After blood has been filtered in the kidneys, the filtrate undergoes a series of reabsorptions and exudation throughout the length of the convoluted tubules. The resulting liquid then passes to the collecting tubules, after which it enters the collecting duct. From the collecting ducts, the urine passes from the calyces to the renal pelvis, which marks the beginning of the ureters. These are bilateral muscular, tubular structures, each responsible for taking urine from one kidney to the urinary bladder for storage, prior to excretion. The arterial supply to the ureters comes directly and indirectly from the abdominal aorta. There are no ganglia on the ureters; however, it receives both sympathetic and parasympathetic innervation.
Gross and Histological Perspective
The ureters are collapsible S-shaped channels, each about 25 cm in length. They are widest at the renal pelvis and narrow progressively as they enter the urinary bladder in the concavity of the true pelvis. The lumen of each ureter is lined by a mucosal layer of transitional epithelium, which accommodates the increase in pressure that accompanies increases in the volume of urine leaving the kidney; thereby aiding to minimize the risk of rupturing the ureters. These conduits have several in-folding caused by multiple layers of smooth muscle throughout the ureteral wall. From a histological perspective, there are two muscular layers in the wall of the ureter: a longitudinal and a circular layer. In the lower segment of the ureters, another longitudinal layer can be found proximal to the bladder. Also interesting to note is that urine is propelled along the ureters by peristaltic motions initiated by pacemaker cells in the proximal renal pelvis. The whitish, non-pulsatile exterior along with the peristaltic waves helps to distinguish between ureters and blood vessels in vivo.
The ureters leave the kidneys posterior to the renal vessels. Both ureters pass inferiorly over the abdominal surface of the psoas major, with the genitofemoral nerve behind it and the vessels of the gonads in front. As the right ureter travels towards the bladder, it travels posterior to the duodenum and further down it is crossed by branches of the superior mesenteric vessels. The left ureter, however, travels laterally to the inferior mesenteric vessels and is subsequently crossed by its branches. Eventually, the vessels leave the psoas major as the common iliac arteries bifurcate to enter the true pelvis. The ureter pierces through the wall of the urinary bladder from lateral to medial and posterior to anterior. So, this entrance is oblique. It forms the orifice of the ureter in the urinary bladder at the ureterovesicular junction.
The ureters have an expansive anastomosing network of arterial supply and venous drainage along their length. The proximal end receives arterial supply from the ureteric branch of the renal artery. Contributions from the ovarian artery (testicular artery in males) as well as a direct ureteric branch from the abdominal aorta supply the middle segment. The distal portion receives its arterial supply from ureteric branches from both the superior and inferior vesical arteries. They are drained by accompanying veins.
Neuronal supply to the ureters comes from both divisions of the autonomic nervous system. Thoracolumbar outflow from T10-L1 provides sympathetic innervation via the renal plexus and ganglia, renal and upper ureteric branches from the intermesenteric plexus proximally, and the middle ureteric branch of the intermesenteric plexus in the middle segment. In the true pelvis, the ureter receives parasympathetic supply from the pelvic splanchnic nerves and from the inferior hypogastric plexus. These innervations are not pertinent to the generation and maintenance of peristaltic action along the ureters; as this arises from pacemaker cells in the renal pelvis and calyces. However, both the sympathetic and parasympathetic divisions are capable of increasing ureteric peristalsis.
The lymphatic drainage of the ureters is similar to that of the kidneys. Most of the nodes are named relative to the segment of the aorta they travel adjacent to. Distally, they drain to the internal and external iliac nodes. The middle segment drains to the common iliac and precaval nodes. Proximally, they drain to the lumbar (lateral aortic) and precaval nodes. From there, lymphatic fluid travels to the cisterna chyli and thoracic duct before returning to systemic circulation.
Obstruction of the ureter can result in pooling of water in the renal pelvis, a condition called hydronephrosis. Consequently, the ureters become distended and the patient will present with pain along the tip of the 9th costal cartilage to the end of the common iliac artery (on the anterior abdominal surface). Dilation can occur over a short period of time and if left untreated, may result in dysfunction and possibly acute renal failure. Consequent to obstruction, renal atrophy may also follow. Renal obstruction may be caused by the formation of kidney stones or they may be blocked as a result of extensive convolution arising from ectopic kidneys. Cysts arising at the ureteropelvic junction can also result in renal obstruction.