The empty bladder is a pyramidal-shaped organ which lies entirely within the pelvic cavity. Upon filling (capacity approximately 500 ml) the bladder assumes a more ovoid shape, rises out of the pelvis and separates the peritoneum from the anterior abdominal wall. The bladder has an apex (anteriorly), a base (posteriorly), a superior surface (the dome) and two inferolateral surfaces. The apex is anchored to the anterior abdominal wall by the urachus, a fibrous embryological remnant which, during development, connects the bladder to the allantois. The superior bladder surface is covered by peritoneum, which reflects onto the anterior abdominal and lateral pelvic walls. The base is triangular in shape, limited superolaterally by the entrances of the ureters into the bladder and inferiorly by the urethral orifice. The area immediately adjacent to the urethral orifice is known as the bladder neck. The seminal vesicles and vasa deferentia lie immediately posterior to the bladder base (Figure 29.1).

The mucosal surface lining the bladder base is known as the trigone. It is distinct in that, because of firm adherence to the underlying muscle coat, its surface is always smooth, in contrast to the remainder of the mucosa which, when the bladder is empty, assumes an undulated appearance.

The bladder is lined by transitional epithelium or urothelium, usually six cell layers thick. This rests on a thick layer of fibroelastotic connective tissue, allowing considerable distention. Below this is the ill-defined muscularis mucosae, composed of wispy irregular bundles of smooth muscle. The main muscular coat of the bladder, the muscularis propria or detrusor muscle, is composed of interlacing bundles of larger smooth muscle fibres loosely arranged into inner longitudinal, middle circular and outer longitudinal layers. At the bladder neck the circular layer is thickened, forming a preprostatic sphincter which is responsible for maintaining urinary continence. This muscle is richly ennervated by sympathetic nerve fibres.

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