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the organ in which urine accumulates before being excreted from the body in certain invertebrates, in most vertebrates, and in man.
The urinary bladder takes several forms in invertebrates: rotifers have an unpaired bladder, oligochaetes and hirudineans (leeches) have multiple bladders, and higher crustaceans have paired bladders. In the vertebrates, the urinary bladder is usually unpaired. In female elasmobranchs, paired urinary bladders, or urinary sinuses, occur as widenings of the ureters; these sinuses merge in the posterior end of the body into an unpaired urinary sinus that opens into the cloaca. In most teleosts, the urinary bladder occurs as a widening of the ureters that merge into a terminal unpaired duct; this duct can have its own external orifice, or it can share an orifice with the terminal sections of the gonaducts in males of certain species.
In most terrestrial vertebrates, the urinary bladder is formed from the embryonic membrane called the allantois; amphibians are the only class in which the bladder arises from a ventral diverticulum of the cloaca. In amphibians, reptiles, and monotremes, the urinary bladder opens into the cloaca opposite the cloacal openings of the ureters. In metatherians (marsupials) and eutherians (placental mammals), the ureters empty directly into the urinary bladder, which in turn is usually connected to the exterior by the urethra. The ureters empty into the urogenital sinus only in the females of certain animals. Birds have no urinary bladder, and crocodilians and Serpentes (snakes) are the only reptiles in which this structure is rudimentary.
In man, the urinary bladder is a hollow, muscular organ situated in the lesser pelvis. Its average capacity is 500 cc, but when distended, it can hold much more urine. The bladder’s walls are lined with mucosa. Urine passes through the ureters from the kidneys and is deposited into the urinary bladder every 10–20 seconds. The neck of the urinary bladder opens into the first section of the urethra; ring-shaped muscles are located in the neck and form two sphincters that surround and close the urethra. Contraction of the sphincters prevents the urine from flowing out of the bladder. When the bladder is full, neural elements in the bladder wall elicit reflex contractions of the bladder muscles, as well as relaxation of the sphincteral muscles; these simultaneous processes result in the outflow of urine through the urethra.
In males, the posterior wall of the bladder is adjacent to the rectum; the neck of the bladder and the first section of the urethra are surrounded by the prostate gland. Diseases of the prostate can interfere with and even prevent urination. In females, the posterior wall of the bladder is adjacent to the uterine cervix and to the vagina.
The mucous membrane of the bladder is extremely susceptible to infection. The short length of the female urethra facilitates the spread of infections from the genitalia to the bladder. In males, the bladder can become infected because of an impairment in the excretory function, as occurs in diseases of the prostate.