Bladder
The bladder is a hollow muscular organ located in the pelvis behind the pubic symphysis. Its main function is the accumulation of urine and periodic emptying. The capacity of the bladder in an adult is 250–500 ml, although with pathology it can stretch to 1 liter or more.
Wall structure
The bladder wall consists of:
- Mucous membrane - lined with transitional epithelium, which can stretch when filled. In an empty bladder, the mucous membrane forms folds that straighten when filled.
- Submucosa - ensures the mobility of the mucosa
- Muscular membrane (detrusor) - three layers of smooth muscle (external and internal longitudinal, middle circular)
- Adventitia/tunica serosa - the upper part is covered with peritoneum
Lieto Triangle
On the posterior wall of the bladder there is the vesical triangle (Lieto's triangle) - a smooth area between the orifices of the ureters (top) and the internal opening of the urethra (bottom). Here the mucosa is devoid of folds and is tightly fused with the muscular layer. Lieto's triangle is the most common site of inflammatory processes.
Sphincters
Two sphincters control urination:
- Internal sphincter - smooth muscle, involuntary, located in the neck of the bladder
- External sphincter - striated, voluntary, located in the urogenital diaphragm
Mechanism of urination
- The bubble fills → the wall stretches
- At a volume of ~200 ml the first urge appears
- Stretch receptors send a signal to the micturition center (sacral spinal cord, S2–S4)
- Parasympathetic nerves cause detrusor contraction
- Both sphincters relax at the same time
- Urine is excreted through the urethra
The cerebral cortex exercises voluntary control: we can delay urination by straining the external sphincter.
Clinical aspects
Cystitis - inflammation of the bladder - is one of the most common urinary tract infections, especially in women (due to a short urethra). Neurogenic bladder - a disruption of innervation due to spinal cord injuries - can manifest itself as both urinary retention and incontinence.
The bladder and surrounding pelvic structures are available for study in the Humio app - the 3D model allows you to see the organ from all sides and understand its topography.