Light micrograph of a section through the wall of the urethra in its contracted (passive) state. The urethra carries urine from from the bladder out of the body. Lining the interior of the urethra is transitional epithelium (purple), which forms an impermeable barrier to prevent toxins from the urine entering the blood stream. Beneath the epithelium is a layer of connective tissue known as the lamina propria. Across bottom are muscle cells (red). During urination, the smooth muscle lining the urethra relaxes in concert with the bladder contractions to forcefully expel the urine in a pressurized stream. Following this, the urethra re-establishes muscle tone by contracting the smooth muscle layer, and the bladder returns to a relaxed, quiescent state. Magnification: x100 when printed at 15cm wide.