf’s Nucleus)
  • Storage Centre in the lateral part of the Pontine Reticular Formation exerts central control on this nucleus
  • During micturition, sphincter relaxes as the Detrusor muscle contracts
  • In the female: the Pubovaginalis part of the Levator Ani assists the external sphincter at the end of micturition
• In spinal cord transection above the level of S2
  • Afferent impulses indicating distension cannot reach consciousness
  • Cortical control of the sacral reflex is lost
  • Relaxation of the Sphincter Urethrae cannot be prevented
  • Sacral centre intact, thus, the Bladder automatically empties when distended
  • If sacral segments themselves are destroyed, the Detrusor muscle is paralysed and the bladder becomes abnormally distended until overflow incontinence occurs

Structure

• Smooth muscle of Bladder wall (Detrusor muscle) composed of interlacing network of fibres running in various directions
• Produce a trabeculated appearance (both externally and internally, beneath the mucous membrane)
  • Exaggerated when muscular hypertrophy occurs as a result of progressive chronic obstruction to micturition
• Well supplied by Parasympathetic (Cholinergic) nerve fibres
• Trigone – possesses a superficial triangular layer of muscle (Superficial Trigonal Muscle) that is histologically and histochemically different from the rest of the Bladder (including the deep part of the Trigone)
  • Extends into the proximal Urethra in both sexes
  • Receives predominantly Sympathetic (Adrenergic) fibres
  • Contraction of this muscle may help close the Ureteral Orifices
• In the male, at the Bladder Neck:
  • Pre-Prostatic Sphincter: Circular smooth muscle fibres form a collar around the Internal Urethral Orifice
  • Extends distally to surround the proximal part of the Prostatic Urethra (the Pre- prostatic part)
  • Profusely supplied with Sympathetic (Adrenergic) fibres
• In the female, at the Bladder Neck:
  • The muscle is arranged longitudinally and extends into the Urethral wall
• Muscle of the Bladder neck has nothing to do with urinary continence
  • In the male: Acts to prevent seminal regurgitation into the bladder during ejaculation
• Mucous membrane:
  • Thick
  • Lax
  • Lined by Transitional epithelium
  • Glands usually absent (mucous in shed urine comes from Urethral glands)
  • No muscularis mucosae

Development

• Endoderm of the Vesicourethral part of the Urogenital Sinus becomes the Bladder epithelium
• Surrounding mesenchyme forms the muscle and connective tissue
• Continued growth leads to incorporation of the lower ends of the mesodermal mesonephric ducts into the posterior part of the bladder – forming the Trigone
• The mesonephric ducts in the male end up at a lower level as the Ejaculatory Ducts entering the Urethra
• Allantois regresses to form a fibrous cord, the Urachus (median umbilical ligament)