Urinary bladder

The urinary bladder is a smooth, collapsible, muscular sac that stores urine temporarily, It is located retroperitoneally on the pelvic floor just posterior to the pubic symphisis. The prostate (part of the male  reproductive system) lies inferior to the bladder neck, which empties into the urethra. In females, the bladder is anterior to the vagina and uterus.

The inferior of the bladder has openings for both ureters and the urethra. The smooth, triangular region of the bladder base outlined by these three openings is the trigone (trigone=triangle), important clinically because infection tend to persist in this region.

The bladder wall has three layers: a mucosa containing transitional epithelium, a thick muscular layer, and a fibrous adventitia (except  on its superior surface, where it is covered by the peritoneum). The muscular layer, called the destrusor muscle, consist of intermingled smooth muscle fibers arranged in inner and outer longitudinal layers and a middle circular layer.

The bladder is very distensible and uniquely suited for its function of urine storage. When empty, the bladder collapses into its basic pyramidal shape and its wall are thick and thrown into folds (rugae). as urine accumulates, the bladder expands, becomes pear shaped, and rises superiorly in the abdominal cavity. The muscular wall stretches and thins, and rugae dissapear. These changes allow the bladder to store more urine without a significant  rise in internal pressure.

A moderately full bladder is about 12 cm (5 inches) long and holds approximately 500 ml (1 pint) of urine, but it can hold nearly double that if necessary. When tense with urine, it can be palpated well above the pubic symphysis. The maximum capacity of the bladder is 800-1000 ml and when it is overdistended, it may burst. Although urine is formed continuosly by the kidneys, it is usually stored in the bladder until its release is convenient.