The internal aspect of the base of the urinary bladder features a smooth, inverted triangular-shaped area known as the trigone. This area does not contain rugae, and here, the muscular layer is firmly fused with the mucosa of the bladder.

Each corner of the trigone contains an opening. In total, the urinary bladder has three openings:

The upper two openings are found at the corners (posterolateral angles) of the base of the triangle. They serve as entry sites for both ureters and are known as ureteric orifices. Both ureteric orifices are connected by a longitudinal mucosal ridge called the interureteric crest or fold. When the bladder is empty, the ureteric orifices are about 0.98 inches (2.5 cm) apart from each other. In contrast, when the bladder is full, the distance between both goes up to 1.97 inches (5 cm). The ureters enter the bladder in an oblique manner.
The lower opening at the apex (inferior angle) of the trigone is known as the internal urethral orifice. It leads into the urethra.

Above the internal urethral orifice, the trigone presents with a small, elevated area that is called the uvula. It is formed by a slight protrusion of the underlying tissue. In males, it is formed by the median lobe of the prostate that protrudes into the trigone. The uvula is more prominent in older males due to benign prostatic hyperplasia (enlargement of the prostate).

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Trigone and openings of urinary bladder