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The act of passing URINE from the BLADDER, also called uresis or micturition. Urination occurs when the urethral sphincter relaxes at the same time the detrusor muscle that forms the middle layer of the bladder wall contracts, squeezing urine into the URETHRA.
The urethra carries the urine to the meatus, its opening on the outer surface of the body. In men the meatus is at the tip of the PENIS; in women the meatus is within the vulva between the CLITORIS and the VAGINA. Urination ends when the bladder sphincter closes and the residual urine in the urethra passes from the body.
Urination is a blend of involuntary and voluntary control. At birth urination is completely under the control of the micturition reflex and the sympathetic nervous system, which regulates involuntary functions. The micturition reflex is the series of events that begins when the filling of the bladder with urine activates specialized nerves in the bladder wall called stretch receptors.
The stretch receptors send nerve signals out to the spinal nerves (S2, S3, and S4) that control the urethral sphincter and the detrusor muscle. The spinal nerves send back the nerve signals that stimulate the detrusor muscle to contract and the urethral sphincter to relax. A structure within the pons of the brainstem, the pontine micturition center (PMC), coordinates these functions to occur simultaneously.
Control Urination and Normal Frequency of Urination
The ability to control urination becomes possible around the ages of three to five when the muscles and nerve paths mature. At this point of development the brain can override the involuntary nerve processes and the pubococcygeal muscle, which is a voluntary muscle, can override the involuntary muscle functions of the bladder.
The normal frequency of urination varies among individuals and with fluid consumption, which largely determines urine volume. A healthy adult produces between 1.5 and 3 liters of urine every 24 hours. Typically the stretch receptors respond when the bladder contains about 200 to 300 milliliters of urine; maximum capacity of the bladder is about 500 ml.
Conditions of Altered Urination
|CONDITIONS OF ALTERED URINATION|
|URINARY FREQUENCY||URINARY INCONTINENCE|
|URINARY RETENTION||URINARY URGENCY|
See also NEUROGENIC BLADDER.