What is micturition?

Micturition is the process of passing urine from the urinary bladder to the outside. The kidneys make urine, the ureters carry it to the bladder, and the bladder stores it until it is released through the urethra. 

The act of releasing urine is called micturition or urination. It is controlled by a reflex that involves the spinal cord and brain centers.

Steps in the micturition reflex

1. Filling: Urine collects in the bladder. Stretch receptors in the bladder wall detect distension once volume rises (often 150–300 mL).
2. Reflex activation: Afferent nerves carry signals to the sacral spinal cord. Parasympathetic fibers stimulate the detrusor muscle to contract.
3. Sphincter control: Internal urethral sphincter (involuntary) relaxes; external urethral sphincter (voluntary, skeletal muscle) is consciously relaxed to allow flow.
4. Voiding: Coordinated detrusor contraction and sphincter relaxation expel urine.

Neural control

• Spinal reflex: Sacral segments (S2–S4) mediate the basic reflex.
• Higher centers: Pontine micturition center coordinates timing; cerebral cortex gives voluntary control over the external sphincter.

Clinical and everyday points

• Incontinence: Loss of voluntary control, common in children learning toilet habits and in some adults after nerve injury.
• Urinary retention: Failure to void completely due to obstruction or nerve issues.
• Hydration: Urine volume and color vary with fluid intake and hormones like ADH.