40 The Ureters, Bladder, and Urethra
The ureters are a pair of thin, muscular tubes that transport urine from the kidneys to the bladder. Beginning at the level of the second lumbar vertebra, the location of the ureters is retroperitoneal. Each ureter runs inferiorly and enters the posterolateral wall of the urinary bladder. This angle of entry is important, because it helps prevent urine from flowing back into the ureters when the bladder fills with urine. In addition, accumulating urine increases the internal pressure of the bladder, and this pressure compresses and seals the distal portion of the ureters.
There are three layers in the ureter wall. The innermost mucosa lining contains transitional epithelium capable of stretching but is impermeable to urine. The ability to stretch allows the ureter wall to accommodate changing volumes of urine. The middle muscularis layer is composed of two layers of smooth muscle: an inner longitudinal layer and outer circular layer. In the lower third of the ureter, the muscularis has a third outer layer of longitudinal muscle fibers. The muscularis layer is responsible for the peristaltic contractions needed to move urine through the ureters and into the bladder. The external layer of the ureter wall, the adventitia, is made of fibrous connective tissue and helps anchor the ureter to the abdominal wall.
When urine enters and distends the ureters, stretch receptors are stimulated. Reflexive action results in the contraction of the muscularis and, movement of the urine into the bladder. The power and frequency of peristalsis is directly related to the rate of urine formation. Although the ureters are innervated by both sympathetic and parasympathetic fibers, the nervous system does not appear to have major involvement in the transport of urine in these organs.
The Urinary Bladder
The urinary bladder is a hollow, collapsible muscular sac that serves as a temporary storage facility for urine. It is located in the pelvic cavity, just posterior to the pubic symphysis. In females, the bladder lies anterior to the vagina and inferior to the uterus. In males, it is immediately anterior to the rectum. Peritoneal folds hold the bladder in place.
The bladder can hold up to about a liter of urine, although this amount varies from person to person. Despite its capacity to enlarge, an overfull bladder can burst but it is more likely that excess urine will leak out of the urethra. When empty, the bladder collapses into a pyramidal shape. When a small amount of urine accumulates, it is spherical. When a larger volume of urine accumulates, the bladder becomes pear-shaped and ascends in the abdominal cavity. There are three openings in the bladder: two for the ureters and one for the urethra. These openings frame a triangular region at the base of the bladder called the trigone.
The bladder wall is made up of a mucosa with transitional epithelium, a submucosa, a thick muscularis called the detrusor muscle, and a fibrous adventitia. The adventitia is on the inferior surface only. In contrast, the peritoneum covers the superior surface. The detrusor muscle is composed of inner and outer layers of longitudinal smooth muscle fibers and an intermediate layer of circular muscle fibers.
The Urethra
The urethra is a small muscular tube that transports urine from the bladder out of the body. The urethra is five times longer in males (8 inches, 20 cm) than in females (1.6 inches, 4 cm). In males, the urethra is also part of the reproductive system, providing a passageway for semen as well as urine. The course of the urethra also differs between the sexes. In females, fibrous connective tissue binds it to the anterior vaginal wall, and its external urethral orifice (external opening) is located anterior to the vaginal opening and posterior to the clitoris. In males, the urethra is divided into three regions. The prostatic urethra is surrounded by the prostate. The membranous urethra passes through the urogenital diaphragm. The spongy urethra runs through the penis and ends at the external urethral orifice.
At the bladder-urethra junction, the circular fibers of the bladder’s detrusor muscle form the internal urethral sphincter. When urine is not draining from the bladder, this involuntary sphincter closes off the urethra to prevent the leakage of urine when you are not voiding. At the point where the urethra passes through the urogenital diaphragm, it is surrounded by the external urethral sphincter, a skeletal muscle. The external sphincter is voluntarily controlled and is kept contracted until voiding.