muscular sheets. Peristaltic contractions of the smooth muscle move the urine from the minor calyces through the ureter to the bladder.
Each ureter conducts urine from the renal pelvis to the urinary bladder and is approximately 24 to 34 cm long. The distal part of the ureter enters the urinary bladder and follows an oblique path through the wall of the bladder. Transitional epithelium (urothelium) lines the luminal surface of the wall of the ureter. The remainder of the wall is composed of smooth muscle and connective tissue. The smooth
FIGURE 20.28 • Diagrams of the luminal surface of transitional epithelial cells. The upper drawing depicts part of a surface cell in a distended bladder; the lower drawing depicts the same cell as it would appear in a relaxed bladder. The plasma membrane is thickened in regions to form plaques. The interplaque regions consist of membrane that is not thickened. In the relaxed bladder, the plaques are invaginated into the cell, and although they retain their continuity with the surface, the invaginated plaques typically appear as isolated fusiform vesicles in electron micrographs. Filaments attached to the undersurface of the plaques may prevent undue stretching in the distended bladder. (Modified from Staehelin LA, Chlapowski FJ, Bonneville MA. Lumenal plasma membrane of the urinary bladder. I. Three-dimensional reconstruction from freeze-etch images. J Cell Biol 1972;53:73–91.) muscle is arranged in three layers: an inner longitudinal layer, a middle circular layer, and an outer longitudinal layer (Plate 78, page 736). However, the outer longitudinal layer is present only at the distal end of the ureter. Usually, the ureter is embedded in the retroperitoneal adipose tissue. The adipose tissue, vessels, and nerves form the adventitia of the ureter.
As the bladder distends with urine, the openings of the ureters are compressed, reducing the possibility of reflux of urine into the ureters. Contraction of the smooth muscle of the bladder wall also compresses the openings of the ureters into the bladder. This action helps prevent the spread of infection from the bladder and urethra, frequent sites of chronic infection (particularly in females), to the kidney.
In the terminal portion of the ureters, a thick outer layer of longitudinal muscle is present in addition to the two listed above, particularly in the portion of the ureter that passes through the bladder wall. Most descriptions of the bladder musculature indicate that this longitudinal layer continues into the wall of the bladder to form a principal component of its wall. The smooth muscle of the bladder, however, is not as clearly separated into distinctive layers.
chapter 20 Urinary System U R ETE R, U R I NARY B LADD E R, AN D U R ETH RA 725
The urinary bladder is a distensible reservoir for urine, located in the pelvis, posterior to the pubic symphysis; its size and shape change as it fills. It contains three openings, two for the ureters (ureteric orifces) and one for the urethra (internal urethral orifce). The triangular region defined by these three openings, the trigone, is relatively smooth and constant in thickness, whereas the rest of the bladder wall is thick and folded when the bladder is empty and thin and smooth when the bladder is distended. These differences reflect the embryologic origins of the trigone and the rest of the bladder wall: the trigone is derived from the embryonic mesonephric ducts, and the major portion of the wall originates from the cloaca.