Urinary Tract Structure Flashcards
Urinary Tract in the Kidney
- The urinary tract begins within the hilum of the kidney, with the minor calyces, which collect urine from each medullary pyramid.
- Minor calyces join to form major calyces, which in turn converge to form the renal pelvis.
- The renal pelvis narrows to form the ureter.
- Transitional epithelium, or urothelium, lines the lumen of the urinary tract, beginning with the minor calyces and extending through the proximal urethra.
-Transitional epithelium is a stratified epithelium, capable of great extension, and characterized by the presence of urothelial cells (aka umbrella cells, transitional cells, dome cells) at the lumenal surface.
Urothelial Cells
- Urothelial cells have one or two nuclei, and are characteristically acidophilic.
- They express uroplakins, integral membrane proteins that form plaques in the apical membrane.
- Uroplakins are thought to contribute to mechanical stability of the urothelium as well as to its barrier function.
Ureters
- The ureters extend from the kidneys to the bladder, within the posterior body wall, and are retroperitoneal along most of their length.
- They have a star-shaped lumen lined with transitional epithelium, supported by a loose connective tissue lamina propria.
- The muscularis of the ureter has two layers, an inner longitudinal layer and an outer circular layer.
-Peristalsis in the muscularis moves urine continuously towards the bladder.
- The ureters run obliquely through the wall of the bladder as they enter it, and in this way each ureter forms a one-way flap valve which is closed when the bladder is distended, thereby preventing reflux of urine up the ureters to the kidneys.
- In addition, fibers of the detrusor muscle encircle the opening, and their contraction helps to prevent backflow of urine during voiding of the bladder.
Main Sites of Constriction in the Ureter
The lumen of the ureter is susceptible to constriction at 3 main sites, which are therefore prone to obstruction by renal calculi (stones):
- at the superiormost end of the ureter, where it forms by narrowing of renal pelvis (ureteropelvic junction)
- about two thirds of the way between kidney and bladder, at the pelvic brim, where the ureter crosses the common iliac or external iliac artery
- during passage of the ureter through the wall of the bladder (ureterovesicle junction).
Ureter in the Female
In the female, the ureters are crossed superiorly by the uterine arteries lateral to the cervix, and they are vulnerable to injury at this location during hysterectomy.
Urinary Bladder
- The empty urinary bladder lies in the pelvis posterior and slightly superior to the pubic bone. It lies immediately inferior to the peritoneum covering the floor of the abdominal cavity.
- The distended bladder expands in the extraperitoneal tissue of the anterior abdominal wall (between the rectus abdominis and the peritoneum), and when fully distended may reach superiorly as far as the umbilicus.
- The bladder lies anterior to the cervix and vagina in females, and anterior to the seminal vesicles and rectum in males.
Trigone
A triangular area of the bladder wall called the trigone, lies between the ureteral openings and is smoother than the rest of the wall. In females, this area is sometimes lined by stratified squamous epithelium rather than transitional epithelium. The trigone is supported against collapse, and allows complete emptying of the bladder.
Bladder Layers
- The bladder, like the ureters, is lined by transitional epithelium supported by a loose connective tissue lamina propria. More or less sparse strands of smooth muscle may be present in the lamina propria, forming a muscularis mucosae.
- The muscularis (propria) of the bladder is comprised of several layers of very well-developed smooth muscle called the detrusor muscle. Fibers of this muscle are developed in the neck of the bladder at the urethral orifice as an involuntary internal sphincter.
- The outer layer of the bladder is mostly adventitia, but where the bladder directly underlies the peritoneum of the pelvic cavity it is covered by serosa.
Female Urethra
- The female urethra is approximately 4 cm long, and extends from the neck of the bladder inferiorly and anteriorly to open at the urethral meatus, anterior to the vaginal opening and posterior to the clitoris.
- The urethra passes through the pelvic diaphragm, and skeletal muscle fibers of the pelvic diaphragm form the voluntary external urethral sphincter. The urethra is lined by transitional epithelium near its exit from the bladder. This changes for a short distance to pseudostratified columnar then to stratified squamous epithelium for the remainder of its length.
- Skene’s glands, aka periurethral glands, are present in the urethral lamina propria. They secrete mucus and open to the urethral lumen. It is thought that they may be homologous to the prostate gland.
Male Urethra
•The male urethra has three parts:
- The prostatic urethra passes through the prostate gland and is lined by transitional epithelium. -
- The membranous urethra passes through the urogenital diaphragm, and therefore houses the voluntary (external) urethral sphincter.
- The penile urethra extends from immediately inferior to the urogenital diaphragm to the urethral meatus, which is on the glans penis.
- The urethra is surrounded by the corpus spongiosum of the penis, which is comprised of the venous sinuses of erectile tissue.
- There is no sphincter near the urethral meatus in the male.
- The epithelium lining the male urethra is transitional in the prostate then shifts to pseudostratified or stratified columnar epithelium going distally, then to stratified squamous epithelium.
- Periurethral glands (aka glands of Littré), with similar histological appearance to periurethral glands in the female, are associated with the male urethra.
Uretovesicle Junction
The bladder wall forms a flap valve at the ureterovesicle junction.
- Hydronephrosis is the swelling of a kidney due to a build-up of urine. It happens when urine cannot drain out from the kidney to the bladder from a blockage or obstruction.
- Hydronephrosis can occur in one or both kidneys.
Membranous urethra (location of voluntary sphincter)
