The urinary bladder is a hollow, muscular organ in the lower abdomen that serves as the temporary reservoir for urine produced by the kidneys. Its ability to expand and contract makes it a dynamic part of the urinary system. Understanding the specific anatomy of the bladder is important because its various regions handle different functional and structural demands. The dome is one such region, and its unique location and composition are directly related to the bladder’s capacity for storage.
Defining the Location and Structure
The dome of the bladder, also known as the apex, is the superior, rounded portion of the organ. This area points toward the anterior abdominal wall and is the most mobile part of the bladder. It is distinct from the body, the base (or fundus), and the neck, which connects to the urethra.
When the bladder is empty, it rests low within the pelvis, positioned behind the pubic symphysis. As urine volume increases, the dome stretches and expands superiorly, rising out of the pelvis and into the abdominal cavity. This expansion changes the dome’s anatomical relationships, especially concerning the peritoneum, the membrane lining the abdominal cavity.
The superior surface of the dome is covered by the parietal peritoneum, making it an intraperitoneal structure when distended. This contrasts with the lower portions of the bladder, which are covered by adventitia and are thus extraperitoneal. An important anchor point for the dome is the median umbilical ligament, a fibrous cord extending from the dome to the umbilicus. This ligament is the remnant of the urachus, a tube that connected the bladder to the umbilical cord during fetal development.
The Unique Role of Tissue Layers
The wall of the bladder dome is composed of several tissue layers, each playing a role in the organ’s ability to store urine without leakage. The innermost layer is the mucosa, which features the urothelium, or transitional epithelium. This epithelium is highly adaptable, shifting from five to seven cell layers thick when relaxed to only two or three layers when the bladder is full.
The most superficial cells of the urothelium, known as umbrella cells, are connected by tight junctions and coated with uroplakin. This arrangement creates an effective barrier that prevents urine components from diffusing into the underlying tissues. Beneath the urothelium is the lamina propria, connective tissue containing elastic fibers, nerves, and blood vessels.
The bulk of the bladder wall is the muscular layer, known as the detrusor muscle, composed of smooth muscle fibers. In the dome and body, these fibers are arranged in a random, interwoven pattern, unlike the organized layers near the bladder neck. This basket-like arrangement allows the detrusor muscle to stretch uniformly as the dome expands and contract forcefully during voiding.
Clinical Significance of the Bladder Dome
The dome’s unique position and distensibility contribute to its clinical relevance in trauma and congenital anomalies. Because the dome is the most expandable portion and is covered by the peritoneum, it is the most common site for intraperitoneal bladder rupture. This rupture typically occurs from blunt abdominal trauma when the bladder is distended, spilling urine into the abdominal cavity, potentially leading to peritonitis and sepsis.
The dome’s connection to the median umbilical ligament makes this region relevant for conditions related to the urachus. If the urachus fails to close after birth, it can result in urachal remnants, such as cysts, sinuses, or a patent urachus (a direct connection between the dome and the umbilicus). These remnants can become infected, form stones, or lead to adenocarcinoma, requiring surgical management.
The dome’s accessibility when distended also makes it a target for certain surgical procedures, especially for suprapubic access. Surgeons may use the dome for placing a suprapubic catheter or for surgical entry during laparoscopic procedures. However, an undetected urachal remnant presents a risk of injury during the insertion of instruments near the midline.