Explain micturition.

Hint: After its formation in the kidneys, urine is stored in the urinary bladder. The urinary bladder has stretch receptors in its walls. Stretching of its walls as it becomes filled with a particular volume of urine opens the urethral sphincters. It leads to the expulsion of the urine from the bladder to the outside of the body.

Complete answer:
Micturition includes stretch receptors, stretch reflex, and the urinary bladder.
Let’s discuss them one by one.
Before talking about stretch receptors, one should know what the receptors are. They are the specialized cells that are sensitive to the particular type of stimulus. Stretch receptors, as the name suggests, are sensitive to the stretching of the muscles. As the urine is formed in the kidneys, it is passed to the urinary bladder for the temporary storage. The urinary bladder is a muscular organ that has stretch receptors in its muscular wall.

Another peculiar feature of the urinary bladder is the presence of smooth muscles in its walls. Recall that smooth muscles contract involuntarily.
To release the urine outside the body, the urinary bladder should be open to the exterior of the body. This requirement is fulfilled by the urethra, a small tube present in the floor of the urinary bladder that serves as a passageway for the exit of urine from the body. The urinary bladder also has an internal and external sphincter. The sphincter is any muscle that regulates the opening and closing of an organ. The internal sphincter is present around the opening to the urethra while the external one is present inferior to the internal sphincter.

Knowing the prerequisite information, let's discuss the mechanism by which the urine is expelled from the urinary bladder to the out of the body. When the urinary bladder is filled with about 250 ml of the urine, the stretch receptors present in its wall send the information to the spinal cord. The spinal cord signals the simultaneous contraction of the smooth muscles present in the wall of the urinary bladder and the relaxation of the urethral sphincters. It allows the release of the urine from the body and the process is called micturition.

Note: The process of micturition removes the nitrogenous wastes from the body in the form of urine. Kidneys, ureters, urinary bladder, and urethra are the major organs of the urinary system. Kidneys are the primary organ involved in the formation of urine. Ureters are the muscular tubes that pass the urine from the kidneys to the urinary bladder. Also, the process of micturition includes both voluntary and involuntary muscles.