Urination, also known as micturition, voiding, peeing, and more rarely, emiction, is the process of disposing of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans the process of urination is under voluntary control. In infants, elderly individuals and those with neurological injury, urination may occur as an involuntary reflex. In other animals, in addition to urination serving the purpose of expulsion of waste material, the act can serve to mark territory or express submissiveness. Physiologically, micturition involves coordination between the central, autonomic and somatic nervous systems. Brain centers that regulate urination include the pontine micturition center, periaqueductal gray, and the cerebral cortex.
The main organs involved in urination are the bladder and the urethra. The smooth muscle of the bladder, known as the detrusor, is innervated by sympathetic nervous system fibers from the lumbar spinal cord and parasympathetic fibers from the sacral spinal cord. Fibers in the pelvic nerves constitute the main afferent limb of the voiding reflex; the parasympathetic fibers to the bladder that constitute the excitatory efferent limb also travel in these nerves. Part of the urethra is surrounded by the external urethral sphincter, which is innervated by the somatic pudendal nerve originating in the cord, in an area termed Onuf's nucleus.
Smooth 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's epithelium is termed transitional epithelium which contains a superficial layer of dome-like cells and multiple layers of stratified cuboidal cells underneath when evacuated. When the bladder is fully distended the superficial cells become squamous (flat) and the stratification of the cuboidal is reduced in order to provide lateral stretching.
The physiology of micturition and the physiologic basis of its disorders are subjects about which there is much confusion, especially at the supraspinal level. Micturition is fundamentally a spinobulbospinal reflex facilitated and inhibited by higher brain centers such as the pontine micturition center and, like defecation, subject to voluntary facilitation and inhibition.
In healthy individuals, the lower urinary tract has two discrete phases of activity: the storage (or guarding) phase, when urine is stored in the bladder; and the voiding phase, when urine is released through the urethra. The state of the reflex system is dependent on both a conscious signal from the brain and the firing rate of sensory fibers from the bladder and urethra. At low bladder volumes, afferent firing is low, resulting in excitation of the outlet (the sphincter and urethra), and relaxation of the bladder. At high bladder volumes, afferent firing increases, causing a conscious sensation of urinary urge. When the individual is ready to urinate, he or she consciously initiates voiding, causing the bladder to contract and the outlet to relax. Voiding continues until the bladder empties completely, at which point the bladder relaxes and the outlet contracts to re-initiate storage. The muscles controlling micturition are controlled by the autonomic and somatic nervous systems. During the storage phase the internal urethral sphincter remains tense and the detrusor muscle relaxed by sympathetic stimulation. During micturition, parasympathetic stimulation causes the detrusor muscle to contract and the internal urethral sphincter to relax. The external urethral sphincter (sphincter urethrae) is under somatic control and is consciously relaxed during micturition.
It is commonly believed that in infants, voiding occurs involuntarily (as a reflex). However, the practice of elimination communication suggests otherwise. The ability to voluntarily inhibit micturition develops by the age of 2–3 years, as control at higher levels of the central nervous system develops. In the adult, the volume of urine in the bladder that normally initiates a reflex contraction is about 300-400 ml.
During storage, bladder pressure stays low, because of the bladder's highly compliant nature. A plot of bladder (intravesical) pressure against the depressant of fluid in the bladder (called a cystometrogram) will show a very slight rise as the bladder is filled. This phenomenon is a manifestation of the law of Laplace, which 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. The bladder's 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.
Action potentials carried by sensory neurons from stretch receptors in the urinary bladder wall travel to the sacral segments of the spinal cord through the pelvic nerves. Since bladder wall stretch is low during the storage phase, these afferent neurons fire at low frequencies. Low-frequency afferent signals cause relaxation of the bladder by inhibiting sacral parasympathetic preganglionic neurons and exciting lumbar sympathetic preganglionic neurons. Conversely, afferent input causes contraction of the sphincter through excitation of Onuf's nucleus, and contraction of the bladder neck and urethra through excitation of the sympathetic preganglionic neurons.
Diuresis (production of urine by the kidney) occurs constantly, and as the bladder becomes full, afferent firing increases, yet the micturition reflex can be voluntarily inhibited until it is appropriate to begin voiding.
Voiding begins when a voluntary signal is sent from the brain to begin urination, and continues until the bladder is empty.
Bladder afferent signals ascend the spinal cord to the periaqueductal gray, where they project both to the pontine micturition center and to the cerebrum. At a certain level of afferent activity, the conscious urge to void becomes difficult to ignore. Once the voluntary signal to begin voiding has been issued, neurons in pontine micturition center fire maximally, causing excitation of sacral preganglionic neurons. The firing of these neurons causes the wall of the bladder to contract; as a result, a sudden, sharp rise in pressure in intravesical pressure occurs. The pontine micturition center also causes inhibition of Onuf's nucleus, resulting in relaxation of the external urinary sphincter. When the external urinary sphincter is relaxed urine flows 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.
The flow of urine through the urethra has an overall excitatory role in micturition, which helps sustain voiding until the bladder is empty.
After urination, the female urethra empties by gravity. Urine remaining in the urethra of the male is expelled by several contractions of the bulbospongiosus muscle.
The mechanism by which voluntary urination is initiated remains unsettled. One possibility is that the voluntary relaxation of the muscles of the pelvic floor causes a sufficient downward tug on the detrusor muscle to initiate its contraction. Another possibility is the excitation or disinhibition of neurons in the pontine micturition center, which causes concurrent contraction of the bladder and relaxation of the sphincter.
There is an inhibitory area for micturition 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 pressure applied to the urinary bladder wall, but voiding can be initiated without straining even when the bladder is nearly empty.
Voiding can also be consciously interrupted once it has begun, through a contraction of the perineal muscles and the external sphincter can be contracted voluntarily, which will prevent urine from passing down the urethra.
The other way around, voiding can be facilitated by immersing a hand in a cup or sink full of warm water. The mechanism is unclear. The phenomenon (and perhaps immersion diuresis) has given rise to the trick of immersing the hand of a sleeping person in water to make this victim urinate in sleep, although the efficacy of the trick is disputed.
The need to urinate is experienced as an uncomfortable, full, feeling. It is highly correlated with the fullness of the bladder. In males the feeling of the need to urinate can be sensed at the base of the penis, even though the neural activity associated with a full bladder comes from the bladder itself. In females the need to urinate is felt in the lower abdomen region when the bladder is full. When the bladder becomes too full, the sphincter muscles will involuntarily relax, allowing urine to pass from the bladder.
Release of urine is experienced as a lessening of the uncomfortable, full, feeling. In most people this release is experienced as a relief.
Many clinical conditions can cause disturbances to normal urination. Here is a partial list:
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.
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.
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.
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.
Because of the flexible and protruding nature of the penis, it is simple to control the direction of the urine stream. This makes it easy to urinate while standing; most males urinate this way. Extant foreskin (acropostheon) may block the direct path of the outgoing stream by causing turbulence, resulting in a slower but thicker stream of urine that may also dribble. A retracted or absent foreskin may have a more focused stream of urine that travels at the same speed as it exits the urethra. When a male is done urinating, he will usually shake and/or gently squeeze his penis to expel the excess urine trapped in the opening of the foreskin or on the glans. Trousers commonly have a fly allowing men to urinate without lowering the whole trousers. Trousers without a fly, like some jogging trousers, usually have an elastic waist band allowing lowering the front side like underpants. Some men also urinate sitting down, especially if defecation has to be done at the same time.
In females, 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. Hence, urination will normally have to take place while sitting down. When sitting, it helps if the female leans forward and keeps her legs together, as this helps direct her urine stream downwards. When not urinating into a toilet, squatting is the easiest way for a female to direct the urine stream. If done this way, the urine will go forward downwards. Some females use one or both hands to focus the direction of the urine stream, which is more easily achieved while in the squatting position.
It is also possible for many females to urinate standing while spreading their legs. This technique is not uncommon for a female who wears a sarong, skirt, or other such open bottomed garments, and either wears no underwear, or removes it. In Africa, even signs which forbid public urination often show a picture of a female urinating like this. Laos is a country where it is common that women urinate standing. There are reports that seem to indicate that in Togo women urinate using this method, while men do it squatting. Herodotus described a similar custom in ancient Egypt. Though uncommon, it is also possible for females to urinate standing up, in a way similar to that of males. This may be done by spreading the labia minora open and orienting the pelvis at an angle, and rapidly forcing the urine stream out. Reports indicate that it is common that women in the Ivory Coast use this method when they urinate. Some sources seem to indicate that women urinate this way in West Africa in general. Other reports indicate, however, that it seems to be less common for women to urinate standing in West Africa, while it has been reported that it is common that women urinate standing up in the streets in Malawi. An alternative method for women to urinate standing is to use a tool known as a female urination device to assist.
Occasionally, if a male's penis is damaged or removed, or a
female's 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
urination while seated/squatting, though a permanent
urinary catheter may be used in rare cases.
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. Consequently, young children sometimes suffer from nocturnal enuresis.
It is socially more accepted and more environmentally hygienic for adults and older children to urinate in a toilet. In some countries, public toilets are separated for men and women, and may be partitioned for reasons of cultural modesty.
Public restrooms may have urinals for male users. Female urinals, though rare, allow females to urinate while standing through the use of a special tool or through the finger-assist method. Urinals for either sex may have partitions between them in order to increase privacy. People with a mild form of paruresis, or "shy bladder syndrome," may have difficulty urinating in the presence of others and will consequently avoid using urinals directly adjacent to another person. Alternatively, they may opt for the privacy of a stall or simply avoid public restrooms altogether.
Acceptability of outdoor urination in a public place other than at a public urinal varies with the situation and with customs. Typically males do this standing up, while females squat. Potential disadvantages include the smell of urine, and some exposure of genitals. The latter can be unpleasant for the one who exposes them (modesty, lack of privacy) and/or those who can see them; it can be avoided or mitigated by going to a quiet place, and/or, for males, facing a tree or wall and for females, hiding the back behind walls, bushes, or a tree. The more developed and crowded a place is the more urination tends to be objectionable. In the countryside it is more acceptable than in a street in a town. In the latter case it is a common transgression. Often this is done after the consumption of alcoholic beverages. The alcohol causes production of additional urine as well as a reduction of inhibitions. In most places, public urination is punishable by fine, though attitudes vary widely by country.
It is common to find both men and women urinating outdoors in Africa, Vietnam, Laos and Cambodia, and, in one report, also in the United Kingdom, because of lack of toilet facilities. However in India women are told to control urine, whereas men of all ages urinate in public, because no facilities are found.
Females typically feel a need to urinate more than males on account of their (usually) smaller bladders. Resisting the urge to urinate because of lack of facilities can promote urinary tract infection in women.
Sometimes urination is done in a container such as a bottle, urinal, bedpan or chamber pot, also known as a gazunder, e.g., in case of lying sick in bed, in the case that the urine has to be examined (for medical reasons, or for a drug test), or when no toilet is available, and there is 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 traveling etc.) is a special disposable bag containing absorbent material that solidifies the urine within ten seconds, making it convenient and safe to keep. If used in the presence of others, there is still the issue of privacy; it is hardly suitable in public transport, because if there is no toilet there is typically no other place to withdraw either; depending on social aspects and/or possibilities to withdraw it may or may not be suitable in a car, boat, private plane, etc., shared with others. Such a bag can also be used for vomiting, should one experience motion sickness.
It is possible for both sexes to urinate into bottles in case of emergencies. The technique can help the sickly and the children to urinate discreetly inside cars and in other places without being seen by others.
In many societies and in many social classes, even mentioning the need to urinate is seen as a social transgression, despite it being a universal need. Even today, many adults avoid stating that they need to urinate.
Urination is often referred to as "peeing", a euphemism for "piss" which is considered more vulgar. It is also referred to as "weeing", "squirting", "taking a leak", or "doing/having a wee-wee".
"To whiz" is common in the U.S. "To piddle" is common in the U.K., as well as "to have a slash", which originates from the Scottish word "slash" meaning a large splash of liquid. An English term is to 'splash (one's) boots'. In the UK, common euphemisms include "Spending a penny" (a reference to coin-operated pay toilets), "Going to see my aunt", or "Going to see a man about a dog." Others of note are "tinkle" and "potty" - both of which are often used with children. An English language expression predominantly used by younger women (and men) for outdoor female urination is "popping a squat" - referring to the squatting position many women adapt while urinating outdoors. One expression used in some circles in Australia is to say "I am off to take a Chinese singing lesson," derived from the tinkling sound of urination against the China porcelain of a toilet bowl. The phrase 'to spend a penny' is common in the UK as a euphemism for going to urinate. Its origin is in the old British custom of charging women 1d (pre-decimal penny) in public toilets.
While the primary purpose of urination is the same across the animal kingdom, urination often serves a social purpose beyond the expulsion of waste material. In dogs and other animals, urination can mark territory or express submissiveness. In small rodents such as rats and mice, it marks familiar paths.
The urine and urination of animals of differing physiology or sex sometimes have different characteristics. For example, the urine of birds and reptiles is whitish, consisting of a pastelike suspension of uric acid crystals, and discharged with the feces of the animal via the cloaca, whereas mammals' urine is a yellowish colour, the uric acid having been chemically converted into urea, and is discharged via the urethra, separately from the feces. Some animals' (example: carnivores') urine possesses a strong odour, especially when it is used to mark territory or communicate in other ways. A male fox raises one hind leg and its urine is sprayed forward in front of it, whereas a female fox squats down so that the urine is sprayed in the ground between the hind legs. Four-legged animals usually kneel, or lift or spread one or more legs, to complete full urination.
References to peeing and pissing are commonly used in crass slang. Usage includes:
The content of this section is licensed under the GNU Free Documentation License (local copy). It uses material from the Wikipedia article "Urination" modified November 23, 2009 with previous authors listed in its history.