The ureters are two thin, slender tubes 25 cm. to 30 cm long which run from the hilius
of each kidney to the posterior area of the bladder. They freshly collected urine from the
kidneys to the bladder. Layers of muscle tissue line the walls of the ureters. These
muscles help "push" urine from the kidneys to the bladder. When urine has
entered the bladder, there is a number of valve-like mucosa that prevents
urine from traveling back up the ureters.
 The
urethra is a narrow, hollow passageway that flows from the bladder to the to an opening
outside the body. At the junction of the bladder and urethra, smooth muscle forms the
internal urethral sphincter. Which is an involuntary muscle that prevents urine from
flowing up the urethra to the bladder. At the end of the urethra is the external urethral
sphincter. This sphincter is voluntary and regulates urine flowing from the bladder to an
opening outside the body.
 When urine has become concentrated, solutes form
crystals within the renal pelvis. These crystals are known as renal calculi, or more
commonly kidney stones. When he walls of the ureters try to close in on the calculi, which
are extremely sharp, a tremendous amount of pain is emitted. This pain continues as the
renal calculi pass through the ureter. Alkaline urine, bacterial infections, and/or
urinary retention aid in the process of renal calculi formation.
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