The hollow muscular reservoir located directly behind the pubic symphysis is the urinary bladder. This flexible structure functions as the temporary storage vessel for urine before it is eliminated from the body. Its position, tucked within the bony pelvis and shielded by the front pelvic joint, is an important anatomical arrangement for protection and function.
Anatomical Identification and Location
The symphysis pubis is a joint found at the front of the pelvis where the left and right pubic bones meet. This midline joint is a fibrocartilaginous connection that provides stability to the pelvic girdle. The urinary bladder sits immediately posterior to this joint, nestled within the lesser pelvis when empty.
When the bladder is empty, it maintains a small, collapsed, pyramidal shape and remains entirely within the confines of the bony pelvis. As it fills with urine, however, it expands superiorly, rising into the abdominal cavity above the pubic bone. The typical adult bladder can comfortably hold between 500 and 700 milliliters of urine before the urge to void becomes intense.
The position of the bladder varies slightly depending on sex due to the arrangement of other pelvic organs. In males, the bladder rests directly on the prostate gland, with the seminal vesicles and rectum positioned immediately behind it. In females, the bladder is situated anterior to the uterus and the upper part of the vagina. In both cases, a space known as the retropubic space separates the bladder from the pubic symphysis.
Structure and Composition of the Bladder Wall
The bladder wall is constructed to allow for extreme distension and powerful contraction. The wall consists of several distinct layers, which grant the bladder elasticity and strength. The innermost lining, called the urothelium or transitional epithelium, is a specialized type of cell that prevents urine components from leaking into the body tissues.
When the bladder is empty, this internal lining forms numerous folds known as rugae, which flatten out as the organ expands. Beneath the urothelium is the lamina propria, a layer of connective tissue that contains blood vessels and nerves. This layer provides structural support and flexibility to the inner lining.
The thickest layer is the muscularis propria, composed of interwoven bundles of smooth muscle fibers called the detrusor muscle. The detrusor muscle contracts forcefully to expel urine during elimination, working involuntarily. At the base of the bladder lies a smooth, triangular area known as the trigone, which is structurally fixed and does not expand like the rest of the bladder wall.
The Process of Urine Storage and Elimination
The primary function of the urinary bladder is the storage and controlled elimination of urine through a coordinated process called the micturition reflex. During the storage phase, the detrusor muscle remains relaxed to allow for filling, while the internal and external urethral sphincters remain contracted. The internal sphincter is composed of involuntary smooth muscle located at the bladder neck, which prevents leakage.
As urine accumulates and the bladder wall stretches, sensory stretch receptors are activated, sending signals to the spinal cord and brain. A mild urge to urinate begins when the bladder volume reaches about 150 milliliters. The storage phase is regulated by the sympathetic nervous system, which promotes detrusor relaxation and sphincter contraction.
The elimination, or voiding, phase is initiated when an individual consciously decides to urinate. This involves the voluntary relaxation of the external urethral sphincter. Simultaneously, the parasympathetic nervous system is activated, causing the detrusor muscle to contract strongly and the involuntary internal sphincter to relax. This coordinated action forces urine out through the urethra.