What is the bladder able to expand as urine accumulates within due presence of?

The bladder is able to expand as urine accumulates within due presence of detrusor muscle relaxation.

Detrusor muscle relaxation allows the bladder to accommodate increasing volumes of urine without causing significant pressure increases. This relaxation is mediated by the parasympathetic nervous system, which releases neurotransmitters that cause smooth muscle relaxation in the detrusor muscle.

As the bladder fills, the detrusor muscle stretches, activating stretch receptors that send signals to the spinal cord and brain. This stretch reflex triggers the contraction of the detrusor muscle and relaxation of the urethral sphincter, which allows for urination.