The bladder wall is composed of four layers:

Mucosa (mucosal membrane) - the inner layer that is lined with transitional epithelium; the mucosa of an empty bladder forms many visible folds (rugae) as the cells are larger and rounder than when the bladder is full; cells change their shape and size depending on the amount of urine in the bladder; the trigone of the bladder does not contain folds;
Submucosa - loose connective tissue layer beneath the mucosa, connects the muscular layer with the mucosal membrane;
Muscular layer - composed of three layers of smooth muscle cells; the inner and outer layers are longitudinal, while the middle is circular; the middle layer around the inner opening of the urethra forms the inner or internal urethral sphincter; the muscular layer of the urinary bladder is named the detrusor muscle;
Serosa / adventitia - serosa is the outer fibrous layer lined with simple squamous epithelium, which covers the bladder from the front, as well as its fundus part (when it is empty); the posterior and lateral walls of the bladder are covered by the adventitia - formed by loose connective tissue and is covered by the peritoneum.

There are three openings present on the lower part of the urinary bladder, all close to each other. The upper two are the openings of the ureters - ureteral openings or orifices, while the lower is the inner opening of the urethra - internal urethral orifice.

Both ureteral orifices are connected by the interuretric fold.

A smooth triangle called trigone forms on the bladder wall between these three openings. In the trigone the muscular layer is strongly fused with the mucosa of the bladder.

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Internal structure of bladder