Bladder

The bladder () is a hollow organ in humans and other vertebrates that stores urine from the kidneys. In placental mammals, urine enters the bladder via the ureters and exits via the urethra during urination. In humans, the bladder is a distensible organ that sits on the pelvic floor. The typical adult human bladder will hold between 300 and (10 and ) before the urge to empty occurs, but can hold considerably more.

The Latin phrase for "urinary bladder" is vesica urinaria, and the term vesical or prefix vesico- appear in connection with associated structures such as vesical veins. The modern Latin word for "bladder" – cystis – appears in associated terms such as cystitis (inflammation of the bladder).

Structure

thumb|Male and female urinary bladders in lateral cross-section

In humans, the bladder is a hollow muscular organ situated at the base of the pelvis. <!-- Shape -->In gross anatomy, the bladder can be divided into a broad (base), a body, an apex, and a neck. In males, the neck of the urinary bladder is next to the prostate gland.

<!-- Openings -->The bladder has three openings. The two ureters enter the bladder at ureteric orifices, and the urethra enters at the trigone of the bladder. These ureteric openings have mucosal flaps in front of them that act as valves in preventing the backflow of urine into the ureters, known as vesicoureteral reflux. Between the two ureteric openings is a raised area of tissue called the interureteric crest. 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.

<!-- Walls -->The walls of the bladder have a series of ridges, thick mucosal folds known as rugae that allow for the expansion of the bladder. The detrusor muscle is the muscular layer of the wall made of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles. The wall of the urinary bladder is normally 3–5&nbsp;mm thick.

Blood and lymph supply

The bladder receives blood by the vesical arteries and drained into a network of vesical veins.

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.

The inner wall of the bladder is called urothelium, a type of transitional epithelium formed by three to six layers of cells; the cells may become more cuboidal or flatter depending on whether the bladder is empty or full. The epithelium lies on a thin basement membrane, and a lamina propria.

These layers are surrounded by three layers of muscle fibres arranged as an inner layer of fibres orientated longitudinally, a middle layer of circular fibres, and an outermost layer of longitudinal fibres; these form the detrusor muscle, which can be seen with the naked eye.

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File:Gray1141.png|Vertical section of bladder wall

File:Urinary bladder.JPG|Layers of the bladder wall and cross-section of the detrusor muscle

File:2605 The Bladder.jpg|Anatomy of the male bladder, showing transitional epithelium and part of the wall in a histological cut-out

</gallery>

Development

In the developing embryo, at the hind end lies a cloaca. 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. The urogenital sinus divides into three parts, with the upper and largest part becoming the bladder; the middle part becoming the urethra, and the lower part changes depending on the sex of the embryo.

Function

thumb|Female urinary bladderUrine is excreted by the kidneys and flows into the bladder through the ureters, where it is stored until urination (micturition). Urination involves coordinated muscle changes involving a reflex based in the spine, with higher inputs from the brain.

The urge to pass urine stems from stretch receptors that activate when between 300 and 400 mL urine is held within the bladder. Urination is controlled by the pontine micturition center in the brainstem.

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 distended. 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.

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.

Clinical significance

Inflammation and infection

thumb|right|260px|Calcifications on bladder wall caused by urinary [[schistosomiasis]]

Cystitis refers to infection or inflammation of the bladder. It commonly occurs as part of a urinary tract infection. In adults, it is more common in women than men, owing to a shorter urethra. It is common in males during early childhood, and in older men where an enlarged prostate may cause urinary retention. 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. Infections that involve the bladder can cause pain in the lower abdomen (above the pubic symphysis, so called "suprapubic" pain), particularly before and after passing urine, and a desire to pass urine frequently and with little warning (urinary urgency). Infections are usually due to bacteria, of which the most common is E coli.

When a urinary tract infection or cystitis is suspected, a medical practitioner may request a urine sample. A dipstick placed in the urine may be used to see if the urine has white blood cells, or the presence of nitrates which may indicate an infection. The urine specimen may be also sent for microbial culture and sensitivity to assess if a particular bacteria grows in the urine, and identify its antibiotic sensitivities. Sometimes, additional investigations may be requested. These might include testing the function of the kidneys by assessing electrolytes and creatinine; investigating for blockages or narrowing of the renal tract with an ultrasound, and testing for an enlarged prostate with a digital rectal examination.

Urinary tract infections or cystitis are treated with antibiotics, many of which are consumed by mouth. Serious infections may require treatment with intravenous antibiotics.

Interstitial cystitis refers to a condition in which the bladder is infected due to a cause that is not bacteria.

Incontinence and retention

thumb|Urinary bladder (black butterfly-like shape) and hyperplastic [[prostate (BPH) visualized by medical ultrasound]]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. 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, 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
• underactive bladder, a condition with its main symptom being urinary retention.

Disorders of bladder function may be dealt with surgically, by redirecting 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. Ultrasound can be used to estimate bladder volumes.

Cancer

thumb|231x231px|Cross-section of the male [[genitourinary system showing a cancer within the bladder. When a cancer occurs it is most likely to be a transitional cell carcinoma.]]

Cancer of the bladder is known as bladder cancer. It is usually due to cancer of the urothelium, the cells that line the surface of the bladder. Bladder cancer is more common after the age of 40, and more common in men than women; other risk factors include smoking and exposure to dyes such as aromatic amines and aldehydes.

Investigation

thumb|A [[diverticulum of the bladder]]A number of investigations are used to examine the bladder. The investigations that are ordered will depend on the taking of a medical history and an examination. The examination may involve a medical practitioner feeling in the suprapubic area for tenderness or fullness that might indicate an inflamed or full bladder. Blood tests may be ordered that may indicate inflammation; for example a full blood count may demonstrate elevated white blood cells, or a C-reactive protein may be elevated in an infection.

Some forms of medical imaging exist to visualise the bladder. A bladder ultrasound may be conducted to view how much urine is within the bladder, indicating urinary retention. A urinary tract ultrasound, conducted by a more trained operator, may be conducted to view whether there are stones, tumours or sites of obstruction within the bladder and urinary tract. A CT scan may also be ordered.

A flexible internal camera, called a cystoscope, can be inserted to view the internal appearance of the bladder and take a biopsy if required.

Urodynamic testing can help to explain the symptoms.

Other animals

Mammals

All species of mammal have a urinary bladder. This structure begins as an embryonic cloaca. In the vast majority of species, it eventually becomes differentiated into a dorsal part, connected to the intestine, and a ventral part, 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.

The mammalian bladder is an organ that regularly stores a hyperosmotic concentration of urine. It therefore is relatively impermeable and has a multi-layer epithelium. The urinary bladders of cetaceans (whales and dolphins) are proportionally smaller than those of land-dwelling mammals.

Reptiles

In all reptiles, the urinogenital ducts and the rectum both empty into the organ called the cloaca. In some reptiles, a midventral wall in the cloaca opens into a urinary bladder. The urinary bladder exists in all species of turtle and tortoise and most species of lizard. Monitor lizards, the legless lizards, snakes, alligators, and crocodiles do not have urinary bladders. Other desert-dwelling reptiles have large bladders, which can hold long-term reserves of water for several months and aid in osmoregulation.

Turtles have two or more accessory urinary bladders, beside the neck of the urinary bladder and above the pubis, occupying much of the body cavity. Turtles' bladder is also usually divided into two lobes: the right lobe is under the liver, which prevents large stones from remaining in the lobe; the left lobe is likelier than the right to have calculi.

Amphibians

Most aquatic and semi-aquatic amphibians can absorb water directly through their skin. Some semi-aquatic animals also have similarly permeable bladder membranes. They tend to have high rates of urine production, to offset this high water intake; and the dissolved salts in their urine are highly dilute. The urinary bladder helps these animals to retain salts. Some aquatic amphibians, such as Xenopus, do not reabsorb water from their urine, to prevent excessive water influx. For land-dwelling amphibians, dehydration results in reduced urine output.

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.

Fish

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. In freshwater fish the bladder is a key site of absorption for many major ions in marine fish urine is held in the bladder for extended periods to maximise water absorption.. 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.

Birds

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

Crustaceans

Unlike the urinary bladder of vertebrates, the urinary bladder of crustaceans both stores and modifies urine. 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.