Anatomy Of Urinary Continence - Urinary Incontinence: Bladder Conditions

Content provided by the Faculty of the Harvard Medical School

Anatomy of urinary continence

The job of your urinary system is to remove waste products from your blood and help maintain the proper balance of salts and water in your body. The kidneys and ureters constitute the upper urinary system, and the bladder and urethra make up the lower urinary tract (see Figures 1 and 2). Your kidneys filter your blood and create urine to carry away the waste. Urine is a mixture of water, salts, and urea (a waste product produced from protein metabolism). It leaves the kidneys via the ureters, the muscular tubes that deliver urine to the bladder in small, steady amounts.

Figure 1: Female urinary function The bladder's first job is to store urine that has been filtered by the kidneys and delivered through the ureter. Once the bladder is nearly full, the nerves send a signal to the brain. To urinate, you relax the sphincter muscle and the brain signals the bladder muscle (detrusor) to contract to push the urine out. In women, incontinence results from three main problems: (1) The bladder's detrusor muscle can become unstable or overactive, which can cause urge and frequency incontinence; (2) the urinary sphincter that normally opens and closes at your command may become weakened, leading to stress incontinence; or (3) the pelvic muscles (urogenital diaphragm) may be weakened, which can also cause stress incontinence.

The bladder. Your bladder is the storage tank that holds the urine and allows its release at the appropriate time. In the wall of this balloon-like organ is a layer of smooth muscle, called the detrusor muscle, which allows the bladder to expand and store the liquid until urination. A woman's bladder lies in front of the vagina near the uterus (where its ability to expand is noticeably lessened during pregnancy, as the uterus enlarges). A man's bladder is located below his abdomen, between the pubic bone and the rectum.

The urethra. Leading from the bladder to the outside of the body is the urethra, a slim and flexible tube surrounded by smooth muscle. At the junction where the bladder meets the urethra (called the bladder neck), two sets of muscles help hold urine in. The internal urethral sphincter is made up of involuntary muscles that keep a constant pressure on the urethra without conscious effort. The external, or outer, sphincter muscles are under voluntary control. This area is also supported by collagen fibers.

In women, the urethra is only about an inch and a half long. Estrogen-sensitive cells line the urethra, producing secretions that help create a tight seal. In men, the urethra is about 8 inches long, extending from between the scrotum and rectum to the tip of the penis. Near the bladder, a man's urethra passes through his prostate gland. The prostate, normally the size of a walnut, helps to support the urethra and help with continence, but it can also create problems if it enlarges or requires removal or other surgery.

Urinary support. The urinary system, the intestines, and the reproductive organs are supported by ligaments and the pelvic floor. The pelvic floor is an important network of muscles that extends from your pubic bone to your tailbone, with openings for the urethra and anus, as well as the vagina in women. Most of the time, certain pelvic muscles stay contracted to hold the pelvic organs in place against the pull of gravity.

When you are active or trying to avoid urination, you can voluntarily tighten these muscles, along with the external urethral sphincter, to provide added support and prevent leakage. When you urinate, your pelvic floor muscles and urethral sphincter relax, and your bladder muscles contract, sending the urine from the bladder, through the urethra, and out of the body. When the bladder is empty, the bladder muscles relax, and the sphincter and pelvic muscles tighten.

Urinary nerves. The nervous system helps control urination. Before toilet training, a simple reflex controls the timing of urination. Stretch-detecting sensory nerves in the detrusor muscle signal the spinal cord that the bladder is filling with urine. In response, motor nerves from the spine signal the bladder muscles to contract and the urethra and pelvic floor muscles to relax, allowing urination. As the brain matures, it is able to override the spinal nerves' automatic signal for the bladder to empty. The bladder gradually becomes capable of storing greater amounts of urine, and a child learns to urinate when it is convenient and socially acceptable.

Figure 2: Male urinary function The normal male bladder stores urine as it comes from the kidneys through the ureter. Once the bladder is nearly full, the nerves signal "fullness" to the brain. When you are ready, you relax your urethral sphincter, allowing the urine to flow through the urethra, past the prostate gland, and out through the penis. Incontinence can occur when (1) prostate enlargement constricts the urethra, inhibiting the flow of urine, causing difficulty urinating and sometimes causing the bladder to overfill. Prostate surgeries may cause damage to crucial nerves, (2) detrusor overactivity, (3) weakened pelvic floor muscles, or (4) sphincter dysfunction.

Last updated:	September 05, 2008