The urinary bladder is a hollow muscular organ in many animals, that collects and stores urine from the kidneys before disposal by urination. In the human the bladder is a hollow muscular, and distensible (or elastic) organ, that sits on the pelvic floor. Urine enters the bladder via the ureters and exits via the urethra. The typical human bladder will hold between 300 and 500 mL (10.14 and 16.91 fl oz) before the urge to empty occurs, but can hold considerably more.
Structure
In the human, the bladder is a hollow muscular organ situated at the base of the pelvis. Urine collects in the bladder, fed from the two ureters that are connected to the kidneys. Urine leaves the bladder via the urethra, a single muscular tube which ends in an opening â€" the urinary meatus, where it exits the body.
Anatomically, the bladder is divided into a broad fundus, a body, an apex, and a neck. The apex (prev.vertex) is directed forward toward the upper part of the pubic symphysis, and from there the median umbilical ligament is continued upward on the back of the anterior abdominal wall to the umbilicus. The peritoneum is carried by it from the apex on to the abdominal wall to form the middle umbilical fold. 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. In the male the neck of the urinary bladder is adjacent to the prostate gland.
The three openings, two ureteric orifices, and the internal urethral orifice mark the triangular area called the trigone of the bladder. These 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. This makes the upper boundary of the trigone. The trigone is a smooth muscle area that forms the floor of the bladder above the urethra. 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.
In men, the prostate 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 be enlarged when the prostate is enlarged.
The bladder is situated below the peritoneal cavity near the pelvic floor and behind the pubic symphysis. In men, it lies in front of the rectum, separated by the recto-vesical pouch, and is supported by fibres of the levator ani and the prostate gland. In women, it lies in front of the uterus separated by the vesico-uterine pouch, and is supported by the elevator ani and the upper part of the vagina. The wall of the urinary bladder is normally 3â€"5 mm thick. When well distended, the wall is normally less than 3 mm.
The inner walls have a series of ridges, thick mucosal folds known as rugae that allow for the expansion of the bladder.
The Latin for urinary bladder is vesica urinaria and the term vesical or prefix vesico - is used in connection with associated structures such as vesical veins. Modern Latin for bladder is cystis also used in associated terms such as cystitis â€" inflammation of the bladder.
Microanatomy
The outside of the bladder is protected by a serous membrane. The bladder wall itself is smooth muscle. The inner side of the bladder is lined with a mucosal 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. The mucosal lining also offers a urothelial barrier against the passing of infections.
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. 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. 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 responding. The main relaxant pathway is via the adenylyl cyclase cAMP pathway, activated by β2 adrenergic receptors. 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.
Blood and lymph supply
The bladder is supplied by the vesical arteries and drained by the vesical veins. The superior vesical artery supplies blood to the upper part of the bladder, and the inferior vesical artery, supplies the lower part. Both are branches of the internal iliac arteries. The uterine arteries and vaginal arteries also supply blood in females. Venous drainage of blood supplying the bladder begins in a network of small vessels on the outer and back surfaces of the bladder. These coalesce and pass backwards along the lateral ligaments of the bladder into the internal iliac veins.
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.
Nerve supply
The bladder receives motor innervation from both sympathetic fibers, most of which arise from the superior and inferior hypogastric plexuses and nerves, and parasympathetic fibers, which come from the pelvic splanchnic nerves.
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.
For the urine to exit the bladder, both the autonomically controlled internal sphincter (in the male) and the voluntarily controlled external sphincter must be opened. Problems with these muscles can lead to incontinence.
Development
The human urinary bladder is derived 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. At this time the ureters move from the mesonephric ducts to the trigone. 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.
Function
Urine, excreted by the kidneys, collects in the bladder before disposal by urination (micturition). The urinary bladder usually holds 300-350 ml of urine. As urine accumulates, the rugae 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. Urination is controlled by the pontine micturition center in the brainstem.
Clinical significance
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.
Bacteriuria is the presence of bacteria in the urine which can indicate a urinary tract infection such as cystitis.
Disorders of or related to the bladder include:
- Bladder cancer
- 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
- Interstitial cystitis
- 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.
Other animals
Reptiles
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.
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. Such adaptations are the result of environments such as remote islands and deserts where water is very scarce. 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.
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. 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.
Amphibians
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. As as 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. For land-dwelling amphibians, dehydration results in reduced urine output.
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.
Fish
Although the gills of most teleost fish assist with the elimination of ammonia from the body and although fish virtually by definition live in an environment surrounded by water, 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 ones. 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.
Mammals
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 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 multiple epithelial layers. The urinary bladder of the cetaceans (whales and dolphins) is proportionally smaller than that of land-dwelling mammals.
Birds
In the whole class of Aves 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.
Additional images
See also
- Alpha blocker
- Cystitis glandularis
- Ureterocele
References
- Books
- editor-in-chief, Susan Standring ; section editors, Neil R. Borley; et al. (2008). Gray's anatomy : the anatomical basis of clinical practice (40th ed.). London: Churchill Livingstone. ISBN 978-0-8089-2371-8. CS1 maint: Explicit use of et al. (link)CS1 maint: Multiple names: authors list (link)CS1 maint: Extra text: authors list (link)
External links
- Histology at KUMC epithel-epith09 "Urinary Bladder"
- Anatomy photo: Urinary/mammal/bladder/bladder1 - Comparative Organology at University of California, Davis â€" "Mammal, bladder (LM, Medium)"
- Virtual Slidebox at Univ. Iowa Slide 445
- Anatomy photo:43:07-0100 at the SUNY Downstate Medical Center â€" "The Female Pelvis: The Urinary bladder"
- Anatomy photo:44:04-0103 at the SUNY Downstate Medical Center â€" "The Male Pelvis: The Urinary bladder"
- Bladder (ISSN 2327-2120) â€" An open-access journal on bladder biology and diseases.