Question #320
On filling of the bladder:
The bladder is an extraperitoneal structure which, in the adult lies in the pelvis. As it fills with urine it expands and extends up into the abdomen. The inferior edge and the urethra remain fixed. The superior edge pulls the parietal peritoneum off the anterior abdominal wall and with it ensures bowel is also retracted upwards. As a result no bowel lies anterior to the full bladder and it is safe to insert a suprapubic catheter without any form of ultrasound guidance.
The internal sphincter relaxes in when the bladder is full. In infants the detrusor muscle contracts reflexively when the bladder is full.
The antero-superior edge is attached to the umbilicus by the median umbilical ligament – also known as the urachus. This structure may remain patent in adult life and rarely urine may flow up to the umbilicus.