urination

Urination, also known as micturition or voiding, is the process by which urine is expelled from the body. Urine is produced by the kidneys, stored in the bladder, and released through the urethra when a person voids.

Anatomy and flow: The kidneys filter blood to form urine, which travels via the ureters to the bladder. The bladder stores urine until a voluntary decision to urinate. Outflow is controlled by two sphincters: the internal urethral sphincter (involuntary, smooth muscle) and the external urethral sphincter (voluntary, skeletal muscle). The urethra serves as the conduit to the exterior.

Physiology and control: The bladder wall contains stretch receptors that signal fullness. A reflex arc involving the spinal cord and brainstem coordinates detrusor muscle contraction and relaxation of the internal sphincter. Voluntary relaxation of the external sphincter allows urination. Higher brain centers provide conscious control to delay or permit voiding.

Development and variation: In newborns, urination is largely reflexive; continence develops with age. Adult bladder capacity typically ranges in the hundreds of milliliters. Age, pregnancy, neurological conditions, medications, and lifestyle factors can influence storage and voiding function.

Clinical aspects: Common topics include urinary tract infections, urinary incontinence (loss of bladder control), urinary retention, dysuria (painful urination), nocturia (nighttime urination), and hematuria (blood in urine). Evaluation may involve urinalysis, imaging studies, and functional tests such as urodynamics.

Management and context: Approaches emphasize general health, hydration, bladder training, and treatment of underlying conditions. Medications or procedures may be used for specific disorders, with care guided by clinical evaluation. Urination is a normal, essential bodily function.