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Urination, also called micturition, is the process of disposing urine from the urinary bladder through the urethra to the outside of the body. The process of urination is usually under voluntary control. When control over urination is lost or absent, this is called urinary incontinence. Urinary retention refers to the inability to urinate.
Urinary System
The micturition reflex is activated when the urinary bladder wall is stretched; it results in urination. This reflex occurs in the spinal cord, specifically in the sacral region that is modified by the higher centers in the brain: the pons and cerebrum. The presence of urine in the bladder stimulates the stretch receptors, which produces action potential.
Mechanism of Urination
The action potentials are carried by sensory neurons to the sacral segments of the spinal cord through the pelvic nerves the parasympathetic fibers carry the action potentials to the urinary bladder in the pelvic nerves. This causes the wall of the bladder to contract. In addition, decreased somatic motor action potentials cause the external urinary sphincter, which consists of skeletal muscle, to relax. When the external urinary sphincter is relaxed urine will flow from the urinary bladder when the pressure there is great enough to force urine to flow through the urethra. The micturition reflex normally produces a series of contractions of the urinary bladder.Action potentials carried by sensory neurons from stretch receptors in the urinary bladder wall also ascend the spinal cord to a micturition center in the pons and to the cerebrum. Descending potentials are sent from these areas of the brain to the sacral region of the spinal cord, where they modify the activity of the micturition reflex in the spinal cord. The micturition reflex, integrated in the spinal cord, predominates in infants. The ability to voluntarily inhibit micturition develops at the age of 2-3 years, and subsequently, the influence of the pons and cerebrum on the spinal micturition reflex predominates. The micturition reflex integrated in the spinal cord is automatic, but it is either stimulated or inhibited by descending action potentials. Higher brain centers prevent micturition by sending action potentials from the cerebrum and pons through spinal pathways to inhibit the spinal micturition reflex. Consequently, parasympathetic stimulation of the urinary bladder is inhibited and somatic motor neurons that keep the external urinary sphincter contracted are stimulated.
The pressure in the urinary bladder increases rapidly once its volume exceeds approximately 400-500 ml, and there is an increase in the frequency of action potentials carried by sensory neurons. The increased frequency of action potentials conducted by the ascending spinal pathways to the pons and cerebrum results in an increased desire to urinate.
Voluntary initiation of micturition involves an increase in action potentials sent from the cerebrum to facilitate the micturition reflex and to voluntarily relax the external urinary sphincter. In addition to facilitating the micturition reflex, there is an increased voluntary contraction of abdominal muscles, which causes an increase in abdominal pressure. This enhances the micturition reflex by increasing the pressure applied to the urinary bladder wall. Anatomic Considerations
The smooth muscle of the bladder, like that of the ureters, is arranged in spiral, longitudinal, and circular bundles. Contraction of this muscle, which is called the detrusor muscle, is mainly responsible for emptying the bladder during urination (micturition). Muscle bundles pass on either side of the urethra, and these fibers are sometimes called the internal urethral sphincter, although they do not encircle the urethra. Farther along the urethra is a sphincter of skeletal muscle, the sphincter of the membranous urethra (external urethral sphincter). The bladder epithelium is made up of a superficial layer of flat cells and a deep layer of cuboidal cells.
Micturition
The physiology of micturition and the physiologic basis of its disorders are subjects about which there is much confusion. Micturition is fundamentally a spinal reflex facilitated and inhibited by higher brain centers and, like defecation, subject to voluntary facilitation and inhibition. Urine enters the bladder without producing much increase in intravesical pressure until the viscus is well filled. In addition, like other types of smooth muscle, the bladder muscle has the property of plasticity; when it is stretched, the tension initially produced is not maintained. The relation between intravesical pressure and volume can be studied by inserting a catheter and emptying the bladder, then recording the pressure while the bladder is filled with 50-mL increments of water or air (cystometry). A plot of intravesical pressure against the volume of fluid in the bladder is called a cystometrogram . The curve shows an initial slight rise in pressure when the first increments in volume are produced; a long, nearly flat segment as further increments are produced; and a sudden, sharp rise in pressure as the micturition reflex is triggered. These three components are sometimes called segments Ia, Ib, and II. The first urge to void is felt at a bladder volume of about 150 mL, and a marked sense of fullness at about 400 mL. The flatness of segment Ib is a manifestation of the law of Laplace . This law states that the pressure in a spherical viscus is equal to twice the wall tension divided by the radius. In the case of the bladder, the tension increases as the organ fills, but so does the radius. Therefore, the pressure increase is slight until the organ is relatively full.
During micturition, the perineal muscles and external urethral sphincter are relaxed; the detrusor muscle contracts; and urine passes out through the urethra. The bands of smooth muscle on either side of the urethra apparently play no role in micturition, and their main function is believed to be the prevention of reflux of semen into the bladder during ejaculation.
The mechanism by which voluntary urination is initiated remains unsettled. One of the initial events is relaxation of the muscles of the pelvic floor, and this may cause a sufficient downward tug on the detrusor muscle to initiate its contraction. The perineal muscles and external sphincter can be contracted voluntarily, preventing urine from passing down the urethra or interrupting the flow once urination has begun. It is through the learned ability to maintain the external sphincter in a contracted state that adults are able to delay urination until the opportunity to void presents itself. After urination, the female urethra empties by gravity. Urine remaining in the urethra of the male is expelled by several contractions of the bulbocavernosus muscle.
Reflex Control
The bladder smooth muscle has some inherent contractile activity; however, when its nerve supply is intact, stretch receptors in the bladder wall initiate a reflex contraction that has a lower threshold than the inherent contractile response of the muscle. Fibers in the pelvic nerves are the afferent limb of the voiding reflex, and the parasympathetic fibers to the bladder that constitute the efferent limb also travel in these nerves. The reflex is integrated in the sacral portion of the spinal cord. In the adult, the volume of urine in the bladder that normally initiates a reflex contraction is about 300-400 mL. The sympathetic nerves to the bladder play no part in micturition, but they do mediate the contraction of the bladder muscle that prevents semen from entering the bladder during ejaculation.
There is no small motor nerve system to the stretch receptors in the bladder wall; but the threshold for the voiding reflex, like the stretch reflexes, is adjusted by the activity of facilitatory and inhibitory centers in the brain stem. There is a facilitatory area in the pontine region and an inhibitory area in the midbrain. After transection of the brain stem just above the pons, the threshold is lowered and less bladder filling is required to trigger it, whereas after transection at the top of the midbrain, the threshold for the reflex is essentially normal. There is another facilitatory area in the posterior hypothalamus. In humans with lesions in the superior frontal gyrus, the desire to urinate is reduced and there is also difficulty in stopping micturition once it has commenced. However, stimulation experiments in animals indicate that other cortical areas also affect the process. The bladder can be made to contract by voluntary facilitation of the spinal voiding reflex when it contains only a few milliliters of urine. Voluntary contraction of the abdominal muscles aids the expulsion of urine by increasing the intra-abdominal pressure, but voiding can be initiated without straining even when the bladder is nearly empty.
Abnormalities of Micturition
There are three major types of bladder dysfunction due to neural lesions: (1) the type due to interruption of the afferent nerves from the bladder; (2) the type due to interruption of both afferent and efferent nerves; and (3) the type due to interruption of facilitatory and inhibitory pathways descending from the brain. In all three types the bladder contracts, but the contractions are generally not sufficient to empty the viscus completely, and residual urine is left in the bladder. Paruresis, also known as shy bladder syndrome, is an example of a bladder interruption from the brain that often causes total interruption until the person has left a public area.
Effects of Deafferentation
When the sacral dorsal roots are cut in experimental animals or interrupted by diseases of the dorsal roots such as tabes dorsalis in humans, all reflex contractions of the bladder are abolished. The bladder becomes distended, thin-walled, and hypotonic, but there are some contractions because of the intrinsic response of the smooth muscle to stretch.
Effects of Denervation
When the afferent and efferent nerves are both destroyed, as they may be by tumors of the cauda equina or filum terminale, the bladder is flaccid and distended for a while. Gradually, however, the muscle of the "decentralized bladder" becomes active, with many contraction waves that expel dribbles of urine out of the urethra. The bladder becomes shrunken and the bladder wall hypertrophied. The reason for the difference between the small, hypertrophic bladder seen in this condition and the distended, hypotonic bladder seen when only the afferent nerves are interrupted is not known. The hyperactive state in the former condition suggests the development of denervation hypersensitization even though the neurons interrupted are preganglionic rather than postganglionic.
Effects of Spinal Cord Transection
During spinal shock, the bladder is flaccid and unresponsive. It becomes overfilled, and urine dribbles through the sphincters (overflow incontinence). After spinal shock has passed, the voiding reflex returns, although there is, of course, no voluntary control and no inhibition or facilitation from higher centers when the spinal cord is transected. Some paraplegic patients train themselves to initiate voiding by pinching or stroking their thighs, provoking a mild mass reflex . In some instances, the voiding reflex becomes hyperactive. Bladder capacity is reduced, and the wall becomes hypertrophied. This type of bladder is sometimes called the spastic neurogenic bladder. The reflex hyperactivity is made worse by, and may be caused by, infection in the bladder wall.
Causes of Incontinence
The initiation of urination is caused by the stretch in the wall of the bladder. But also irritation such as bacterial infections of the urinary bladder or the urethera or other conditions can initiate the desire to urinate, even when the urinary bladder is nearly empty. Consumption of alcoholic beverages or those containing caffeine may irritate the lining of the bladder, initiating a need to urinate.During urination, muscles in the wall of the bladder contract, forcing urine out of the bladder and into the urethra. At the same time, sphincter muscles surrounding the urethra relax, letting urine pass out of the body. Urinary incontinence will occur if the bladder muscles suddenly contract or muscles surrounding the urethra suddenly relax.
Stress Incontinence
Stress Inconinence occurs when a person is laughing or cannot maintain a normal volume of urine, for women during pregnancy and sometimes following childbirth. This type of incontinence is sometimes helped by Kegel exercises to strengthen the pelvic floor. Secondary incontinence refers to incontinence caused by disease or condition. Any damage to the nervous system, such as may occur with a spinal cord injury or a demyelinating disease like multiple sclerosis, may result in loss of urinary control.Urge Incontinence
Urge Incontinence is when urine leaks before an individual can reach a toilet (when you have 'urgency'). This may not be pathological and maybe actually be related to a mobility issue i.e. the individual is physically unable to get to a toilet before they begin to mitchurate.Overflow Incontinence
General
This is caused by any obstruction in the lower urinary tract (Bladder or Urethra). There are several causes, common ones include prostate hypertrophy and Calculi (Kidney stones), these cause blockage and lead to a subsequent increase in back pressure in the lower urinary tract.Acute
Generally presents as colicky pain, exacerbated by attempts at voiding. Obliteration of the obstruction will relive symptoms in the most part.Chronic
Generally presents as abdominal distension, with some pain. The pathology of this condition is different in that the back pressure results in reflux of urine in to the Renal Calices, causing permanent damage to the kidneys function.Coping With Incontinence
An indwelling Foley catheter may be inserted in extreme cases. There are many types of urine absorbing pads on the market. Both women and men may be able to learn to self-catheterize, thereby voiding on their own schedule.Urination techniques
Due to the differences in where the urethra ends, men and women use different techniques for urination.
Male urination
Because of the flexible and protruding nature of the penis, it is easy to control the urine stream. This makes it easy to urinate standing up, and most men urinate this way. But it is also possible to urinate sitting down on a toilet bowl. Some men prefer to urinate this way, and it also has the advantage that defecation can easily be done at the same time when sitting down. Urination can sometimes cause shivers down the spine whenever the urinating process is almost completed. This phenomenon is common amongst males although no scientific research has been done to explain the reason for this.Female urination
In women, the urethra opens straight into the vulva. Because of this, the urine does not exit at a distance from the body and is, therefore, hard to control. Because of surface tension in the urine, the easiest method is to just rely on gravity to take over once the urine has exited. This can easily be achieved if the woman is seated when urinating, although some women choose to squat, hover, or stand above the toilet-seat. Those alternative choices are sometimes made due to the perceived or actual unsanitary conditions at the location where the female is urinating. When sitting, it helps if the woman is leaning forward and keeping her legs together, as this helps direct the stream downwards * when not urinating into a toilet, squatting is the easiest way for a woman to direct her urine stream.it is also possible for women to urinate standing up. It is possible to do this by manipulating the genitalia in a certain way, orienting the pelvis at an angle, and rapidly forcing the urine stream out *
Animals
Animals usually kneel or, in males, lift legs to complete full urination.Other
Occasionally, if a males penis is damaged or removed, or a females genitals/urinary tract is damaged, other urination techniques must be used. Most often in such cases, doctors will reposition the urethra to a location where urination can still be accomplished, usually in a position that would only promote "female urination", or urination while seated/squatting, though a permanent urinary catheter may be used in some cases, however this technique is rarely used.Length of urination
The time it takes to urinate differs from person to person and from urination session to urination session. For example, it may take some people up to several minutes to fully void, while it may take others 5-10 seconds. The average time is much closer to the lower end of the scale. Normally, this depends on how long it has been and how much and what type of liquid the person has consumed since the last urination. Other factors include a size of the Prostate in men and the strength of the Urethral sphincter
Social factors
Toilet training
Babies have no socialized control over urination within societies that do not practice elimination communication and instead use diapers. Toilet training is the process of learning to restrict urination to socially approved times and situations. Many young children suffer from nocturnal enuresis.Toilets
It is socially more accepted and more hygienic for adults and older children to urinate at a toilet (also known euphemistically as a "bathroom"). Public toilets are often separate for men and women, and may be partitioned for reasons of modesty in some countries.Urinals
- Main article: urinal
Clothing designs
Trousers usually have a fly allowing men to urinate without lowering the whole trousers. The fly has buttons or a zipper. Either just the fly is opened or also the fastening at the waist (including, if present, the belt). Additionally, the fly of the underpants is used or their front-side is lowered. All combinations are possible. Trousers without fly, like some jogging trousers, have usually an elastic waist band allowing lowering the front side like underpants.Social transgressions
A common transgression is urinating in the street (except at a public urinal). Often this is done after consumption of alcoholic beverage: the alcohol causes production of additional urine as well as a reduction of inhibitions. In New Orleans, urination on the street is sometimes referred to as a "New Orleans Piss". In most places, public urination is punishable by fine.Alternatives to toilets
Sometimes urination is done in a container such as a bottle, urinal, bedpan or chamber pot, e.g. in case of lying sick in bed, in the case that the urine has to be examined (for medical reasons, or as a doping or other drugs test), or in the case that there is no toilet or it is inconvenient to go there, and no other possibility to dispose of the urine right away. See also Bedpan use and output measurement.For the latter application a more expensive solution (hence for special occasions while travelling etc.) is a special disposable bag containing absorbent material that solidifies the urine in 5 to 10 seconds, making it convenient and safe to keep (leakproof, puncture resistant, odorless) until there is an opportunity to dispose of it (conveniently like regular garbage). It can also be used for vomiting. As well, it is not uncommon for people who do not have access to toilets to simply urinate on the ground. The local flora such as a tree or bush can be used for added privacy. Females will usually seek some form of privacy while doing this, while males tend to be less inhibited. This is partially because of females having to fully expose their genitalia, and usually their buttocks as well, when urinating, and partially because it is more sociably acceptable for males to be less inhibited by public urination.
Animals
In dogs and other animals, urination can mark territory or express submissiveness. In small rodents such as rats and mice, it marks familiar paths.Slang
Urination is almost always referred to as "Peeing", a euphemism for "Piss" which is considered vulgar.Urination fetishes
Some pornography depicts urination. See also: Urolagnia, Pornography Introduction.References
Review of Medical Physiology,twentieth edition ,William F. Ganong, MDSee also
- Post-micturition convulsion syndrome ("piss shiver")
- Micturition Syncope
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"Urination"