In human anatomy, theureters are muscular tubes that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually 2530cm (1012in) long and ~3-4 mm in diameter. In humans, the ureters arise from the renal pelvis on the medial aspect of each kidney before descending towards the bladder on the front of thepsoas majormuscle. The ureters cross the pelvic brim near the bifurcation of the iliac arteries (which they run over). This pelviureteric junction is a common site for the impaction ofkidney stones(the other being the ureterovesical valve). The ureters run posteroinferiorly on the lateral walls of the pelvis They then curve anteriormedially to enter the bladder through the back, at the vesicoureteric junction, running within the wall of the bladder for a few centimeters. The backflow of urine is prevented by valves known as ureterovesical valves. In the female, the ureters pass through the mesometrium on the way to the urinary bladder. Ureters are also found in all other amniote species, although different ducts fulfil the same role in amphibians and fish. Histology The ureteric lumen is star-shaped. Like the bladder, it is lined with transitional epithelium, and contains layers of smooth muscle, thereby being under autonomic control. The epithelial cells of the ureter are stratified (in many layers), are normally round in shape but become squamous (flat) when stretched. The lamina propria is thick and elastic (as it is important that it is impermeable). There are two spiral layers of smooth muscle in the ureter wall, an inner loose spiral, and an outer tight spiral. The inner loose spiral is sometimes described aslongitudinal, and the outer ascircular. The distal third of the ureter contains another layer of outer longitudinal muscle. The adventitia of the ureter, like elsewhere is composed of fibrous connective tissue, that binds it to adjacent tissues. Urinary bladder The organ designed for the storage of urine is known as theurinary bladder. It looks like a sac that shrinks and expand depending oh how much urine it is holding. Slightly posterior to the symphysis pubis, the urinary bladder is found anterior to the rectum. In the human female body, the bladder is directly adjacent to the uterus and the vagina. In the human male body, the bladder lies just above the prostate. The bladders discernable shape is dependant entirely on how much urine it contains at any given time. An empty bladder will appear to have the shape of a pyramid while a bladder that is ready to be emptied and is filled with urine will appear ovular. When a bladder is full it extends upward into the abdominal cavity. The anterior and superior edges of the bladder are lined with the median umbilicus ligament. The median umbilicus ligament is the left over fibers of the umbilical cord. The ureters run directly into the base of the urinary bladder while the urethra runs out of the bladder at the apex, also known as the inferior angle. The neck of the bladder refers to the area encompassing the urethral opening. Four distinct layers create the wall of the urinary bladder. The inner layer is known as the mucosa layer, and this layer is created by transitional epithelium which gradually thins as the bladder distends, creating thinner walls. Rugae, or specialized folds within the mucosa layer, allow the bladder to continually stretch as it fills. These folds can be observed clearly when the bladder is empty. The mucosa is equipped with additional flaps which are designed to act as valves to prevent the possible backwash of urine. Without these flaps, urine that fills the bladder may end up backing up into the kidneys once again. The two uretal openings and the solitary opening for the urethra create a triangular area known as the trigone. The trigone does not alter its shape as the bladder fills and empties. It appears smooth from its lack of rugae. The submucosa is designed as structural support for the mucosa layer. This is the middle layer of the structural design of the bladder. The third layer, the muscularis layer, is created by the interlacing of three smooth muscle layers. These muscles together are known as the detrusor muscle. The detrusor muscle undergoes a transformation when it reaches the neck of the urinary bladder. This transformation creates the superior muscular sphincter that encompasses the urethra. There are two of these sphincter muscles. The superior sphincter muscle is known as the internal urethra sphincter. The final outer layer of the urinary bladder is the adventitia. This can only be discerned on the bladders superior surface and in reality is a continuation of the parietal peritoneum. The urinary bladder receives its blood supply from the superior and inferior vesicular arteries. These arteries are tributaries of the internal iliac arteries. The vesicular venous plexus is responsible for the venous drainage of the bladder. This then converges with the internal iliac veins. The pelvic plexus is responsible for supplying the urinary bladder with autonomic nerves. The last thoracic and the first and second lumbar nerves create the necessary innervation to the bladder. The sympathetic innervation is directed to the blood vessels, urethral openings, and the trigone. Parasympathetic innervation is derived from second, third, and fourth sacral nerves. These are aimed at serving the detrusor muscle. The pelvic spinal nerves are responsible for responding to the sensory response of a full bladder, which responds to the impulses sent via the central nervous system.