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| A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
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urinary bladder
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The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae. The rugae and transitional epithelium allow the bladder to expand as it fills.
The second layer in the walls is the submucosa that supports the mucous membrane. It is composed of connective tissue with elastic fibers.
The next layer is the muscularis, which is composed of smooth muscle. The smooth muscle fibers are interwoven in all directions and collectively these are called the detrusor muscle. Contraction of this muscle expels urine from the bladder. On the superior surface, the outer layer of the bladder wall is parietal peritoneum. In all other regions, the outer layer is fibrous connective tissue.
There is a triangular area, called the trigone, formed by three openings in the floor of the urinary bladder. Two of the openings are from the ureters and form the base of the trigone. Small flaps of mucosa cover these openings and act as valves that allow urine to enter the bladder but prevent it from backing up from the bladder into the ureters. The third opening, at the apex of the trigone, is the opening into the urethra. A band of the detrusor muscle encircles this opening to form the internal urethral sphincter.
Related entries
• urinary system
• bladder cancer
• bladder cancer treatment
Related category
• ANATOMY AND PHYSIOLOGY
Source: US National Cancer Institute
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