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The urethra is a tube that transports urine from the bladder to the tip of the penis. Normally, the urethra is widely patent (open) throughout, and therefore, as the bladder empties, there is no restriction of flow.
A urethral stricture is a disease process associated with a narrowing or stenosis of the involved segment of the urethra. Although this is referred to as a "disease", it is not contagious or generally associated with other medical problems. The consequence of a urethral stricture is obstructive urination. This means that as the bladder contracts (squeezes to empty), the flow through the urethra is impaired the way a kink/pinch in a garden hose is associated with a major reduction of water flow.
Urine is made in the kidneys. The urine then travels through tubes called ureters to the bladder as shown in the illustration below.
|The bladder then fills with urine. Once the bladder is full, the distension of the bladder is sensed as an urge to urinate. During urination, the bladder contracts (squeezes), this contraction continues until the bladder is completely empty. Immediately after urination, there should be no urine or very little residual urine remaining in the bladder.|
As the bladder contracts, urine first passes through an opening called the bladder neck, a connection between the bladder and the prostate.
This is an important area as this is a main area responsible for continence (not leaking urine). The bladder neck is closed most of the time and this keeps the urine in the bladder. However, when the bladder is contracting, the bladder neck reflexively opens, allowing the urine to pass through.
The urine then travels through the prostate (a gland that surrounds the urethra). This portion of the urethra, closest to the bladder, is called the prostatic urethra. As a side note, during ejaculation but not urination, secretions from the prostate and other glands called seminal vesicles enter the urethra here along with sperm produced by the testicles. The sperm made in the testicle travels though a structure attached to the testicle called the epididymis and then into a tubular structure called the vas deference or "vas" for short, and the vas connects to the prostatic urethra as a duct called the ejaculatory duct. When a man has a vasectomy the vas is cut and tied on both sides.
As the urine continues to travel towards the penis, it enters a short segment called the membranous urethra, which is surrounded by a muscle called the external urinary sphincter. The voluntary contraction of the external sphincter allows a patient to stop voiding "mid-stream" once urination has been initiated. Most of the time, the external sphincter is closed.
During urination, as the bladder contracts, the external sphincter relaxes, allowing urine to pass through. This is the portion of the urethra often damaged when the pelvic is fractured during a motor vehicle accident or pelvic crush injury. Just a couple of details - The bladder neck and the external sphincter are two distinct continence mechanisms that prevent involuntary urine leakage (incontinence). As long as one is functional, a man will not be incontinent. In a way, it is similar to the main water line and a faucet connected to a garden hose in the home. If the water is turned off at the main water line, then water will not come out of the garden hose, regardless of the setting at the faucet. Moreover, if a working faucet is in the "off" position, water will not exit the hose when the main water line is not turned off. One other detail is that the prostatic urethra and the membranous urethra are together called the posterior urethra.
As the urine travels further towards the penis, it enters the bulbar urethra, the portion of the urethra between the sphincter area and the base of the penis. This portion of the urethra travels under the skin in the area between the scrotum and anus (called the perineum) and within the scrotum between and deep to the testicles. When a man forcefully straddles a fence or a bicycle bar, the bulbar urethra is at risk for injury. This is because the urethra in this area is not well protected and is close to the skin. Impact injuries from straddle trauma or a man being kicked crush the urethra and surrounding tissue against the bone, leading to subsequent scarring and an associated narrowing of the urethra.
The urine then enters the penile urethra that courses along the undersurface of the penis.
The head of the penis is called the glans penis. The opening of the urethra at the tip of the glans is called the urethral meatus. The urethra in the area where the head and shaft of the penis connect is called the fossa navicularis.
The bulbar and penile urethra, fossa navicularis, and meatus together represent the anterior urethra. The anterior urethra is generally 18-25 cm (9-10 inches) long and the posterior urethra is approximately 3-6 cm (1-2 inches) long. Although the posterior urethra contains 2 distinct sphincter mechanisms responsible for continence, the anterior urethra is mostly a tube, like the garden hose. Its function is simply to serve as a conduit for urine as it travels and exits at the tip of the penis. The anterior urethra is surrounded by spongy tissue with a rich blood supply called the corpus spongiosum. This spongy tissue is enclosed by a tissue covering. Anterior urethral strictures are usually associated with a process called spongiofibrosis, where the soft vascular spongy tissue surrounding the tissue develops scarring and there is an associated narrowing of the urethra. Many patients mistakenly believe their strictures represent scarring inside of the urethra that can be "scraped out" the way debris in a pipe can be cleaned out. This is not the case with urethral strictures as the urethra itself is narrow.