Editing Urination (section)

====Voiding phase====
[[File:Litres of pee (4212939949).jpg|thumb|An [[elephant]]'s bladder can store up to 18 litres of urine.<ref>{{Cite journal |last1=Yang |first1=Patricia J. |last2=Pham |first2=Jonathan |last3=Choo |first3=Jerome |last4=Hu |first4=David L. |date=2014-08-19 |title=Duration of urination does not change with body size |journal=Proceedings of the National Academy of Sciences |language=en |volume=111 |issue=33 |pages=11932–11937 |bibcode=2014PNAS..11111932Y |doi=10.1073/pnas.1402289111 |issn=0027-8424 |pmc=4143032 |pmid=24969420 |doi-access=free}}</ref>]]
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.<ref>{{cite journal |vauthors=Blok BF, Holstege G | title = Direct projections from the periaqueductal gray to the pontine micturition center (M-region). An anterograde and retrograde tracing study in the cat | journal = Neurosci. Lett. | volume = 166 | issue = 1 | pages = 93–6 | date = January 1994 | pmid = 7514777 | doi = 10.1016/0304-3940(94)90848-6 | s2cid = 41146134 }}</ref> At a certain level of afferent activity, the conscious '''urge to void''' or '''urination urgency''',  becomes difficult to ignore. Once the voluntary signal to begin voiding has been issued, neurons in the 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 intravesical pressure occurs. The pontine micturition center also causes inhibition of Onuf's nucleus, resulting in relaxation of the external urinary sphincter.<ref>{{cite journal |vauthors=Sie JA, Blok BF, de Weerd H, Holstege G | title = Ultrastructural evidence for direct projections from the pontine micturition center to glycine-immunoreactive neurons in the sacral dorsal gray commissure in the cat | journal = J. Comp. Neurol. | volume = 429 | issue = 4 | pages = 631–7 | year = 2001 | pmid = 11135240 | doi = 10.1002/1096-9861(20010122)429:4<631::AID-CNE9>3.0.CO;2-M | s2cid = 7570375 }}</ref> When the external urinary sphincter is relaxed urine is released from the urinary bladder when the pressure there is great enough to force urine to flow out of 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.<ref>{{cite journal |last1=Jung |first1=Suk Young |last2=Fraser |first2=Matthew O. |last3=Ozawa |first3=Hideo |last4=Yokoyama |first4=Osamu |last5=Yoshiyama |first5=Mitsuharu |last6=De Groat |first6=William C. |last7=Chancellor |first7=Michael B. |title=Urethral afferent nerve activity affects the micturition reflex; implication for the relationship between stress incontinence and detrusor instability |journal=Journal of Urology |date=July 1999 |volume=162 |issue=1 |pages=204–212 |doi=10.1097/00005392-199907000-00069 |pmid=10379788 }}</ref>

Many men, and some women, may sometimes [[Pee shivers|briefly shiver]] after or during urination.<ref>{{cite news |last1=Briggs |first1=Bill |title=Pee shivers: You know you're curious |url=https://www.nbcnews.com/healthmain/pee-shivers-you-know-youre-curious-688401 |work=NBC News |date=9 April 2012 }}</ref>

After urination, the [[female urethra]] empties partially by gravity, with assistance from muscles.{{clarify|date=October 2012}} Urine remaining in the [[male urethra]] is expelled by several contractions of the [[bulbospongiosus muscle]], and, by some men, manual squeezing along the length of the penis to expel the rest of the urine.

For land mammals over 1 kilogram, the duration of urination does not vary with body mass, being dispersed around an average of 21 seconds (standard deviation 13 seconds), despite a 4 order of magnitude (1000×) difference in bladder volume.<ref name=law>{{cite arXiv |eprint=1310.3737 |first1=Patricia J. |last1=Yang |first2=Jonathan C. |last2=Pham |first3=Jerome |last3=Choo |first4=David L. |last4=Hu |title=Law of Urination: all mammals empty their bladders over the same duration |class= physics |year= 2013 }}</ref><ref name=ng>{{cite news |last1=Arnold |first1=Carrie |title=New Law of Urination: Mammals Take 20 Seconds to Pee |url=https://www.nationalgeographic.com/animals/article/mammals-pee-20-seconds-biology-urination |work=National Geographic |date=23 October 2013 }}</ref> This is due to increased urethra length of large animals, which amplifies gravitational force (hence flow rate), and increased urethra width, which increases flow rate. For smaller mammals a different phenomenon occurs, where urine is discharged as droplets, and urination in smaller mammals, such as mice and rats, can occur in less than a second.<ref name=ng /> The posited benefits of faster voiding are decreased risk of predation (while voiding) and decreased risk of urinary tract infection.