Science Fair Project Encyclopedia
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.
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.
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.
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 chose to squat or hover above the toilet-seat.
A little known fact is that it is 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 . An alternative method is to use a tool to assist .
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.
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.
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  or with the finger-assist method . 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 additional urine as well as a reduction of inhibitions. This is sometimes referred to as a "New Orleans Piss".
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 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.
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