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The act of urination goes like this (Visible Body): Micturition, or urination, is the act of emptying the bladder. When the bladder is full of urine, stretch receptors in the bladder wall trigger the micturition reflex. The detrusor muscle that surrounds the bladder contracts. The internal urethral sphincter relaxes, allowing for urine to pass out ...


There's no extra muscle involved - it's like squeezing an almost-empty toothpaste tube to get the last bits of toothpaste out.

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