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The lower urinary tract consists of two ureters, one urinary bladder, and one urethra. An extensive area of contact exists between the urinary and cardiovascular systems. After the kidneys filter blood plasma, the majority of water and solutes return to the bloodstream. The remaining water and solutes constitute urine, which passes through the ureters, to then be stored in the urinary bladder until it is excreted from the body through the urethra.
The ureters, which drain the kidneys to the bladder, are formed from intermediate mesoderm. The most proximal part of the ureteric bud forms the transitional epithelium lining each ureter. The supporting connective tissue and smooth muscle of the ureter walls are formed by the intermediate mesoderm of the metanephric cap. The distal portions of the ureteric buds form the collecting structures within the kidneys, the renal pelvis and renal calyces, which drain into the ureters.
The bladder and urethra are formed by a portion of the hindgut endoderm, which gives rise to the epithelial lining of the bladder and urethra, and by the splanchnic mesoderm, which forms the connective tissue, smooth muscle and blood vessel components. During development, the cloaca divides into a urogenital sinus, into which urinary and genital ducts empty, and into a rectum, which discharges into the anal canal. The urinary bladder develops from the urogenital sinus. In females, the urethra develops as a result of lengthening of the short duct that extends from the urinary bladder to the urogenital sinus. In males, the urethra is considerably longer and more complicated, but it is also derived from the urogenital sinus.
The branched ureteric bud gives rise to the renal collecting ducts and to the ureters, which drain the urine from the kidney. The mesonephric ducts and ureters open into the bladder by a process in which the mesonephric ducts open into the dorsal wall of the urogenital sinus at a point which will become the posterior bladder wall. As the bladder grows, its expanding wall first incorporates the mesonephric ducts and then their ureteric buds, resulting in these structures opening separately into the bladder. The ureters subsequently migrate to enter at a more cranial location than the mesonephric ducts.