Urinary Bladder- Urinary Bladder relation-Micturition-

Urinary Bladder

The urinary bladder lies immediately behind the pubic bones inside the pelvis. Its function is storage urine and in the adult has a maximum capacity of about 500 mL. The bladder has a strong muscular wall. Its shape and relations vary according to the amount of urine that it contains.

When the bladder is empty  it is pyramidal, having an apex, a base, and a superior and two inferolateral surfaces; it also has a neck. in the adult lies entirely within the pelvis; as the bladder fills, its superior wall rises up into the hypogastric region. In the young child, the empty bladder projects above the pelvic inlet; later, when the pelvic cavity enlarges, the bladder sinks into the pelvis to take up the adult position.

The apex of the bladder points anteriorly and lies behind the upper margin of the symphysis pubis. It is connected to the umbilicus by the median umbilical ligament (remains of urachus). The base, or posterior surface of the bladder, faces posteriorly and is triangular. The superolateral angles are joined by the ureters, and the inferior angle gives rise to the urethra. The two vasa deferentia lie side by side on the posterior surface of the bladder and separate the seminal vesicles from each other. The upper part of the posterior surface of the bladder is covered by peritoneum, which forms the anterior wall of the rectovesical pouch. The lower part of the posterior surface is separated from the rectum by the vasa deferentia, the seminal vesicles, and the rectovesical fascia. The superior surface of the bladder is covered with peritoneum and is related to coils of ileum or sigmoid colon. Along the lateral margins of this surface, the peritoneum passes to the lateral pelvic walls.

As the bladder fills, it becomes ovoid, and the superior surface bulges upward into the abdominal cavity. The peritoneal covering is peeled off the lower part of the anterior abdominal wall so that the bladder comes into direct contact with the anterior abdominal wall.

The inferolateral surfaces are related in front to the retropubic pad of fat and the pubic bones. More posteriorly, they lie in contact with the obturator internus muscle above and the levator ani muscle below.

The neck of the bladder lies inferiorly and rests on the upper surface of the prostate. Here, the smooth muscle fibers of the bladder wall are continuous with those of the prostate. The neck of the bladder is held in position by the puboprostatic ligaments in the male; these are called the pubovesical ligaments in the female. These ligaments are thickenings of the pelvic fascia.
When the bladder fills, the posterior surface and neck remain more or less unchanged in position, but the superior surface rises into the abdomen The mucous membrane of the greater part of the empty bladder is thrown into folds that disappear when the bladder is full. The area of mucous membrane covering the internal surface of the base of the bladder is called the trigone. Here, the mucous membrane is always smooth, even when the viscus is empty, because the mucous membrane is firmly adherent to the underlying muscular coat. The superior angles of the trigone correspond to the openings of the ureters, and the inferior angle to the internal urethral orifice. The ureters pierce the bladder wall obliquely, and this provides a valvelike action, which prevents a reverse flow of urine toward the kidneys

as the bladder fills. The trigone is limited above by a muscular ridge, which runs from the opening of one ureter to that of the other and is known as the interureteric ridge. The uvula vesicae is a small elevation situated immediately behind the urethral orifice, which is produced by the underlying median lobe of the prostate.

The muscular coat of the bladder is composed of smooth muscle and is arranged as three layers of interlacing bundles known as the detrusor muscle. At the neck of the bladder, the circular component of the muscle coat is thickened to form the sphincter vesicae.

 

Micturition

Micturition is a reflex action that, in the toilet-trained individual, is controlled by higher centers in the brain. The reflex is initiated when the volume of urine reaches about 300 mL; stretch receptors in the bladder wall are stimulated and transmit impulses to the central nervous system, and the individual has a conscious desire to micturate. Most afferent impulses pass up the pelvic splanchnic nerves and enter the 2nd, 3rd, and 4th sacral segments of the spinal cord. Some afferent impulses travel with the sympathetic nerves via the hypogastric plexuses and enter the first and second lumbar segments of the spinal cord. Efferent parasympathetic impulses leave the cord from the second, third, and fourth sacral segments and pass via the parasympathetic preganglionic nerve fibers through the pelvic splanchnic nerves and the inferior hypogastric plexuses to the bladder wall, where they synapse with postganglionic neurons. By means of this nervous pathway, the smooth muscle of the bladder wall (the detrusor muscle) is made to contract, and the sphincter vesicae is made to relax. Efferent impulses also pass to the urethral sphincter via the pudendal nerve (S2, 3, and 4), and this undergoes relaxation. Once urine enters the urethra, additional afferent impulses pass to the spinal cord from the urethra and reinforce the reflex action. Micturition can be assisted by contraction of the abdominal muscles to raise the intra-abdominal and pelvic pressures and exert external pressure on the bladder. In young children, micturition is a simple reflex act and takes place whenever the bladder becomes distended. In the adult, this simple stretch reflex is inhibited by the activity of the cerebral cortex until the time and place for micturition are favorable. The inhibitory fibers pass downward with the corticospinal tracts to the 2nd, 3rd, and 4th sacral segments of the cord. Voluntary control of micturition is accomplished by contracting the sphincter urethrae, which closes the urethra; this is assisted by the sphincter vesicae, which compresses the bladder neck. Voluntary control of micturition is normally developed during the second or third year of life.