Urinary Bladder - MBBS 10 Marks Answer

1. Introduction

• Capacity: Normally ~500 mL; the urge to void begins at ~300 mL
• When empty: bowl-shaped, lies entirely in the pelvis
• When full: ovoid, apex rises above the pubic symphysis into the abdomen

2. Gross Anatomy

• A smooth triangular region on the internal floor of the bladder
• Bounded by the two ureteric orifices superolaterally and the internal urethral orifice inferiorly
• The mucosa here is firmly attached to the underlying muscle - no folds form even when empty
• Derived embryologically from the mesonephric ducts (unlike the rest of the bladder)
• Clinically important: most common site for bladder tumours and infections

• Anterior: pubic symphysis and retropubic space (cave of Retzius)
• Superior: covered by peritoneum; small intestine rests on it when full
• Posterior (male): rectovesical pouch, seminal vesicles, vas deferens, rectum
• Posterior (female): uterovesical pouch, uterus, anterior vaginal wall
• Inferior: prostate gland (male); urogenital diaphragm (female)

3. Microanatomy (Histology)

a) Mucosa

• Lined by transitional epithelium (urothelium) - typically 4-6 cells thick when empty, 2-3 cells thick when distended
• The surface cells are large, dome-shaped "umbrella cells" or "facet cells"
• Lamina propria: loose connective tissue beneath the epithelium
• The mucosa forms folds (rugae) when the bladder is empty; these disappear when full
• Exception - Trigone: no muscularis mucosae or submucosal layer; mucosa is flat and firmly adherent

b) Submucosa

• Loose connective tissue; allows the mucosa to fold and unfold

c) Muscularis (Detrusor Muscle)

• Three indistinct layers of smooth muscle - inner and outer longitudinal, middle circular
• Collectively called the detrusor muscle
• At the bladder neck, the circular layer forms the involuntary internal urethral sphincter
• At the trigone: only two layers (continuation of the ureteric musculature)

d) Serosa / Adventitia

• The superior surface and part of the posterior surface are covered by peritoneum (serosa)
• The rest is covered by adventitia (fibrous connective tissue)

4. Nerve Supply (Innervation)

5. Blood Supply

• Arterial supply:
  • Superior vesical artery - from the umbilical artery (a branch of the internal iliac artery) - supplies the superior and anterosuperior parts
  • Inferior vesical artery - from the internal iliac artery - supplies the fundus and neck (in males also supplies the prostate)
  • Additional small branches from the obturator, inferior gluteal, and (in females) uterine and vaginal arteries
• Venous drainage:
  • The vesical venous plexus surrounds the fundus of the bladder
  • Drains into the internal iliac veins

6. Lymphatic Drainage

• Superior bladder: external iliac nodes
• Inferolateral and neck: internal iliac nodes
• Trigone: external and internal iliac nodes

7. Embryological Development

1. The cloaca is divided by the descending urorectal septum into the urogenital sinus (ventral) and the rectum (dorsal)
2. The urogenital sinus has three parts: vesical (upper), pelvic (middle), phallic (lower)
3. The vesical part expands to form most of the bladder; its epithelium is derived from endoderm; the muscular wall is from splanchnic mesenchyme
4. The bladder is initially continuous superiorly with the allantois, which later obliterates to form the urachus, then the median umbilical ligament in adults
5. The trigone is formed by incorporation of the distal mesonephric (Wolffian) ducts into the posterior bladder wall - hence it has a different embryological origin than the rest of the bladder

• Patent urachus - if urachus fails to close, urine drips from the umbilicus
• Urachal cyst - partial obliteration leaves a midline cyst
• Extrophy of bladder - failure of mesodermal migration leads to an exposed bladder on the anterior abdominal wall

8. Physiology - Micturition Reflex

• At 0-50 mL: pressure rises to 5-10 cmH₂O
• At 50-300 mL: pressure stays nearly constant (intrinsic wall tone)
• Beyond 300-400 mL: pressure rises sharply; urgent desire to void

1. Bladder fills → stretch receptors in the wall (especially posterior urethra) are activated
2. Afferent signals travel via pelvic nerves to the sacral spinal cord (S2-S4) and also ascend to the pontine micturition center (PMC)
3. Parasympathetic efferents fire → detrusor contracts via M3 receptors (ACh)
4. Simultaneously: sympathetic inhibited (detrusor relaxes its basal tone), internal sphincter opens (NO release)
5. Somatic pudendal inhibition → external sphincter relaxes
6. The reflex is self-regenerating - once initiated, stretch signals increase further, amplifying the contraction until voiding is complete
7. Voluntary control is exerted via higher cortical centres through the PMC - cortex can inhibit or facilitate the reflex

• Spinal cord lesion above sacral cord → automatic (uninhibited) bladder - reflexive voiding, no voluntary control
• Sacral cord/pelvic nerve lesion → autonomous (atonic) bladder - overflow incontinence, large residual urine

9. Clinical Correlations

Summary Table

• Guyton and Hall Textbook of Medical Physiology, Chapter 26
• Histology: A Text and Atlas (Gartner & Hiatt), Chapter on Urinary System
• Color Atlas of Human Anatomy, Vol. 2 - Internal Organs (Thieme)
• The Developing Human: Clinically Oriented Embryology (Moore & Persaud), Chapter 12

Urinary Bladder - Anatomy (MBBS 10 Marks)

1. Introduction

• Capacity: ~500 mL (normal); desire to void begins at ~300 mL
• Empty state: Tetrahedral, bowl-shaped, lies entirely in the pelvis
• Full state: Ovoid; apex rises above the pubic symphysis into the lower abdomen

2. Parts of the Urinary Bladder

3. Surfaces of the Urinary Bladder

1. Superior surface - covered by peritoneum; in contact with coils of small intestine and sigmoid colon
2. Two inferolateral surfaces - lie against the pelvic fascia and the obturator internus and levator ani muscles; no peritoneum covers these
3. Posterior surface (base/fundus) - directed backward; related to seminal vesicles, vas deferens, and rectum in males; uterus and vagina in females

4. Relations

In the Male:

In the Female:

5. Interior of the Bladder

Rugae (Mucosal Folds)

• The mucosa of most of the bladder forms irregular folds (rugae) when empty because it is loosely attached to the underlying muscle
• The rugae disappear completely when the bladder is distended

Trigone of the Bladder

• A smooth, triangular area on the base (fundus) of the bladder
• Bounded by three orifices:
  • Two ureteric orifices (superolateral corners) - where the ureters enter the bladder obliquely
  • Internal urethral orifice (inferior apex) - where the urethra begins
• The interureteric fold (bar of Mercier) is a raised ridge connecting the two ureteric orifices - this is the superior boundary of the trigone
• The mucosa of the trigone is always smooth and firmly attached to the underlying muscle - no rugae form here
• The trigone is derived from the mesonephric ducts (different embryological origin from rest of bladder)

Uvula of the Bladder (Male Only)

• A small conical elevation at the internal urethral orifice produced by the underlying median lobe of the prostate

6. Coats (Layers) of the Bladder Wall

a) Mucosa

• Lined by transitional epithelium (urothelium)
• Rests on lamina propria (loose connective tissue)
• Forms rugae everywhere except the trigone
• Trigone exception: no muscularis mucosae or submucosa - mucosa is flat and non-folding

b) Submucosa

• Loose areolar connective tissue
• Allows the mucosa to fold and glide over the muscular layer
• Absent at the trigone

c) Muscularis (Detrusor Muscle)

• Three layers of smooth muscle - inner longitudinal, middle circular, outer longitudinal
• The three layers are not distinct and interdigitate with each other (unlike the GI tract)
• Collectively called the detrusor muscle
• At the bladder neck, the circular fibers condense to form the internal urethral sphincter (involuntary smooth muscle)
• At the trigone, the musculature is a continuation of the smooth muscle of the ureters (only two layers)

d) Serosa / Adventitia

• Serosa (peritoneum): covers only the superior surface and the upper part of the posterior surface
• Adventitia (fibrous connective tissue): covers all other surfaces

7. Blood Supply

Arteries (all branches of the Internal Iliac Artery):

Veins:

• The vesical venous plexus surrounds the fundus and inferolateral surfaces
• Drains into the internal iliac veins
• In males, the vesical plexus is continuous with the prostatic venous plexus
• In females, it is continuous with the vaginal venous plexus

8. Lymphatic Drainage

9. Nerve Supply

10. Peritoneal Relations and Ligaments

• The peritoneum reflects from the anterior abdominal wall onto the superior surface of the bladder only, then passes backwards onto the uterus (female) or rectum (male), forming the vesicouterine pouch or rectovesical pouch respectively
• The anterior and lateral walls of the bladder are extraperitoneal
• Ligaments of the bladder:
  • Median umbilical ligament (urachus) - apex to umbilicus
  • Lateral umbilical ligaments (obliterated umbilical arteries) - lateral walls to umbilicus
  • Puboprostatic ligaments (male) / Pubovesical ligaments (female) - neck to pubis
  • Lateral ligaments of bladder - from posterolateral bladder to pelvic wall (contain the inferior vesical vessels)

11. Applied Anatomy (Clinical Points)

Quick Revision Summary

POSITION:     Lesser pelvis, behind pubic symphysis (full → rises above pubis)
CAPACITY:     ~500 mL; desire to void at ~300 mL
PARTS:        Apex, Body, Fundus (base), Neck
COATS:        Mucosa (TCC) → Submucosa → Detrusor muscle → Serosa/Adventitia
TRIGONE:      Between 2 ureteric orifices + internal urethral orifice; always smooth
ARTERIES:     Superior + Inferior vesical (branches of internal iliac artery)
VEINS:        Vesical venous plexus → Internal iliac veins
LYMPHATICS:   External iliac, internal iliac, obturator nodes
NERVES:       Para S2-S4 (micturition); Symp L1-L2 (storage); Pudendal S2-S4 (ext. sphincter)
PERITONEUM:   Covers only superior surface and upper posterior surface

• Color Atlas of Human Anatomy, Vol. 2 - Internal Organs (Thieme), p. 405
• Fischer's Mastery of Surgery, 8th ed., Chapter 281
• Sabiston Textbook of Surgery, 21st ed., Chapter on Bladder and Prostate
• Bailey & Love's Short Practice of Surgery, 28th ed., Applied Anatomy of the Bladder
• Histology: A Text and Atlas (Gartner & Hiatt), Chapter on Urinary System