Final answer:
The detrusor muscle is responsible for compressing the urinary bladder to expel urine, with the help of the internal urethral sphincter that relaxes to allow urine flow.
Step-by-step explanation:
The muscle responsible for compressing the urinary bladder to expel its contents is the detrusor muscle. This muscle, found in the wall of the bladder, is made of smooth muscle fibers and is regulated by both the autonomic and somatic nervous systems. During urination, conscious signals cause the detrusor muscle to contract, consequently, the internal urethral sphincter relaxes to allow urine to pass through the urethra and out of the body. The contraction of the detrusor muscle is a forceful process in the young, and its potency diminishes with age, but additional force can be applied via voluntary contractions of abdominal skeletal muscles, aiding in more complete bladder emptying.