Urinary bladder is the sac-like hollow muscular and distensible organ that collects urine formed in the kidney and temporarily stores it before it can be excreted from the body. Urinary bladder is situated immediately behind the pubic bones within the pelvis. It has strong muscular walls with three openings: two for ureters and one for urethra.
Urinary bladder of an adult person has a maximum capacity of about 500ml. Its shape and relations vary according to the amount of urine that it contains. Empty bladder lies entirely within the pelvis, however, as it fills with urine, superior wall rises up into the hypogastric region.
Empty bladder is pyramidal in shape. It has an apex, a base, a superior surface and two inferolateral surfaces. The apex points anteriorly and lies behind the upper margin of the symphysis pubis. It is connected to the umbilicus by the median umbilical ligament, which represents the remains of the urachus. The base of the bladder is triangular and faces posteriorly.
The superolateral angles of the bladder are joined by the ureters and the inferior angle is joined by the urethra. The vasa deferentia lie side by side on the posterior surface, separating the seminal vesicles from one another.
The upper part of the posterior surface of the bladder is covered by peritoneum. This part forms the anterior wall of the rectovesical pouch in males. The lower part of the posterior surface is devoid of peritoneum.
The superior surface of the bladder is covered completely by peritoneum. Along the lateral margins, the peritoneum passes onto the lateral walls of the pelvis. This means that anterior and lateral parts of the bladder are devoid of peritoneum.
As the bladder fills with urine, the peritoneum peels off from the anterior wall of pelvis so that the bladder comes in direct contact with the anterior abdominal wall.
Neck of bladder:
Neck of the urinary bladder is its inferior part that rests on the prostate gland. Here the smooth muscle fibers of the bladder are continuous with those of the prostate. The neck is also supported by the puboprostatic ligaments (in males) or pubovesical ligaments (in females). These ligaments are thickenings of pelvic fascia. When the bladder gets filled with urine, the support of the neck makes it stationary and only the superior surface of the bladder rises into the abdominal cavity.
Trigone is the triangular area of mucous membrane covering the internal surface of the base of the bladder between the two ureteral orifices superiorly and the urethral orifice inferiorly. Unlike the rest of the bladder, the mucous membrane of trigone is always devoid of folds, even when the bladder is empty. The reason is that in this region the mucous membrane is firmly adherent to the underlying muscular coat.
Embryologically, the trigone has a different origin than the rest of the bladder. It arises from mesonephric ducts (mesodermal in origin) while the rest of the bladder is endodermal in origin.
The layer of bladder wall made of smooth muscle is collectively called detrusor muscle. It is composed of three layers of smooth muscle fibers: spiral, longitudinal and circular. It is the detrusor muscle whose contraction causes expulsion of urine from the bladder, under the action of parasympathetic nervous system.
- Arterial supply is through superior and inferior vesical arteries, which are both branches of internal iliac artery.
- Venous drainage occurs by the way of vesical venous plexus, which drains into the internal iliac nodes.
Lymph from the bladder drains into internal and external iliac nodes.
Nerve supply to bladder comes from the inferior hypogastric plexus.
- Sympathetic fibers originate in 1st and 2nd lumbar ganglia
- Parasympathetic fibers arise as pelvic splanchnic nerves
- Afferent sensory fibers reach the central nervous system via the pelvic splanchnic nerves
Generally, the sympathetic fibers inhibit contraction of the bladder wall and close the sphincter. Thus they inhibit micturition. On the other hand, parasympathetic fibers contract the bladder wall and relax the sphincters, thus they promote micturition.