The bladder (from Old English blædre 'bladder, blister, pimple') is a hollow organ in humans and other vertebrates that stores urine from the kidneys.

In gross anatomy, the bladder can be divided into a broad fundus (base), a body, an apex, and a neck.

[5] The apex (also called the vertex) is directed forward toward the upper part of the pubic symphysis, and from there the median umbilical ligament continues upward on the back of the anterior abdominal wall to the umbilicus.

The neck of the bladder is the area at the base of the trigone that surrounds the internal urethral orifice that leads to the urethra.

These ureteric openings have mucosal flaps in front of them that act as valves in preventing the backflow of urine into the ureters,[6] known as vesicoureteral reflux.

[7] It is an area of smooth tissue for the easy flow of urine into and from this part of the bladder - in contrast to the irregular surface formed by the rugae.

The middle lobe of the prostate causes an elevation in the mucous membrane behind the internal urethral orifice called the uvula of urinary bladder.

In males, it lies in front of the rectum, separated by the rectovesical pouch, and is supported by fibres of the levator ani and of the prostate gland.

In females, it lies in front of the uterus, separated by the vesicouterine pouch, and is supported by the elevator ani and the upper part of the vagina.

[11] The lymph drained from the bladder begins in a series of networks throughout the mucosal, muscular and serosal layers.

[11] The bladder receives both sensory and motor supply from sympathetic and the parasympathetic nervous systems.

[12] Sensation from the bladder, relating to distension or to irritation (such as by infection or a stone) is transmitted primarily through the parasympathetic nervous system.

[11] When viewed under a microscope, the bladder can be seen to have an inner lining (called epithelium), three layers of muscle fibres, and an outer adventitia.

[8] Additionally, these are lined with a mucous membrane consisting of a surface glycocalyx that protects the cells beneath it from urine.

This, over the fourth to the seventh week, divides into a urogenital sinus and the beginnings of the anal canal, with a wall forming between these two inpouchings called the urorectal septum.

In females, the bladder sits inferior to the uterus and anterior to the vagina; thus its maximum capacity is lower than in males.

[25] Other risk factors include other causes of blockage or narrowing, such as prostate cancer or the presence of vesico-ureteric reflux; the presence of outside structures in the urinary tract, such as urinary catheters; and neurologic problems that make passing urine difficult.

[25] When a urinary tract infection or cystitis is suspected, a medical practitioner may request a urine sample.

Though both urinary frequency and volumes have been shown to have a circadian rhythm, meaning day and night cycles,[29] it is not entirely clear how these are disturbed in the overactive bladder.

[33] When cancer is present, the most common symptom in an affected person is blood in the urine; a physical medical examination may be otherwise normal, except in late disease.

[33] Investigations performed usually include collecting a sample of urine for an inspection for malignant cells under a microscope, called cytology, as well as medical imaging by a CT urogram or ultrasound.

Cancer present only in the bladder may be removed surgically via cystoscopy; an injection of the chemotherapeutic mitomycin C may be performed at the same time.

[33] Cancers that are high grade may be treated with an injection of the BCG vaccine into the bladder wall, and may require surgical removal if it does not resolve.

[38] Other desert-dwelling reptiles have large bladders, which can hold long-term reserves of water for several months and aid in osmoregulation.

[44] The amphibian bladder is usually highly distensible; among some land-dwelling species of frogs and salamanders, it may account for 20%–50% of total body weight.

[46] The urinary bladders of fish and tetrapods are thought to be analogous while the former's swim-bladders and latter's lungs are considered homologous.

Most fish also have an organ called a swim-bladder which is unrelated to the urinary bladder except in its membranous nature.

The loaches, pilchards, and herrings are among the few types of fish in which a urinary bladder is poorly developed.

It is largest in those fish which lack an air bladder, and is situated in front of the oviducts and behind the rectum.

[48] Although all birds have kidneys, the ureters open directly into a cloaca which serves as a reservoir for urine, fecal matter, and eggs.