12.9: Urination

Elimination of urine from the bladder is called urination, micturition, or voiding. This process is similar in operation to the elimination of feces. When the bladder is empty, its muscular wall is relaxed and the internal urethral sphincter is contracted. As urine from the ureters enters the bladder, the bladder stretches outward. After 200 to 300 ml of urine has entered the bladder, pressure in the bladder rises and is detected by sensory neurons. Impulses are sent by nerves to the spinal cord and brain, causing the person to perceive the need to void. At this point autonomic impulses from the spinal cord reflexively stimulate contraction of the detrusor muscle and relaxation of the internal urethral sphincter, causing urine to flow out through the urethra. However, urination can be prevented by voluntary impulses from the brain that suppress the impulses from the spinal cord and cause contractions of the external urethral sphincter and muscles in the pelvic floor.

If urination is voluntarily prevented, the perception of fullness and pressure in the bladder subsides. Bladder filling, bladder stretching, and increasing pressure continue until the sensory neurons are stimulated enough to again cause the sensation of fullness. Once again reflex voiding is initiated, and bladder emptying can be voluntarily prevented. This process can be repeated until the pressure rises high enough and impulses from neurons that detect bladder pressure become powerful enough to override efforts to retain the urine.

In most circumstances, maximum bladder filling and very high pressures do not develop because voluntary impulses that suppress voiding are purposely stopped. Then reflex contraction of the bladder, together with relaxation of both urethral sphincters and the pelvic floor muscles, results in forceful elimination of urine through the urethra. Once initiated, voiding usually continues reflexively until the bladder is empty, at which point the bladder relaxes and the internal urethral sphincter contracts again. Voiding can be stopped before complete emptying has been achieved by voluntarily contracting the external urethral sphincter and pelvic floor muscles.

Urination can occur voluntarily as long as the bladder contains some urine. Then voluntary contraction of abdominal muscles causes bladder pressure to rise, initiating the voiding reflex. Then voluntary relaxation of the external sphincter and pelvic floor muscles permits urine flow.

Age Changes

Age changes in the sensory nerves associated with the bladder cause a declining ability to detect bladder stretching and pressure; some individuals lose all ability to perceive bladder fullness. These sensory changes increase the risk of prolonged urine retention and therefore urinary incontinence. However, the effects of age changes in the bladder usually override the effects of changes in the sensory neurons and cause voiding to occur more frequently and at lower bladder volumes.

Urinary Incontinence

Adequate control of urination is retained regardless of age unless abnormal or disease conditions reduce it. Since the incidence and severity of many of these conditions and diseases increase with age, the incidence of abnormal and inadequate control of urination also rises with age.

One form of inadequate control that becomes more common as age increases is urinary incontinence. Estimates of its incidence vary widely depending on both the strictness applied in defining this condition and the techniques used to identify it. Among noninstitutionalized people over age 65, 5 percent to 15 percent of men and 11 percent to 50 percent of women have at least temporary urinary incontinence. However, at least 50 percent of institutionalized elderly people have urinary incontinence. The ratio of occurrence between elderly hospitalized women and men is approximately 2:1.

The very high incidence of urinary incontinence among institutionalized individuals occurs because incontinence is a main reason for institutionalizing older individuals and because many other conditions leading to institutionalization contribute to it. Examples include dementia, strokes, and severe physical disability.

Types

Four distinct types of urinary incontinence can be identified. Some individuals may have two or more types simultaneously. Overflow incontinence is due to excess pressure in the bladder caused by excessive urine retention. This type of incontinence, which is less common than the other types, may or may not be accompanied by a strong sensation of bladder fullness. Urge incontinence is accompanied by a strong perception that urination is necessary even though the bladder is not filled to capacity. It is often due to excess bladder contractions. Stress incontinence involves urine loss from factors that weaken muscles in the sphincter and pelvic floor. Incontinent events often occur when a rise in abdominal pressure causes higher bladder pressure, such as during coughing, laughing, sneezing, and strenuous effort such as standing up and lifting a heavy object. Stress incontinence is much more common in women than in men because women have shorter urethras and postmenopausal thinning and weakening of structures used for retaining urine. Functional incontinence results from factors that reduce the cognitive functions needed to control urination. Factors include dementia, stroke, and strongly psychoactive medications. This type of incontinence involves no abnormalities or diseases of the urinary system. Some people have more than one type of urinary incontinence, a condition called mixed incontinence.

Overflow incontinence causes elimination of small volumes of urine. Stress, urge, and functional incontinence may result in loss of urine volumes ranging from a few drops to several hundred milliliters. Urge and functional urinary incontinence may cause complete bladder emptying.

Contributing Factors

Urinary incontinence results from excess bladder pressure caused by excess urine production, urine retention, or stimulation of the bladder; from inadequate contraction of pelvic floor muscles due to muscle weakness or nervous system malfunction; or from a combination of these conditions. A person may have two or more factors acting simultaneously or in various sequences.

Effects and Complications

Urinary incontinence has the same undesirable results that characterize fecal incontinence. These include skin inflammation, sores, and infection; social and psychological disruptions; and institutionalization. Costs for devices and supplies (e.g., absorbent undergarments) for adults with urinary incontinence reach 10 billion dollars per year.

Prevention and Treatments

Some cases of urinary incontinence can be prevented by avoiding factors that substantially increase the risk of developing this condition. Examples include certain medications (e.g., diuretics, psychoactive drugs) and limited access to toilet facilities.

Many individuals with urinary incontinence can reduce their incidents of incontinence substantially or can be cured. As with fecal incontinence, the nature and extent of interactions between care givers and persons with urinary incontinence can influence the degree of success achieved. Steps can also be taken to reduce the impact of incidents of urinary incontinence. The first step is to identify the factors leading to incontinence. This procedure may involve taking a patient history, performing a physical examination that includes special tests for urinary function, evaluating nervous system function, and scrutinizing the medications being taken. Once the type of incontinence and the contributing factors have been identified, an individualized care plan can be developed (Table 12.1).

Table 12.1 TREATMENTS FOR URINARY INCONTINENCE

Regulate intake of fluids and diuretics (e.g., alcohol, caffeine, drugs) to reduce urine formation

Regulate all medications affecting urinary or nervous system functioning

Assure accessibility to facilities such as bedpans, urinals, and care giver assistance

Urinate at scheduled times

Cure urinary tract infections to reduce bladder instability

Exercise sphincter and pelvic floor muscles to increase strength (e.g., Kegel exercises)

Use estrogen therapy in women to increase urethral strength

Take medications to modify bladder and internal sphincter function

Undergo surgery to remove obstructions (e.g., prostate surgery), enlarge the bladder, denervate the bladder, or implant an artificial sphincter

Use behavioral modification and training

Use biofeedback control to increase awareness of need to void and gain better control of muscles

Use electrical stimulators to control muscles

Use absorbent pads or male condom catheters to catch urine

Perform skin care to avoid complications

Use catheters to drain urine (can lead to complications such as infections and bladder instability)