The correct answer is option 'D': void urine when desired.

The urinary bladder is a hollow, muscular organ located in the pelvic region. Its main function is to store urine until it is voluntarily released from the body. When the bladder is filled with urine and reaches its maximum capacity, it sends signals to the brain indicating the need to urinate. The brain then initiates the process of voiding urine.

The process of voiding urine involves several steps:

1. Filling of the bladder: The bladder is initially empty. As the kidneys filter waste products from the blood, urine is produced and transported to the bladder through tubes called ureters. The bladder gradually fills up as urine is collected.

2. Sensory signals: As the bladder fills, specialized nerve endings in the bladder wall send sensory signals to the brain, indicating the level of bladder fullness. These signals are interpreted by the brain, which triggers the sensation of needing to urinate.

3. Voluntary control: The brain has voluntary control over the process of urination. When it is convenient or appropriate to empty the bladder, the brain sends signals to the bladder muscles to contract and the urethral sphincters to relax. This allows urine to flow out of the bladder and through the urethra.

4. Micturition reflex: The process of voiding urine is also regulated by the micturition reflex, which is an automatic response of the body when the bladder is full. This reflex involves coordinated contractions of the bladder muscles and relaxation of the urethral sphincters, leading to the expulsion of urine.

In summary, the urinary bladder helps to store urine until the brain signals the need to urinate. It then contracts to expel the urine through the urethra. This process allows for voluntary control over the timing and location of urination, ensuring that it occurs when desired and in a socially acceptable manner.

The correct answer is option 'D': the urinary bladder helps to void urine when desired. The urinary bladder is an important organ in the urinary system responsible for storing and eliminating urine from the body. It plays a crucial role in maintaining the body's overall fluid balance and excreting waste products.

The process of voiding urine, also known as urination or micturition, is a complex physiological process that involves the coordination of various muscles and nerves. Here is a detailed explanation of how the urinary bladder helps in voiding urine:

1. Storage of Urine:
- The primary function of the urinary bladder is to store urine produced by the kidneys until it can be eliminated.
- As the kidneys continuously filter waste products and excess water from the blood, urine accumulates in the bladder.
- The urinary bladder has a highly elastic and expandable wall, allowing it to stretch and accommodate increasing amounts of urine.

2. Sensory Signals:
- When the bladder becomes distended and reaches its capacity, specialized nerve endings called stretch receptors in the bladder wall are stimulated.
- These receptors send sensory signals to the brain, indicating a need to empty the bladder.

3. Coordination of Muscles:
- The process of voiding urine involves the coordination of both voluntary and involuntary muscles.
- The detrusor muscle, which forms the wall of the bladder, contracts under the influence of the parasympathetic nervous system.
- This contraction of the detrusor muscle helps to generate pressure within the bladder and expel urine.

4. Relaxation of Sphincters:
- Simultaneously, the external urethral sphincter, a ring of skeletal muscle surrounding the urethra, needs to relax to allow the passage of urine.
- The external sphincter is under voluntary control and is regulated by the somatic nervous system.
- When the appropriate signals are received from the brain, the external sphincter relaxes, allowing urine to flow out of the bladder through the urethra.

Overall, the urinary bladder acts as a reservoir for urine and facilitates its elimination from the body when desired. The coordination of various muscles and nerves ensures the proper storage and controlled release of urine, maintaining the body's fluid balance and eliminating waste products.