How does the urinary tract function under normal conditions?
After urine is made in the kidneys, it flows down the ureters into the bladder. The bladder is a hollow, balloon-like organ with muscular walls. As the bladder fills, the muscle relaxes so that the bladder expands and holds urine. During urination, the bladder muscle contracts to squeeze out the urine. The urethra is the tube through which urine passes from the bladder to the outside. The urethra has a muscle, the sphincter, which is completely different from the bladder muscle. The sphincter normally stays closed and makes a seal to keep urine from leaking. During urination, the sphincter opens and lets urine pass. The bladder and urethra have a specialized lining called the epithelium. The epithelium forms a barrier between the urine and the bladder muscle. The epithelium also helps to keep bacteria from sticking to the bladder, so it helps to prevent bladder infections.
