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The bladder has supportive structures and tissues to help maintain control of urine. These structures mainly involve sphincters that are supported by muscle and tissue systems. These structures act like doors to keep urine in.
The Urethra
The main structure is the urethra (the tube or pipeline that urine flows through). In the urethra, the vesical neck and urethral sphincters tighten to keep the urethra closed, maintaining higher pressure in the urethra than in the bladder. Additionally, the tissues around the urethra help create a tight seal, assisted by the connective tissues in the urethral wall.
Aging and having children can decrease the number of muscle fibers which makes the muscle weaker.
Levator Ani : Supportive "Hammock"
Beneath the urethra and vesical neck, there is a supportive "hammock" made of muscles and tissues that provides a firm base. This tissue compresses the urethra during increased abdominal pressure to help keep it closed.
The Levator ani "Hammock" and the cardinal ligaments attach to the front wall of the vagina and also attach to the pelvic bones.
When resting: helps keep the opening closed against pressure from gravity and slight abdominal pressure.
With activity: work harder to keep the opening closed against the pressure from movements and increases in abdominal pressure due to actions like breathing and moving.
Urinary Incontinence
Urinary incontinence, which affects 8.5% to 38% of women depending on age and childbirth history, is mostly stress urinary incontinence (SUI), often combined with urge incontinence. Both types are caused by a weak urethral sphincter that doesn't create enough pressure to prevent leaks.
Treatments usually start with exercises and other non-surgical methods, but surgery may be needed if these don't work.
Despite how common SUI is, there hasn't been much progress in understanding its causes over the past 40 years. Most surgeries aim to improve support for the bladder neck. Choosing the right treatment requires understanding which muscles, nerves, and tissues are involved.
How is urinary continence maintained?
To keep urine in the bladder and stop leaks, the pressure in the urethra has to be higher than the pressure in the bladder, both when you’re resting and when there's more pressure in your abdomen, like when you cough. The muscles around the urethra help keep this balance by having a strong enough resting tone. The main thing that controls this is called the maximum urethral closure pressure, which is managed by the urethral sphincter.
When you do things like cough, the bladder pressure goes up a lot, but the urethral closure pressure also increases to keep the urethra closed and prevent leaks. The strength of the urethral pressure when resting and how much the pressure in the abdomen increases during a cough both affect when leaks might happen.
Studying how the resting closure pressure and pressure changes can help understand the basics, but it doesn’t show how specific injuries affect the urethra’s ability to close. For that, a detailed look at the urethral sphincter and its support structures is needed.
The Urinary Sphincter Closure System
The urethra stays closed mainly due to three things working together:
urethral striated muscles
urethral smooth muscle
blood vessels within the urethral lining.
How the Urethral Muscles Work
The urethral muscles help keep urine from leaking in several ways.
Detrusor smooth muscle forms a U-shape around the upper part of the urethra, helping to keep it closed.
Internal Urethral sphincter is an inner layer made of smooth muscle, that are running lengthwise and circular. These are good at holding constant pressure and can also tighten more when needed to prevent leaks. The inner longitudinal smooth muscle might help to open the urethra to start urination rather than keeping it closed, but its exact role is still not completely understood.
3. Urethrovaginal sphincter and the compressor urethrae make up the external sphincter and are striated layers of muscle that help to squeeze the urethra closed.
4. A network of blood vessels around the urethra helps to keep it closed by pressing the mucosal surfaces together, forming a watertight seal.
Impact of Aging
There are significant differences in resting urethral closure pressures between normal women and those with stress urinary incontinence. The severity of incontinence often correlates with these pressures. In lab studies, thinning of the striated muscle layers was particularly noticeable in the urethra in older women. The age-related loss of nerve fibers in these areas matched the muscle fiber loss, supporting the idea that nerve damage contributes to stress urinary incontinence and explaining why poor nerve function affects continence.
Healthy striated muscle can increase its strength by about 30% after 8-12 weeks of intensive resistance training. For example, if an older woman’s resting urethral closure pressure control was 100 cmH₂O when she was young but is now 40 cmH₂O due to muscle loss, exercise may only increase her pressure control to 52 cmH₂O. This is far less than the 100 cmH₂O pressure control needed during a hard cough. This is why the quality of muscle strength training is so important. If the quality of pelvic floor muscle training is poor, then the quality of strength gains would also be poor.
Urethral and Anterior Vaginal Wall Support System
The urethra and the front part of the vaginal wall are supported by a strong connective tissue called the endopelvic fascia. This fascia connects to the pelvic floor muscles and help keep the urethra and the neck of the bladder in place, which is important for controlling urine flow.
Importance of Urethral Support
To stay continent, or to not leak urine, two things are needed:
The urethra must be tightly closed when resting.
It must also close tightly when there’s pressure on the abdomen such as with coughing or sneezing.
The endopelvic fascia provides a firm base that helps the urethra stay closed under pressure. This setup helps prevent stress urinary incontinence.
Structures Involved
Endopelvic Fascia: This is a tough, fibrous tissue that surrounds the vagina and attaches to the arcus tendineus fascia pelvis, which then connects to the pubic bone and the ischial spine.
Arcus Tendineus Fascia Pelvis: These are strong, cable-like structures that help suspend the urethra and vaginal wall like a suspension bridge.
Levator Ani Muscles: These muscles support the pelvic organs and play a crucial role in preventing prolapse (when organs slip down from their normal position).
Levator Ani Muscles
The levator ani muscles have three main parts:
Iliococcygeus: Forms a flat shelf across the pelvis.
Pubovisceral: Comes from the pubic bone and attaches to pelvic organs.
Puborectal: Forms a sling around the rectum.
These muscles help keep the pelvic floor closed and support the organs by pressing them against the pubic bone.
Interaction with Connective Tissue
The levator ani muscles protect the pelvic connective tissues from being overstretched. They support the weight of the pelvic organs and prevent continuous strain on the ligaments and fasciae. If these muscles are damaged, the connective tissues are placed under stress, which can lead to prolapse.
Importance of Muscle and Fascia Health
If the pelvic floor muscles are injured, especially during childbirth, they may not function properly, leading to conditions like stress urinary incontinence and prolapse. MRI studies show that about 20% of first-time mothers have visible damage to the levator ani muscles, which can weaken these muscles and contribute to recurring prolapse even after surgical repair.
References: Bo, K., Berghmans, B., Mørkved, S., & Van Kampen, M. (Eds.). (2023). Evidence-Based Physical Therapy for the Pelvic Floor (3rd ed.). Elsevier.
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