Urinary bladder

The outside of the bladder is protected by a serous membrane.[7] The bladder wall itself is smooth muscle.[7] The inner side of the bladder is lined with a mucous membrane consisting of a surface glycocalyx that protects the cells beneath it from urine, the urothelium (a form of transitional epithelium), a basement membrane, and the lamina propria.[8][7][9] The mucosal lining also offers a urothelial barrier against the passing of infections.[10]

• 
  Vertical section of bladder wall
• 
  Layers of the urinary bladder wall and cross-section of the detrusor muscle
• 
  Anatomy of the male bladder, showing transitional epithelium and part of the wall in a histological cut-out

The detrusor muscle is a layer of the urinary bladder wall made of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles.

Stretch receptors in the bladder signal the parasympathetic nervous system to stimulate the muscarinic receptors in the detrusor to contract the muscle when the bladder is extended.[11] This encourages the bladder to expel urine through the urethra. The main receptor activated is the M3 receptor, although M2 receptors are also involved and whilst outnumbering the M3 receptors they are not so responsive.[12]

The main relaxant pathway is via the adenylyl cyclase cAMP pathway, activated via the β3 adrenergic receptors. The β2 adrenergic receptors are also present in the detrusor and even outnumber β3 receptors, but they do not have as important an effect in relaxing the detrusor smooth muscle.[13][14][15]

The detrusor muscle is unusually able to change its length. It can also contract for a long time whilst voiding, and it stays relaxed whilst the bladder is filling.[13] The urothelium and lamina propria (also termed the bladder mucosa) line the inside of the detrusor muscle, and can release regulators to influence general contractility of the detrusor muscle, maintain normal bladder tone, and potentially mediate bladder diseases such as overactive bladder.[16]

The bladder is supplied by the vesical arteries and drained by the vesical veins.[17] The superior vesical artery supplies blood to the upper part of the bladder. The lower part of the bladder is supplied by the inferior vesical artery in males and by the vaginal artery in females, both of which are branches of the internal iliac arteries.[17] In females, the uterine arteries provides additional blood supply.[17] Venous drainage begins in a network of small vessels on the lateral and posterior surfaces of the bladder, which coalesce and pass backwards along the lateral ligaments of the bladder into the internal iliac veins.[17]

The lymph drained from the bladder begins in a series of networks throughout the mucosal, muscular and serosal layers. These then form three sets of vessels: one set near the trigone draining the bottom of the bladder; one set draining the top of the bladder; and another set draining the outer undersurface of the bladder. The majority of these vessels drain into the external iliac lymph nodes.[17]

The bladder receives motor innervation from both sympathetic fibers, most of which arise from the superior and inferior hypogastric plexuses and nerves, and from parasympathetic fibers, which come from the pelvic splanchnic nerves.[18]

Sensation from the bladder is transmitted to the central nervous system (CNS) via general visceral afferent fibers (GVA). GVA fibers on the superior surface follow the course of the sympathetic efferent nerves back to the CNS, while GVA fibers on the inferior portion of the bladder follow the course of the parasympathetic efferents.[18]

For the urine to exit the bladder, both the autonomically controlled internal sphincter and the voluntarily controlled external sphincter must be opened. The internal sphincter is controlled via the hypogastric nerve, predominantly through the alpha-1 receptor; the external sphincter is controlled by the pudendal nerve.[15] Problems with these sphincters can lead to incontinence.[19]

The human urinary bladder derives from the urogenital sinus, and it is initially continuous with the allantois. The upper and lower parts of the bladder develop separately and join together around the middle part of development.[5] At this time the ureters move from the mesonephric ducts to the trigone.[5] In males, the base of the bladder lies between the rectum and the pubic symphysis. It is superior to the prostate, and separated from the rectum by the recto-vesical pouch. In females, the bladder sits inferior to the uterus and anterior to the vagina; thus its maximum capacity is lower than in males. It is separated from the uterus by the vesico-uterine pouch. In infants and young children the urinary bladder is in the abdomen even when empty.[20]

Calcifications on bladder wall caused by urinary schistosomiasis

Bacteriuria is the presence of bacteria in the urine which can indicate a urinary tract infection such as cystitis.

• Interstitial cystitis

Frequent urination can be due to excessive urine production, small bladder capacity, irritability or incomplete emptying. Males with an enlarged prostate urinate more frequently. One definition of an overactive bladder is when a person urinates more than eight times per day.[23] An overactive bladder can often cause urinary incontinence. Though both urinary frequency and volumes have been shown to have a circadian rhythm, meaning day and night cycles,[24] it is not entirely clear how these are disturbed in the overactive bladder. Urodynamic testing can help to explain the symptoms. An underactive bladder is the condition where there is a difficulty in passing urine and is the main symptom of a neurogenic bladder. Frequent urination at night may indicate the presence of bladder stones.

Disorders of or related to the bladder include:

• Bladder exstrophy
• Bladder sphincter dyssynergia, a condition in which the sufferer cannot coordinate relaxation of the urethra sphincter with the contraction of the bladder muscles
• Paruresis
• Trigonitis
• Urinary retention

Disorders of bladder function may be dealt with surgically, by re-directing the flow of urine or by replacement with an artificial urinary bladder. The volume of the bladder may be increased by bladder augmentation. An obstruction of the bladder neck may be severe enough to warrant surgery.

3D still showing bladder carcinoma

• Bladder cancer – the most common type is transitional cell carcinoma.[25]

A diverticulum of the bladder

Urodynamic testing can help to explain the symptoms.

In all reptiles, the urinogenital ducts and the anus both empty into an organ called a cloaca. In some reptiles, a midventral wall in the cloaca may open into a urinary bladder, but not all. It is present in all turtles and tortoises as well as most lizards but is lacking in the monitor lizard, the legless lizards. It is absent in the snakes, alligators, and crocodiles.[26]^{:p. 474}

Many turtles, tortoises, and lizards have proportionally very large bladders. Charles Darwin noted that the Galapagos tortoise had a bladder which could store up to 20% of its body weight.[27] Such adaptations are the result of environments such as remote islands and deserts where water is very scarce.[28]^:143 Other desert-dwelling reptiles have large bladders that can store a long-term reservoir of water for up to several months and aid in osmoregulation.[29]

Turtles have two or more accessory urinary bladders, located lateral to the neck of the urinary bladder and dorsal to the pubis, occupying a significant portion of their body cavity.[30] Their bladder is also usually bilobed with a left and right section. The right section is located under the liver, which prevents large stones from remaining in that side while the left section is more likely to have calculi.[31]

Most aquatic and semi-aquatic amphibians have a membranous skin which allows them to absorb water directly through it. Some semi-aquatic animals also have similarly permeable bladder membrane.[32] As a result, they tend to have high rates of urine production to offset this high water intake, and have urine which is low in dissolved salts. The urinary bladder assists such animals to retain salts. Some aquatic amphibian such as Xenopus do not reabsorb water, to prevent excessive water influx.[33] For land-dwelling amphibians, dehydration results in reduced urine output.[34]^{:p. 184}

The amphibian bladder is usually highly distensible and among some land-dwelling species of frogs and salamanders may account for between 20% and 50% of their total body weight.[34]^{:p. 184}

The gills of most teleost fish help to eliminate ammonia from the body, and fish live surrounded by water, but most still have a distinct bladder for storing waste fluid. The urinary bladder of teleosts is permeable to water, though this is less true for freshwater dwelling species than saltwater species.[27]^{:p. 219} 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.[35]

All mammals have a urinary bladder. This structure begins as an embryonic cloaca. In the vast majority, this eventually becomes differentiated into a dorsal part connected to the intestine and a ventral part which becomes associated with the urinogenital passage and urinary bladder. The only mammals in which this does not take place are the platypus and the spiny anteater both of which retain the cloaca into adulthood.[26]

The mammalian bladder is an organ that regularly stores a hyperosmotic concentration of urine. It therefore is relatively impermeable and has multiple epithelial layers. The urinary bladder of the cetaceans (whales and dolphins) is proportionally smaller than that of land-dwelling mammals.[36]

In nearly all bird species, there is no urinary bladder per se.[37] Although all birds have kidneys, the ureters open directly into a cloaca which serves as a reservoir for urine, fecal matter, and eggs.[38]

Unlike the urinary bladder of vertebrates, the urinary bladder of crustaceans both stores and modifies urine.[39] The bladder consists of two sets of lateral and central lobes. The central lobes sit near the digestive organs and the lateral lobes extend along the front and sides of the crustacean's body cavity.[39] The tissue of the bladder is thin epithelium.[39]