Structure
The bladder is a hollow, expandable, muscular organ located in the pelvic girdle (fig. 1-59). Although the shape of the bladder is spherical, its shape is altered by the pressures of surrounding organs. When it is empty, the inner walls of the bladder form folds. But as the bladder fills with urine, the walls become smoother.

The internal floor of the bladder includes a triangular area called the trigone (fig. 1-59). The trigone has three openings at each of its angles. The ureters are attached to the two posterior openings. The anterior opening, at the apex of the trigone, contains a funnel-like continuation called the neck of the bladder. The neck leads to the urethra.

The wall of the bladder consists of four bundles of smooth muscle fibers. These muscle fibers, interlaced, form the detrusor muscle (which surrounds the bladder neck) and comprise what is called the internal urethral sphincter. The internal urethral sphincter prevents urine from escaping the bladder until the pressure inside the bladder reaches a certain level. Parasympathetic nerve fibers in the detrusor muscle function in the micturition (urination) process. The

Figure 1-59.-Urinary bladder and urethra:
A. Frontal section of the female urinary bladder and urethra;
B. Frontal section of the male urinary bladder and urethra.

outer layer (serous coat) of the bladder wall consists of two types of tissue, parietal peritoneum and fibrous connective tissue.