1. Human urinary system: 2. Kidney, 3. Renal pelvis, 4. Ureter, 5. Urinary bladder, 6. Urethra. (Left side with frontal section)
7. Adrenal gland
Vessels: 8. Renal artery and vein, 9. Inferior vena cava, 10. Abdominal aorta, 11. Common iliac artery and vein
With transparency: 12. Liver, 13. Large intestine, 14. Pelvis
|Artery||Superior vesical artery
Inferior vesical artery
|Vein||Vesical venous plexus|
|Nerve||Vesical nervous plexus|
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 can hold between 300 and 500 mL (10.14 and 16.91 fl oz) before the urge to empty occurs, but can hold considerably more.
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 – 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 3–5 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 – inflammation of the bladder.
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.
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.
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.
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.
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.
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 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
- 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.
Urinary bladders occur throughout much of the animal kingdom, but are very diverse in form, and in some cases are not homologous with the urinary bladder in humans. The urinary bladder of chelonians is very thin and cystoscopy permits visualisation of internal organs. The pig bladder is very similar to the human bladder.
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