The urinary bladder contains a specialized, triangular region at its base known as the trigone. This structure is located at the base of the bladder and is integral to the urinary system’s function. It is defined by three points: the two openings where the ureters connect to the bladder and the single opening that leads to the urethra. The trigone has a unique cellular composition and sensitivity that distinguishes it from the rest of the bladder. This allows it to perform specific roles related to storing and expelling urine.
Anatomy and Location of the Trigone
The trigone is a smooth, triangular patch of tissue situated on the inner, lower, posterior wall of the urinary bladder. Its three corners are marked by openings. The top two points are the ureteric orifices, where the ureters enter the bladder. These two openings are connected by a raised ridge of tissue called the interureteric crest. The third point, at the bottom apex of the triangle, is the internal urethral orifice, which is the entrance to the urethra.
A defining characteristic of the trigone is its smooth surface. This contrasts sharply with the rest of the bladder’s inner wall, which is lined with folds known as rugae. These folds allow the bladder to expand as it fills with urine. The trigone, however, remains fixed in position and relatively taut, providing a stable platform at the bladder base.
The tissue composition of the trigone also sets it apart. While the bulk of the bladder wall is made of detrusor muscle, the trigone has a distinct embryological origin. It possesses two distinct muscle layers: a superficial layer that connects with the musculature of the urethra and a deep layer that integrates with the bladder’s main detrusor muscle. This muscular arrangement is related to its functions in the urinary cycle.
The Role of the Trigone in Urinary Function
The trigone is highly sensitive to stretch. As the bladder fills with urine, the trigone is one of the first areas to expand. This stretching activates nerve receptors within its tissue, which then send signals to the brain. These signals are interpreted as the sensation of bladder fullness and create the initial urge to urinate, which intensifies as the bladder continues to fill.
Beyond signaling, the trigone acts as a functional funnel during urination, a process also known as micturition. When the bladder contracts to expel urine, the fixed, smooth shape of the trigone helps guide the flow of urine directly toward the internal urethral orifice. The muscular structure of the trigone assists in this process by helping to open and lower the bladder neck, reducing resistance for outflow.
Another function of the trigone is preventing the backward flow of urine into the ureters, a condition called vesicoureteral reflux. The ureters pass through the bladder wall at an oblique angle. When the bladder contracts during urination, the pressure inside the bladder compresses these tubes at their entry point. The trigone’s muscular structure anchors and supports this junction, ensuring the openings seal shut.
Common Conditions Affecting the Trigone
Inflammation of the trigone, known as trigonitis, is a common condition that causes discomfort. Symptoms often mimic those of a urinary tract infection (UTI), including increased urinary frequency, urgency, and pain in the pelvic region. However, in cases of trigonitis, a urine culture may not show the presence of bacteria. The condition is often associated with Bladder Pain Syndrome/Interstitial Cystitis (BPS/IC), and for many individuals with BPS/IC, inflammation of the trigone is a prominent finding during a cystoscopy.
The trigone is also a location where bladder cancers can develop. Its unique cellular makeup and constant exposure to urine, which may contain carcinogens, contribute to this risk. Symptoms include blood in the urine (hematuria), persistent UTI-like symptoms without a confirmed infection, and changes in urinary habits. Because of its specific tissue characteristics, some types of bladder cancer are more likely to arise in this region.
Diagnosing conditions related to the trigone often requires a direct visual inspection of the bladder lining. A cystoscopy allows a urologist to view the trigone’s surface, identify any abnormalities like inflammation or suspicious growths, and take tissue samples (biopsies) if necessary. Persistent symptoms that do not resolve with standard treatments for UTIs may lead a healthcare provider to suspect an underlying issue with the trigone, prompting further investigation.