Process of excreting urine from the bladder (see urinary system). Nerve centres in the spinal cord, brain stem, and cerebral cortex control it through involuntary and voluntary muscles. The need to void is felt when the bladder holds 3.5–5 oz (100–150 ml) of urine and becomes uncomfortable at a volume of 14–15 oz (350–400 ml). The detrusor contracts and the sphincter (muscular constriction) of the urethra relaxes to empty the bladder. Normally it empties completely, but bladder stones or prostatic disorders can block outflow; poor muscle tone (especially in older women) or certain nervous system disorders can cause loss of control of urination (incontinence).
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"To whiz" or "whizzing" (or, alternatively, "to squizzle" or "squizzling") 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'. Others of note are "tinkle" and "potty" - both of which are often used with children.
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 urinary sphincter, which is innervated by somatic fibers originating in the sacral cord, in an area termed Onuf's nucleus.
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.
In healthy individuals, the lower urinary tract has two discrete phases of activity: the storage 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.
In infants, voiding occurs involuntarily (as a reflex). The ability to voluntarily inhibit micturition develops at 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.
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 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.
As the bladder becomes full, afferent firing increases, yet the micturition reflex can be voluntarily inhibited until it is appropriate to begin voiding (e.g. a bathroom is reached).
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.
Release of urine is experienced as a lessening of the uncomfortable, full, feeling. In most people this release is experienced as a relief.
Post-micturition convulsion syndrome, the feeling of a shiver running down the spine following urination, occurs in more than 80% of males, but also occurs in more than 55% of females. Its explanation is unknown.
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; many 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 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 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. If underpants are worn, the fly of the underpants is used or their front-side is lowered. All combinations are possible. Trousers without a fly, like some jogging trousers, usually have an elastic waist band allowing lowering the front side like underpants. When out in public, many men urinate into a urinal, while at home they will usually urinate into a toilet. Urinals are uncommon in private dwellings, although their lack of standing water is environmentally beneficial and makes urination into them quiet.
In females, the urethra opens straight into the vulva. Because of this, the urine does not exit at a distance from her body and is therefore very hard to control. Because of surface tension in the urine, the easiest method is to rely simply on gravitational force to take over once the urine has exited her body. This can easily be achieved if the female is sitting on a toilet. When sitting, it helps if the female leans forward and keeps her legs together, as this helps direct her stream downwards. When not urinating into a toilet, squatting is the easiest way for a female to direct her urine stream. 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.
Women who wear pants, trousers, or shorts will need to lower these garments to urinate. Women wearing skirts or dresses only need to raise them to their waists to urinate, just lowering the underpants. While urinating in the squatting position, pants are often just lowered to the midst of the thighs, and some women lift the midst of the lowered underpants up. Some women hold the front of their underpants or bikinipants to the side, hence not having to lower them.
It is possible for many females to urinate standing while spreading their legs. This technique for urinating while standing can be common when females often wear a sarong, skirt, or other such open bottomed garments, and either wear no underwear, or remove it. It is considered normal for females to urinate like this in many parts of Africa, whereas in contrast, public urination by females is not allowed for modesty reasons even when toilets are unavailable in places like India though public urination by males is accepted . In Africa, even signs which forbid public urination often show a picture of a female urinating while standing. . It is mostly in West Africa, like Ghana and Nigeria, that it is considered normal for a female to urinate standing up. In many other parts it mostly occurs in the countryside or not at all.
Though uncommon, it is 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 in a certain way and orienting the pelvis at an angle and rapidly forcing the urine stream out. An alternative method is to use a tool known as a female urination device to assist.
It is socially more accepted and more 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.
Acceptability of outdoor urination in a public place other than at a public urinal varies with the situation and with customs. Typically men do this standing up, and women squatting. Potential disadvantages are the "dirtyness", including 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 is avoided or mitigated by going to a quiet place, and/or, for men, facing a tree or wall. The more developed and crowded a place is the more urination tends to be objectional. 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.
Urination can also be seen and/or intended as a sign of disrespect or contempt for someone or something. In popular culture, signs of a cartoon figure (sometimes Calvin) urinating on another object (usually a car brand) are common.
It is common to find both men and women urinating in places like Africa, Vietnam and Cambodia because of lack of toilet facilities whereas in some places like India public urination is practiced by men only.
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 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 traveling 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. If used in the presence of others there is still the privacy issue: 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. It can also be used for vomiting.
It is possible for both genders to urinate into bottles in case of emergencies. Wide mouthed bottles can be used for girls. The technique can help the sickly and the children to urinate discreetly inside cars and in other places without getting seen by others.
Throughout the animal kingdom, urination often serves a utilitarian purpose. In dogs and other animals, urination can mark territory or express submissiveness. In small rodents such as rats and mice, it marks familiar paths.