Regarding the bladder, which of the following is FALSE?
A. The urachus is the remnant of the allantoisAnswer B
The bladder has a dual function of storing and voiding urine with a capacity of 500 mL. Its shape changes with the amount of urine: a pyramidal shape when empty but ovoid when full. When empty, the apex faces anteriorly and lies just behind the pubic symphysis. It is the point of attachment for the urachus, a fibrous cord and remnant of the allantois, which suspends the bladder from the anterior abdominal wall by fusing with one of the obliterated umbilical arteries near the umbilicus. The base faces posteriorly, is triangular in shape and is in contact with a pair of seminal vesicles separated by the vas deferens. The inferior angle gives rise to the urethra. The bladder neck is 3–4 cm directly behind the pubic symphysis, and its position remains fixed during bladder distention. The superior surface is covered with peritoneum, which in women is further reflected over the uterus (vesicouterine pouch) and continues further back to cover the rectum (rectouterine pouch). On filling, the superior surface rises into the abdominal cavity, pushing the peritoneum away to come into direct contact with the posterior surface of the abdominal wall. The layers of bladder wall include an innermost transitional cell epithelium, made up to 6 cells thick overlying a basement membrane, the lamina propria and the outermost detrusor (smooth) muscle. The fibres of the detrusor are arranged into an inner longitudinal, circular and outer longitudinal fashion that forms the internal (proximal) sphincter. In men, the inner fibres are continuous with those of the urethra. The middle fibres form a ring around the bladder neck with the anterior fibromuscular stroma of the prostate. The outer fibres are abundant in the bladder base and they also form a loop around the bladder neck to provide continence. In females, the arrangement of the inner layer is similar but less is known about the arrangement of the middle and outer fibres. On approaching the bladder, the ureters become covered by a fibromuscular Sheath of Waldeyer before passing obliquely through the wall for about 2 cm and ending at the ureteral orifice. The intramural portion is narrow from compression by the detrusor muscle. This angle of entry and the surrounding detrusor muscle acts to prevent retrograde reflux of urine up the ureter. The two ureteric orifices form a triangular-shaped trigone with the apex at the bladder neck. The longitudinal muscle fibres of the ureter fan out over trigone and deep to this is the fibromuscular sheath of Waldeyer that inserts into the bladder neck. The urothelium is thinnest on the trigone. The fibres between the two orifices are thickened to form the interureteric ridge. The blood supply to the bladder is the superior and inferior vesical arteries. Venous drainage is to the vesical plexus, which drains into the internal iliac vein.