Urology & Urogynecology > Education and Reference

Even though the bladder is often illustrated as a round “balloon,” it is actually an oval, flattened at the top. The abdominal contents press downward on the bladder, causing it to flatten as it empties.

The bladder muscle wall (detrusor) is made of many, thin layers of smooth muscle cells.

Smooth muscle contracts slowly, as the cells bunch together.  Therefore, sharp pressure spikes on the vesical trace of a urodynamic procedure are never caused by detrusor contraction.  Spikes are typically caused by external forces (sudden change in intra-abdominal pressure (coughing, laughing, and movement) equipment artifact (tapping on tubing, electrical spikes, cell phones, etc) or change in catheter position.

The smooth muscles are layered in different directions, contributing to the bladder’s

• impressive elasticity (compliance and capacity)
• ability to sustain contraction of the detrusor during voiding

As an aside, women who need hormone replacement also tend to experience a higher degree of urgency because of deterioration of the mucous lining of the bladder.

The urethral wall is composed of a series of muscle layers with a mucosal lining. The innermost muscle layer is smooth muscle, the outer layers are strieated muscle, predominately at the area of the internal urethral sphincter (bladder neck). Adequate blood supply and mucous production are important to the function of the urethra.

Consideration for hormone replacement therapy (HRT) is important for aging women. If the vagina is dry, so is the urethra.  The stellate structure of the urethra depends upon mucous lubrication to increase the surface tension and keep it closed.

The internal urethral sphincter is composed of a layer of smooth muscle, surrounded by layers of striated muscle.  Striated muscles can contract quickly and strongly (to resist leakage during a cough), but tire easily without proper training. These muscles relax just prior to urination. The combined action of the contraction of the bladder’s smooth muscles and the relaxation of these striated muscles causes the bladder neck to raise and open and the bladder empties.

The external urethral sphincter muscles are :

• Composed of striated muscle.
• Structured and placed differently in males and females.
  

Female

Male