Final answer:
The detrusor muscle is responsible for emptying the bladder, while the internal and external urethral sphincters control the release of urine. The detrusor contracts to expel urine, and the urethral sphincters relax to allow urine to pass during urination.
Step-by-step explanation:
The muscle responsible for emptying the bladder and closing the bladder orifice is the detrusor muscle, which is made of smooth muscle fibers.
This muscle is controlled by both the autonomic and somatic nervous systems. As the bladder fills, the detrusor muscle relaxes to accommodate more urine.
During urination, signals from the nervous system cause the detrusor muscle to contract, expelling urine into the urethra, while the internal urethral sphincter relaxes.
Conversely, for urine retention, the urethral sphincter muscles play a crucial role. The internal urinary sphincter, composed of smooth muscle at the junction of the bladder and urethra, helps retain urine as the bladder fills.
The external urethral sphincter, made of striated muscle and under voluntary control, can be consciously contracted to maintain continence until one is ready to urinate.
The process of urination, also known as the micturition reflex, involves both involuntary and voluntary control mechanisms.
Voluntary urination begins with the conscious relaxation of the external urethral sphincter, which, alongside the contraction of the detrusor muscle, allows urine to flow out of the bladder and the body.