Internal Urethral Orifice

1. Overview

The internal urethral orifice is the anatomical opening located at the junction between the urinary bladder and the urethra. It serves as the gateway for urine to exit the bladder and enter the urethra during the act of micturition. Although relatively small and simple in structure, the internal urethral orifice plays a vital role in urinary continence and is involved in coordinated voiding mechanisms.

2. Location

The internal urethral orifice is located in the bladder neck, at the inferior aspect of the trigone of the bladder, where the bladder transitions into the urethra. Its position differs slightly between sexes:

• In males: It lies just superior to the prostate gland and marks the start of the prostatic urethra.

• In females: It marks the beginning of the short female urethra and is located in the pelvic floor anterior to the vagina.

It lies anterior to the internal ureteric orifices and is one of the three points forming the vesical trigone.

3. Structure

The internal urethral orifice is a rounded or crescent-shaped aperture in the mucosa of the bladder, with surrounding smooth muscle fibers forming part of the internal urethral sphincter. Key components include:

• Mucosa: Lined by transitional epithelium (urothelium), continuous with the bladder mucosa above and urethral mucosa below.

• Muscle layers:

  • Detrusor muscle fibers: Surround the bladder and contribute to micturition.

  • Internal urethral sphincter (smooth muscle): Encircles the orifice and provides involuntary control.

• Autonomic innervation: Sympathetic fibers keep the orifice closed during urine storage; parasympathetic fibers mediate opening during voiding.

4. Function

The internal urethral orifice serves as the regulated outlet for urine from the bladder to the urethra. Its functions include:

• Controlled urine release: Opens during micturition and remains closed during bladder filling.

• Continence preservation: Part of the passive resistance preventing involuntary leakage.

• Fluid direction: Channels urine flow smoothly into the urethra, reducing turbulence and preventing backflow.

5. Physiological Role(s)

The internal urethral orifice is involved in several key physiological mechanisms:

• Storage phase: During bladder filling, sympathetic innervation stimulates the internal sphincter to stay contracted, keeping the orifice closed.

• Voiding phase: Parasympathetic activation causes detrusor contraction and internal sphincter relaxation, allowing the orifice to open.

• Ejaculatory function (in males): During ejaculation, the internal urethral orifice is closed to prevent retrograde flow of semen into the bladder.

6. Clinical Significance

Bladder Outlet Obstruction (BOO)

Hypertrophy or dysfunction of the internal sphincter at the internal urethral orifice can lead to bladder outlet obstruction, commonly seen in:

• Benign prostatic hyperplasia (BPH)

• Urethral stricture

• Neurogenic bladder

This can result in hesitancy, weak stream, incomplete emptying, and increased post-void residual volume.

Urinary Incontinence

Failure of closure at the internal urethral orifice, particularly due to sphincteric weakness, may contribute to urge incontinence or overflow incontinence. Conditions include:

• Detrusor-sphincter dyssynergia

• Post-surgical incontinence (e.g., after prostatectomy)

Retrograde Ejaculation (in Males)

Damage to the sympathetic nerves that control closure of the internal urethral orifice (e.g., after prostate surgery) can result in retrograde ejaculation, where semen flows into the bladder rather than out through the urethra.

Surgical Landmark

In urological and gynecological surgeries, the internal urethral orifice serves as an important landmark when reconstructing or accessing the bladder neck. Careful preservation is crucial to maintain continence and normal urinary flow.

Imaging and Endoscopy

The internal urethral orifice can be visualized during:

• Cystoscopy: Appears as a small, rounded slit at the base of the bladder.

• Voiding cystourethrogram (VCUG): Useful for evaluating function, reflux, or obstruction at the level of the bladder outlet.