The bladder trigone is a specialized, triangular region located at the base of the urinary bladder. It serves as a junction for the flow of urine into and out of the organ, defined by the openings of the urinary tract’s main conduits. Its unique structure and tissue composition allow it to perform functions distinct from the rest of the bladder, providing insight into its importance in urinary health and control.
Unique Anatomical Structure
The trigone is a smooth, triangular patch of tissue situated on the floor of the bladder, just above the opening to the urethra. The three points of this triangle are formed by the internal openings of the urinary tract.
The two upper corners mark the entry points of the ureters, which carry urine from the kidneys. The lower corner is the internal urethral orifice, where urine exits the bladder.
The tissue lining the trigone is unique compared to the surrounding bladder wall, known as the detrusor muscle. The detrusor muscle is composed of interwoven smooth muscle fibers that allow the bladder to stretch dramatically as it fills with urine. Unlike the highly distensible detrusor, the trigone is relatively fixed and non-distensible; it does not stretch significantly as bladder volume increases.
This fixed nature results from the trigone’s muscular composition, which includes fibers from the ureters’ sheaths and the detrusor muscle, creating a more organized and less elastic structure. The mucosal lining remains smooth, even when the rest of the bladder wall collapses into folds (rugae) when empty. This distinct anatomical arrangement provides a stable platform for the ureteral and urethral openings, regulating urine flow.
Function in Urinary Flow Control
The primary function of the bladder trigone is to manage urine flow, specifically by preventing vesicoureteral reflux (VUR). VUR is the abnormal backflow of urine from the bladder up into the ureters and toward the kidneys. This backflow can lead to infection and potential kidney damage. The trigone is the center of this anti-reflux mechanism.
The ureters enter the bladder wall at an oblique angle, creating a tunnel-like passage through the detrusor muscle and across the trigone. As the bladder fills, increasing pressure inside naturally compresses this oblique segment against the firm trigonal muscle. This passive compression acts like a valve, closing the ureteral opening and stopping backflow.
During micturition (urination), the trigone’s musculature actively supports the anti-reflux mechanism. The muscle fibers of the trigone, which are continuous with the ureteral muscle layers, contract and shorten the intramural ureter.
This contraction, coordinated with the detrusor muscle contraction, helps pull the ureteral openings closed. This maintains the integrity of the valve, ensuring urine is expelled only through the urethra.
Common Health Issues Involving the Trigone
The trigone’s specialized location and tissue make it susceptible to specific inflammatory and structural conditions that impact urinary function. One common issue is localized inflammation, often referred to as trigonitis. Trigonitis is a form of chronic cystitis that causes severe irritation and can mimic the symptoms of a typical urinary tract infection (UTI).
When the trigone becomes inflamed, it is often hypersensitive, leading to irritative voiding symptoms. Patients frequently experience a persistent, urgent need to urinate, increased frequency, and burning or painful urination (dysuria).
In some cases, inflammation is associated with squamous metaplasia, a cellular change where the lining cells transform, sometimes linked to hormonal changes in women. The pain from trigonitis is often felt deep in the pelvis and can radiate down the urethra.
Another major concern is the failure of the anti-reflux mechanism, resulting in vesicoureteral reflux (VUR). This condition occurs when the anatomical relationship between the ureter and the trigone is compromised, often due to a shortened intramural ureter tunnel. If the ureter enters the bladder too directly, rising bladder pressure cannot effectively compress the passage, allowing urine to flow backward.
VUR is a serious condition, especially in children, as it facilitates the transport of bacteria from the bladder up to the kidneys. This leads to recurrent kidney infections and potentially causes progressive scarring and long-term kidney damage. Proper development and function of the trigone are necessary for preventing this upward flow and maintaining the health of the upper urinary tract.