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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 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.
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.
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.
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.
Social factors
Babies have no socialized control over urination. Toilet training is the process of learning to restrict urination to socially approved times and situations.
Many young children suffer from nocturnal enuresis.
In wealthy countries, adults and older children usually 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.
In western societies, public toilets often have urinals for men only because it is
culturally less accepted for women to urinate standing up. A novelty, still rare, are public urinals for women, allowing them
also to urinate standing up. This is done using a special tool [1] or with the finger-assist method [2] . Urinals may have partitions between them to avoid men
being able to see the penises of other men, a social taboo in many countries. Men with a mild form of paruresis, or "shy
bladder syndrome," have difficulty using a urinal next to somebody else, and will tend to use a cubicle instead. In more severe
cases even that is difficult.
Men's 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 frontside 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.
A common transgression is urinating in the street. Often this is done after consumption of alcoholic beverage: the alcohol causes production of additonal urine as well as a reduction of
inhibitions. This is sometimes referred to as a "New Orleans Piss".
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 Nurse assistant skills#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.
In dogs and other animals, urination can mark territory or express submissiveness.
Some pornography depicts urination. See also: Urolagnia, Pornography#Introduction.
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