Urinary Bladder - Anatomy & Physiology

()Map URINARY SYSTEM (Map)

The bladder is where urine is stored before being expelled by the body through the micturition reflex.
Without a bladder urinary continence would be impossible.

Anatomy

Histology cross section of a normal guinea pig bladder (© RVC 2008)

It is a hollow, muscular organ
It is divided for descriptive purposes into three parts
- Cranial Pole
- Intermediate body
- Caudal neck
Its wall comprises a muscle layer covered in a transitional epithelial layer.
Its size and posistion are determined by how full it is.
- When empty the bladder wall is wrinkled and thicker
  - It rests on the pubic bones
  - It is entirely in the pelvis of the large species but partly enters the abdomen of the carnivores
  - It is largely retroperiotneal in the larger species
- When full and distended the folds dissapear and the wall appears thinner
  - It then becomes intraperitioneal in the larger species
The paired uteric folds are visible even when the bladder is full.
- They extend from the ureteral opening to the neck of the bladder
- Here they merge
- Forms the urethral crest - continues into the urethra

Muscles of the Bladder

The three muscular components of the bladder described below play a pivotal part in the micturition reflex.

Detrusor Muscle

Network of smooth muscle fibres within the bladder wall which are supplied by both parasympathetic and sympathetic nerves.

Parasympathetic Supply - Detrusor Muscle

S1-S3
Synapse in pelvic plexus or bladder wall
Innvervate the detrusor muscle
Action - excitatory
Function - empty bladder

Sympathetic Supply - Detrusor Muscle

L1-L4
Syanpse in caudal mesenteric ganglion - bladder wall
Receptor - beta
Inhibitory action
Allows bladder filling

Internal Urethral Sphincter

Its a thickening of the bladder musculature found at the neck of the bladder which is continous with the detrusor and is therefore smooth muscle. However unlike the detrusor its innervation is purely from sympathetic fibres.

Sympathetic Supply - Internal Urethral Sphincter

L1-L4
Synapse in caudal mesenteric ganglion
Receptor - alpha
Excitatory action
Function - retain urine through increased urethral tone

External Urethral Sphincter

Unlike the internal sphincter the external sphincter is composed of striated muscle fibres which are under the voluntary control of the somatic nervous system

Somatic Motor Supply - External Urethral Sphincter

S1-S2
Pudendal Nerve
No synapse
Innervates the urethral skeletal muscle - external urethral sphincter
Function is to retain urine

The Ligaments of the Bladder

Two lateral ligaments
- Insert in the dorsal abdominal wall
- Within them are the residual umbilical vessels

Median ligament
- Connects the bladder to pelvic floor and linea alba

Blood Supply, Innervation and Lymphatic Drainage

Blood Supply - Caudal vesicular artery

Sympathetic Innervation - Via hypogastric nerve from the caudal mesenteric ganglion

Parasympathetic Innervation - Pudendal nerve

Lymphatic Drainage - Iliosacral lymph nodes