The Latin phrase for "urinary bladder" isvesica urinaria, and the termvesical or prefixvesico- appear in connection with associated structures such asvesical veins. The modern Latin word for "bladder" –cystis – appears in associated terms such ascystitis (inflammation of the bladder).
Male and female urinary bladders in lateral cross-section
In humans, the bladder is a hollow muscular organ situated at the base of thepelvis. Ingross anatomy, the bladder can be divided into a broadfundus (base), a body, an apex, and a neck.[5] The apex (also called the vertex) is directed forward toward the upper part of thepubic symphysis, and from there themedian umbilical ligament continues upward on the back of the anterior abdominal wall to theumbilicus. Theperitoneum is carried by it from the apex on to theabdominal wall to form themiddle umbilical fold. The neck of the bladder is the area at the base of thetrigone that surrounds theinternal urethral orifice that leads to the urethra.[5] In males, the neck of the urinary bladder is next to theprostate gland.
The bladder has three openings. The twoureters enter the bladder atureteric orifices, and theurethra enters at thetrigone 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,[6] known asvesicoureteral reflux. Between the two ureteric openings is a raised area of tissue called the interureteric crest.[5] This makes the upper boundary of the trigone. The trigone is an area ofsmooth muscle that forms the floor of the bladder above the urethra.[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 walls of the bladder have a series of ridges, thick mucosal folds known asrugae that allow for the expansion of the bladder. Thedetrusor muscle is the muscular layer of the wall made ofsmooth musclefibers arranged in spiral, longitudinal, and circular bundles.[8] The detrusor muscle is able to change its length. It can also contract for a long time whilstvoiding, and it stays relaxed whilst the bladder is filling.[9] The wall of the urinary bladder is normally 3–5 mm thick.[10] When well distended, the wall is normally less than 3 mm.
Bladder location and associated structures in the male
In males, theprostate gland lies outside the opening for the urethra. The middle lobe of the prostate causes an elevation in the mucous membrane behind the internal urethral orifice called the uvula of urinary bladder. The uvula can enlarge when the prostate becomes enlarged.
Thelymph 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 theexternal iliac lymph nodes.[11]
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] These travel viasacral nerves toS2-4.[13] From here, sensation travels to thebrain via thedorsal columns in thespinal cord.[11]
The inner wall of the bladder is calledurothelium, a type oftransitional 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.[8] Additionally, these are lined with amucous membrane consisting of a surfaceglycocalyx that protects the cells beneath it from urine.[14] The epithelium lies on a thinbasement membrane, and alamina propria.[8] The mucosal lining also offers a urothelial barrier against the passing of infections.[15]
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.[8]
In the developingembryo, at the hind end lies acloaca. This, over the fourth to the seventh week, divides into aurogenital sinus and the beginnings of theanal canal, with a wall forming between these two inpouchings called theurorectal septum.[17] The urogenital sinus divides into three parts, with the upper and largest part becoming the bladder; the middle part becoming theurethra, and the lower part changes depending on the biological sex of the embryo.[17]
The human bladder derives from theurogenital sinus, and it is initially continuous with theallantois. The upper and lower parts of the bladder develop separately and join around the middle part ofdevelopment.[7] At this time the ureters move from themesonephric ducts to the trigone.[7] In males, the base of the bladder lies between the rectum and the pubic symphysis. It is superior to theprostate, and separated from therectum by therecto-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 theuterus by thevesico-uterine pouch. Ininfants and young children the urinary bladder is in theabdomen even when empty.[18]
Urine is excreted by thekidneys and flows into the bladder through theureters, where it is stored untilurination (micturition).[13] Urination involves coordinated muscle changes involving a reflex based in the spine, with higher inputs from the brain.[13] During urination, the detrusor muscle contracts, the external urinary sphincter and muscles of theperineum relax, and urine flows through theurethra[13] and exits thepenis orvulva through theurinary meatus.[11]
The urge to pass urine stems fromstretch receptors that activate when between 300 - 400 mL urine is held within the bladder.[13] As urine accumulates, therugae flatten and the wall of the bladder thins as it stretches, allowing the bladder to store larger amounts of urine without a significant rise in internal pressure.[19] Urination is controlled by thepontine micturition center in thebrainstem.[20]
Stretch receptors in the bladder signal theparasympathetic nervous system to stimulate themuscarinic receptors in the detrusor to contract the muscle when the bladder is distended.[21] This encourages the bladder to expel urine through the urethra. The main receptor activated is theM3 receptor, althoughM2 receptors are also involved and whilst outnumbering the M3 receptors they are not so responsive.[22]
The main relaxant pathway is via theadenylyl cyclasecAMP pathway, activated via the β3 adrenergic receptors. Theβ2 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.[9][23][24]
Calcifications on bladder wall caused by urinaryschistosomiasis
Cystitis refers to infection or inflammation of the bladder. It commonly occurs as part of aurinary tract infection.[25] In adults, it is more common in women than men, owing to a shorterurethra. It is common in males during childhood, and in older men wherean enlarged prostate may cause urinary retention.[25] Other risk factors include other causes of blockage or narrowing, such asprostate cancer or the presence ofvesico-ureteric reflux; the presence of outside structures in the urinary tract, such asurinary catheters; and neurologic problems that make passing urine difficult.[25] Infections that involve the bladder can cause pain in the lower abdomen (above thepubic symphysis, so called "suprapubic" pain), particularly before and after passing urine, and a desire topass urine frequently and with little warning (urinary urgency).[25] Infections are usually due tobacteria, of which the most common isE coli.[25]
Urinary tract infections or cystitis are treated withantibiotics, many of which areconsumed by mouth. Serious infections may require treatment withintravenous antibiotics.[25]
Interstitial cystitis refers to a condition in which the bladder is infected due to a cause that is not bacteria.[26][27]
Frequent urination can be due to excessive urine production, small bladder capacity, irritability or incomplete emptying. Males with anenlarged prostate urinate more frequently. One definition of anoveractive bladder is when a person urinates more than eight times per day.[28] An overactive bladder can often causeurinary incontinence. 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.Urodynamic testing can help to explain the symptoms. Anunderactive bladder is the condition where there is a difficulty in passing urine and is the main symptom of aneurogenic bladder. Frequent urination at night may indicate the presence ofbladder stones.
bladder sphincter dyssynergia, a condition in which the sufferer cannot coordinate relaxation of the urethra sphincter with the contraction of the bladder muscles
Cancer of the bladder is known asbladder cancer. It is usually due to cancer of theurothelium, 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;[33] other risk factors includesmoking and exposure todyes such asaromatic amines andaldehydes.[33] When cancer is present, the most common symptom in an affected person isblood in the urine; a physicalmedical examination may be otherwise normal, except in late disease.[33] Bladder cancer is most often due to cancer of the cells lining the ureter, calledtransitional cell carcinoma, although it can more rarely occur as asquamous cell carcinoma if the type of cells lining the urethra have changed due to chronic inflammation, such as due to stones orschistosomiasis.[33]
Treatment depends on the cancer'sstage. Cancer present only in the bladder may be removed surgically viacystoscopy; an injection of thechemotherapeuticmitomycin C may be performed at the same time.[33] Cancers that arehigh grade may be treated with an injection of theBCG vaccine into the bladder wall, and may require surgical removal if it does not resolve.[33] Cancer that is invading through the bladder wall may be managed by complete surgical removal of the bladder (radical cystectomy), with the ureters diverted into a segment of part ofileum connected to astoma bag on the skin.[33] Prognosis can vary markedly depending on the cancer's stage and grade, with a better prognosis associated with tumours found only in the bladder, that are low grade, that do not invade through the bladder wall, and that ispapillary in visual appearance.[33]
A number of investigations are used to examine the bladder. The investigations that are ordered will depend on the taking of amedical history and an examination. The examination may involve amedical practitioner feeling in the suprapubic area for tenderness or fullness that might indicate an inflamed or full bladder.[citation needed] Blood tests may be ordered that may indicate inflammation; for example afull blood count may demonstrate elevatedwhite blood cells, or aC-reactive protein may be elevated in an infection.[citation needed]
Some forms ofmedical imaging exist to visualise the bladder. Abladder ultrasound may be conducted to view how much urine is within the bladder, indicatingurinary retention. Aurinary 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. ACT scan may also be ordered.
A flexible internal camera, called acystoscope, can be inserted to view the internal appearance of the bladder and take abiopsy if required.
Bladder, prostate, and seminal vesicles of astallion
All species of mammal have a urinary bladder.[34] This structure begins as anembryonic 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 theplatypus and thespiny anteater, both of which retain the cloaca into adulthood.[35]
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 ofcetaceans (whales and dolphins) are proportionally smaller than those of land-dwelling mammals.[36]
In all reptiles, the urinogenital ducts and therectum both empty into the organ called thecloaca. 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, thelegless lizards, snakes, alligators, and crocodiles do not have urinary bladders.[35]: p. 474
Many turtles, tortoises, and lizards have proportionally very large bladders.Charles Darwin noted that the bladder of theGalapagos tortoise could store urine weighing up to 20% of the tortoise's body weight.[37] Such adaptations are the result of environments, such as remote islands and deserts, where fresh water is very scarce.[38] Other desert-dwelling reptiles have large bladders, which can hold long-term reserves of water for several months and aid inosmoregulation.[39]
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.[40] 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 havecalculi.[41]
Most aquatic and semi-aquatic amphibians can absorb water directly through their skin. Some semi-aquatic animals also have similarly permeable bladder membranes.[42] 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 asXenopus, do not reabsorb water from their urine, to prevent excessive water influx.[43] For land-dwelling amphibians, dehydration results in reduced urine output.[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.[44] Urine flows from the kidneys through the ureters into the bladder and is periodically released from the bladder to the cloaca.[45]
The gills of mostteleost 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 ofteleosts is permeable to water, though this is less true for freshwater dwelling species than saltwater species.[37]: p. 219 In freshwater fish the bladder is a key site of absorption for many major ions[46] in marine fish urine is held in the bladder for extended periods to maximise water absorption.[46] The urinary bladders of fish andtetrapods are thought to be analogous while the former's swim-bladders and latter'slungs are considered homologous.
Most fish also have an organ called aswim-bladder which is unrelated to the urinary bladder except in its membranous nature. Theloaches,pilchards, andherrings 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 theoviducts and behind therectum.[47]
In nearly all bird species, there is no urinary bladder per se.[48] Although all birds have kidneys, theureters open directly into acloaca which serves as a reservoir for urine, fecal matter, and eggs.[49]
Unlike the urinary bladder of vertebrates, the urinary bladder ofcrustaceans both stores and modifies urine.[50] 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.[50] The tissue of the bladder is thinepithelium.[50]
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