In the human body, the internal and external urethral sphincters are muscles that help control the flow of the urine from the body. The internal one squeezes the urethra shut just outside of the bladder, and the external sphincter squeezes the urethra shut where it leaves the body. External ones in women simply stop urine from flowing out of the body, but these same muscles in men also aid in ejaculation and prevent semen from entering the bladder. Urinary incontinence can occur when either of these muscles are damaged.
The urethra is the small tube that urine passes through when it leaves the bladder and exits the body. A urethral sphincter is a smooth muscle that controls whether urine passes through the urethra. Relaxed sphincters allow the urine to pass through the urethra, while constricted ones prevent it. Humans have both internal and external urethral sphincters.
An internal urethral sphincter is located inside the body, just outside of the urinary bladder. It is found just after the bottom portion of the bladder. The muscle contractions of this sphincter are involuntary, meaning that a person can not control them.
External urethral sphincters, on the other hand, are voluntary muscles, which means that a person can contract or relax them consciously. This squeezes the urethra shut where the urethra exits the body. External urethral sphincters are slightly different in females and males.
The female external urethral sphincter is around the opening of the urethra on the outside of the body, and its only purpose is to stop urine from flowing out of the body. External sphincters in males are located at the base of the penis. These muscles not only stop the urine from leaving the body, but they also help expel ejaculate from the body and prevent semen from entering the urinary bladder.
Some individuals may have problems with their urethral sphincters becoming weak. This can have a few causes, such as injury or illness. It also occurs in elderly individuals, particularly women who have carried children. The weakening of these muscles often results in urinary incontinence, or the loss of control over urine leaving the body.
A person may be able to strengthen a urethral sphincter by performing Kegal exercises. These are done by flexing and relaxing these muscles, but it will often only help if the problem is mild. Medication and even surgery may be needed in severe cases.