Bladder

In anatomy, the urinary bladder is a hollow, muscular, and distensible (or elastic) organ that sits on the pelvic floor in mammals. It is the organ that collects urine excreted by the kidneys prior to disposal by urination. Urine enters the bladder via the ureters and exits via the urethra.

In males, the bladder is superior to the prostate, and separated from the rectum by the rectovesical excavation.

In females, the bladder is separated from the rectum by the rectouterine excavation, and it is separated from the uterus by the vesicouterine excavation.

Regions

 * Trigone of urinary bladder: The ureters enter the bladder diagonally from its dorsolateral floor in an area called the trigone, which is a triangular shaped area on the postero-inferior wall of the bladder. The urethra exits at the lowest point of the triangle of the trigone.
 * Apex: The Median umbilical ligament connects to the apex of the bladder.
 * Neck: The Neck is connected to the pubic bone by the pubovesical ligament in women, and by the puboprostatic ligament in men.

Wall
The wall of the urinary bladder consists of three layers:
 * Mucosa: in this instance transitional epithelium & lamina propria
 * Detrusor muscle: consists of an inner and outer longitudinal layer and a middle circular layer of smooth muscle
 * a fibrous adventitia and the visceral peritoneum: lie on superior surface

When the urinary bladder is relaxed, the epithelium is 5-6 cell layers thick and the superficial cells bulge into the lumen. Thick rigid areas of the plasma membrane fold down within the cytoplasm are called fusiform vesicles.

When the urinary bladder is extended, the epithelium is 3-4 layers thick and has squamous epithelial cells without vesicles. The vesicles have been reinserted in the plasma membrane.

Detrusor muscle
The detrusor muscle is a layer of the urinary bladder wall made of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles. When the bladder is stretched, this signals the parasympathetic nervous system to contract the detrusor muscle. This encourages the bladder to expel urine through the urethra.

For the urine to exit the bladder, both the autonomically controlled internal sphincter and the voluntarily controlled external sphincter must be opened. Problems with these muscles can lead to incontinence.

The urinary bladder usually holds 400–620 mL of urine, but it can hold twice this without rupturing if, for example, the outflow is obstructed.

The desire to urinate usually starts when the bladder reaches around 75% of its working volume. If the subject is distracted the desire can fade and return with more urgency as the bladder continues to fill.

Fundus
The fundus of the urinary bladder is the bottom of the bladder. It is lymphatically drained by the external iliac lymph nodes.

Clinical Significance

 * Cystitis cystica is a chronic cystitis glandularis accompanied by the formation of cysts. This disease can cause chronic urinary tract infections.
 * Interstitial cystitis is a bladder disease of unknown etiology characterized by a bladder wall infiltrated by inflammatory cells resulting in ulcerated mucosa and scarring, spasm of the detrusor muscle, hematuria, urgency, increased frequency, and pain on urination.
 * The bladder may rupture if overfilled and not emptied. This can occur in the case of binge drinkers who have consumed large quantities, but are not conscious due to stupor of the need to urinate. This condition is very rare in women, but does occur. Symptoms include localized pain and uraemia (poisoning due to reabsorbed waste).