Urinary incontinence is the inability to control urination. It may be temporary or permanent, and can result from a variety of problems in the urinary tract. Urinary incontinence is generally divided into four types:
- Stress incontinence
- Urge incontinence
- Overflow incontinence
- Functional incontinence
Often, more than one type of incontinence is present. When this occurs, it is called mixed incontinence. Because incontinence is a symptom, rather than a disease, it is often hard to determine the cause. In addition, a variety of conditions may be the cause.
The urinary system helps to maintain proper water and salt balance throughout the body:
Click the icon to see an animation about urination.
- The process of urination begins in the two kidneys, which process fluids and eliminate water and waste products to produce urine.
- Urine flows out of the kidneys into the bladder through two long tubes called ureters.
- The bladder is a sac that acts as a reservoir for urine. It is lined with a tissue membrane and enclosed in a powerful muscle called the detrusor. The bladder rests on top of the pelvic floor. This is a muscular structure similar to a sling running between the pubic bone in front to the base of the spine.
- The bladder stores the urine until it is eliminated from the body via a tube called the urethra, which is the lowest part of the urinary tract. (In men it is enclosed in the penis. In women it leads directly out.)
- The connection between the bladder and the urethra is called the bladder neck. Strong muscles called sphincter muscles encircle the bladder neck (the smooth internal sphincter muscles) and urethra (the fibrous external sphincter muscles).
The Process of Urination
The process of urination depends on a combination of automatic and voluntary muscle actions. There are two phases: the emptying phase and the filling and storage phase.
The Filling and Storage Phase. When a person has completed urination, the bladder is empty. This triggers the filling and storage phase, which includes both automatic and voluntary actions.
- Automatic Actions. The automatic signaling process in the brain relies on a pathway of nerve cells and chemical messengers (neurotransmitters) called the cholinergic and adrenergic systems. Important neurotransmitters include serotonin and noradrenaline. This pathway signals the detrusor muscle surrounding the bladder to relax. As the muscles relax, the bladder expands and allows urine to flow into it from the kidney. As the bladder fills to its capacity (about 8 - 16 oz of fluid) the nerves in the bladder send back signals of fullness to the spinal cord and the brain.
- Voluntary Actions. As the bladder swells, the person becomes conscious of a sensation of fullness. In response, the individual holds the urine back by voluntarily contracting the external sphincter muscles, the muscle group surrounding the urethra. These are the muscles that children learn to control during the toilet training process.
When the need to urinate becomes greater than one's ability to control it, urination (the emptying phase) begins.
The Emptying Phase. This phase also involves automatic and conscious actions.
- Automatic Actions. When a person is ready to urinate, the nervous system initiates the voiding reflex. The nerves in the spinal cord (not the brain) signal the detrusor muscle to contract. At the same time, nerves are also telling the involuntary internal sphincter (a strong muscle encircling the bladder neck) to relax. With the bladder neck now open, the urine flows out of the bladder into the urethra.
- Voluntary Actions. Once the urine enters the urethra, a person consciously relaxes the external sphincter muscles, which allows urine to completely drain from the bladder.
The female and male urinary tracts are relatively the same except for the length of the urethra.