Ureter urinary bladder and urethra

All excretory passages, except the urethra, have the same general organization

On leaving the collecting ducts at the area cribrosa, the urine enters a series of structures that do not modify it but are specialized for its storage and passage to the exterior of the body. The urine flows sequentially to a minor calyx, a major calyx, and the renal pelvis, and leaves each kidney through the ureter to the urinary bladder, where it is stored. The urine is finally voided through the urethra.

All of these excretory passages, except the urethra, have the same general structures, namely, a mucosa (lined by transitional epithelium), muscularis, and adventitia (or, in some regions, a serosa).

Transitional epithelium lines the calyces, ureters, bladder, and the initial segment of the urethra

Transitional epithelium (urothelium) lines the excretory passages leading from the kidney. This stratified epithelium is essentially impermeable to salts and water. The epithelium begins in the minor calyces as two cell layers and increases to an apparent four to five layers in the ureter (Fig. 19.22) and as many as six or more layers in the empty bladder. However, when the bladder is distended, as few as three layers are seen. This change reflects the ability of the cells to accommodate to distension. The cells in the distended bladder, particularly the large surface cells and those in the layers below, flatten and unfold to accommodate the increasing surface area. As the individual cells unfold and flatten, the resulting appearance is the "true" three layers.

Urethral Wart
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