The urinary bladder lies in the anterior pelvis, with differing relations depending on gender.
Structure
The urinary bladder has a pyramidal shape with the base directed posteriorly and inferiorly when deflated. The apex lies anteriorly, in close proximity to the symphysis pubis. It is capable of great distension and is highly folded when empty. When full, it can expand into the abdomen. The bladder wall contains the thick detrusor muscle, which is an important barrier for malignant spread. Aside from the base and apex, the bladder has four surfaces - superior, inferior and two lateral surfaces.
- The apex lies anteriorly, attached to the anterior abdominal wall by the urachus, a ligament that extends superiorly to the umbilicus as the median umbilical ligament.
- The superior surface is mobile depending on the volume of urine stored in the bladder. It is covered externally by peritoneum.
- The inferolateral surfaces are covered by peritoneum superiorly but are retroperitoneal inferiorly.
- The base lies posteriorly, uniting inferiorly with the inferolateral surfaces at the trigone.
The internal surface is loosely attached to the detrusor muscle over the entire bladder wall, except at the trigone where the mucosa is firmly attached. This is a triangular space with the apex directed anteriorly and formed by the internal urethral orifice; the other two points are formed by the ureteric orifices. The trigone is recognisable because the firmly attached mucosa is always smooth, compared to the folded mucosa of the remaining bladder wall.
The bladder neck extends below the trigone and is functionally distinct. In men it contains a preprostatic sphincter which is important for antegrade ejaculation and continence. In women the bladder neck lies above the pelvic floor (levator ani) which provide continence.
Microscopic Structure
The bladder mucosa is lined by urothelium which is a stratified, transitional type of epithelium. There are between 4 - 7 layers of cells:
- The most superficial layer forms an impermeable barrier which prevents exchange of fluid or ion from the urine and the rest of the body. Cells are large and occasionally multinucleated
- The middle layers contain polygonal cells which can alter their shape, allowing the epithelium to stretch or contract as needed during bladder filling.
- The basal layer contains cuboidal cels that function as the stem cells of the bladder.
Beneath the epithelium is the lamina propria, containing neurovascular structures and elastic connective tissue. The muscularis mucosae is poorly defined and not always visible.
The detrusor muscle is thick and formed by smooth muscle cells. Three layers (longitudinal / circular / longitudinal) can be made out. The smooth muscle cells are usually clumped in bundles.
Relations
Superiorly, with the peritoneal cavity and its contents (small and large bowel). In men the peritoneum continues to the base; whereas in women it is reflected on to the uterus. In most women the uterus is directed antegradely; it may lie over the posterior part of the superior surface, separated by the vesicouterine pouch.
Anteriorly, with the anterior abdominal wall. When distended the superior surface elevates above the attachment of the apex to the abdominal wall, forming the supravesical recess
Laterally, the inferolateral surfaces lie against the levator ani (posteriorly) and pubic bone (anteriorly).
Posteriorly, the bladder is in relation to:
- In men, the rectum is posterior and superior, separated by the rectovesical pouch. The seminal glands lie below this at the base of the pouch, covered by peritoneum.
- In women, the uterus is posterior and superior, with the cervix of the uterus lying posterior to the bladder trigone. The vagina lies posterior to the bladder neck.
Inferiorly, the bladder is related to the prostate gland in men, whereas the levator ani and transverse perineal muscles are in close relation for women. The urethra lies inferiorly in both genders.
Variations
Neonatal and childhood
In newborns a large portion of the bladder lies in the abdomen due to a shallow true pelvis. During childhood the pelvis enlarges and the bladder slowly descends until it is fully contained within; the superior part may still ascend into the abdomen when the bladder is significantly distended.
Hypotonic/Flaccid Bladder
Hypotonic bladder is caused by interruption of parasympathetic supply to the detrusor muscle, which most commonly occurs with lower spinal lesions that damage the S2 - S4 nerve roots. This can be induced by cauda equina compression or local diseases in the sacrum. On examination, there is significant distension of the bladder and the patient may have difficulty voiding.
Diverticuli
Diverticuli are outpouchings of bladder mucosa through the detrusor muscle. Two types are seen:
- Congenital diveritucli are rare and occur in boys. They often occur near the trigone and may cause urine to flow retrogradely into the ureters from the bladder.
- Acquired diverticuli occur in older men, usually due to bladder outlet obstruction and resuting increased bladder pressure
Cystocele
Cystocele is a common condition of older women, usually multiparous, where the connective tissue between the vagina and the bladder weakens and the bladder bulges into the vaginal lumen. This can cause problems with continence as the levator ani are unable to properly contract to occlude the bladder neck. Cystocele can be repaired by removing excess tissue from the region or through insertion of a mesh.
Appearance on Imaging
On CT, the thick muscle of the detrusor can often be made out as separate to the fluid filled lumen. The thickness of the muscle depends on the degree of bladder filling as well as on disease processes.
Many nuclear medicine isotopes are excreted via the bladder, and it often shows increased signal (eg. 99mTc whole body bone scan, 18F PET scan). This makes nuclear medicine studies less helpful when pathology exists near the bladder.
Function
The bladder functions as a storage tank for urine, allowing a person to choose an appropriate time to excrete waste products.
Neurovascular Supply
Arterial Supply
Blood is sourced from branches of the internal iliac artery.
- The superior and inferior vesical arteries provide the majority of blood supply to the bladder. They pass anteriorly from the anterior division of the internal iliac artery along the pelvic wall and enter the substance of the bladder.
- In women, the inferior vesical artery is replaced by the uterine artery, which contributes a significant supply to the lower parts of the bladder.
- The obturator and inferior gluteal arteries also provide some supply to the bladder.
Vessels penetrate the detrusor muscle to supply the capillaries of the lamina propria.
Venous Drainage
Veins collect into venous plexuses on each inferolateral surface, and then drain laterally and posterior to the internal iliac veins.
Lymphatics
Unlike the vascular supply, lymphatic drainage generally occurs to external iliac nodes. Lymphatic fluid initially collects in vessels in the lamina propria, within the detrusor muscle and beneath the peritoneal layer superiorly.
- Lymph from the trigone passes externally and then superior and lateral to reach the iliac nodes (external or internal)
- Lymph from the inferolateral and superior surfaces passes externally and the superolaterally to the external iliac nodes.
Innervation
Autonomic fibres are directed to the bladder from the inferior hypogastric plexuses. These cover the viscera of the pelvis as well as the internal iliac vessels. Sympathetic nerves descend from T10 - L2 sympathetic ganglions via the coeliac, aortic and superior hypogastric plexuses. Pelvic splanchnic nerves from S2-4 convey parasympathetic supply.
- There is significant autonomic supply of the detrusor muscle, which contains numerous nerve fibres running around the muscle fibres. Parasympathetic stimulation causes contraction of detrusor and promotes bladder emptying; sympathetic input is minimal and predominately to vessels.
- In men, the bladder neck and pre-prostatic sphincter are supplied by significant numbers of sympathetic neurons, similar to those supplying the prostate, vas deferens and seminal glands. Sympathetic activation (during emission and ejaculation) causes contraction of the smooth muscle in the bladder neck, preventing retrograde travel of semen.
- Interestingly, the function of these nerves in bladder continence is not known
- The bladder neck in women is poorly innervated and is not thought to be of functional significance.
Sensory fibres typically ascend with sympathetic nerves to the lower thoracic and upper lumbar vertebrae.
Potential Routes of Malignant Spread
Local Invasion
Malignancy must spread through the thick detrusor muscle before it can invade other local structures. Local invasion may cause obstruction of the ureteric orifice, causing hydronephrosis; less commonly the bladder neck may be obstructed. Further invasion occurs later in the disease course, with spread into the prostate, seminal glands and rectum (in men) or into the uterus or vagina in women. Transcoelemic spread may occur if the peritoneum is involved.
Lymphatic Spread
The regional lymph nodes of the bladder are predominately the external iliac nodes. Spread may also occur to internal iliac nodes, including the obturator nodes. Disease typically progresses through adjoining lymph node groups - common iliac, retroperitoneal and finally mediastinal.
Distant Spread
Visceral disease usually involves the liver or lungs.
