Urinary Organs of the Pelvis
Urine good hands here at Osmosis, because we care about each other and our learners! Bad puns aside, urine is an important way for us to eliminate waste and it takes quite the journey from the kidneys to the outside world! Let’s look at the path of urine after it leaves the kidneys, and the structures it passes through along its journey.
Let’s start with the ureters, which are paired muscular tubes sitting retroperitoneally that transport urine from the kidneys to the urinary bladder. The ureters are roughly 30 centimeters long, and have two parts; an upper abdominal part and a lower pelvic part.
The abdominal part starts at the kidneys, and descends in the abdomen posterior to the peritoneum to reach the pelvic brim. Here, the ureters cross near the bifurcation of the common iliac arteries - where the external and internal iliac arteries begin - and are now referred to as the pelvic ureters.
The pelvic ureters run on the lateral walls of the pelvis reaching the ischial spines. Then, each pelvic ureter passes anteromedially to enter the posterior wall of the urinary bladder. Now, the ureter is surrounded by many structures that differ between biologically male individuals and biologically female individuals.
In males, the ureters enter the posterior wall of the bladder superior to the seminal vesicles, which are paired glands that secrete parts of the seminal fluid. Also, the ureters run posterior to the ductus deferens, which are paired tubes that carry sperm from the scrotum to the pelvic cavity.
The ductus deferens emerges from the inguinal canal and travels to the posterolateral angle of the bladder, where it passes superior to the ureter. In females, the ureters pass medial to the origin of the uterine artery. Then, at the level of the ischial spine, the uterine artery crosses the ureter. Finally, the ureters run near the lateral part of the upper vagina and enter the bladder.
When the ureters enter the urinary bladder, they travel in an inferomedial direction to drain into the bladder in an oblique orientation. This oblique orientation creates a one-way valve, preventing urine from going back up the ureters.
As the bladder fills with urine, the internal pressure increases, which compresses the part of the ureter that’s embedded in the bladder wall. Also, during micturition, which is a fancy way to say “when peeing”, the bladder wall contracts, closing off the lumens of the ureters where they enter the bladder in a sphincter like fashion, clamping them shut to also prevent reflux of urine into the ureters.
Okay, now the arterial blood supply of the ureters is segmental, meaning that each segment receives blood from different arteries. Within the pelvis, the pelvic ureters are supplied by branches from the internal iliac artery such as the superior vesical artery, while the terminal parts of the ureters are supplied by the inferior vesical arteries in males and the vaginal and uterine arteries in females.
Finally, the ureters are innervated by nearby autonomic nerve plexuses, including the renal, aortic, and the superior and inferior hypogastric plexuses. The ureters lie above the pelvic pain line, meaning that afferent pain fibers travel with sympathetic nerves to the T10 to L2 spinal segments.
All right, now that we know all about the ureters, let’s look at its next stop; the urinary bladder. The urinary bladder is a hollow organ that temporarily stores urine before micturition .The urinary bladder sits posterior to the pubic bones and the pubic symphysis.
In males, the bladder is anterior to the rectum and superior to the prostate, and in females, its posterior surface rests on the anterior vaginal wall. In adults, an empty bladder lies in the lesser pelvis, posterior and slightly superior to the pubic bones. Now, as the bladder fills with urine, it extends superiorly and ascends into the greater pelvis. In some individuals, a full bladder may even reach the level of the umbilicus.
Before we learn more about the bladder, let’s first look at some of the surrounding structures and how the parietal peritoneum creates folds around the bladder. If you recall, the abdominal cavity is lined by parietal peritoneum, which extends inferiorly into the pelvis and reflects over some pelvic viscera, such as the urinary bladder. These reflections form peritoneal folds that vary between males and females.
Let’s start by first looking at the peritoneal folds in males. First, the peritoneum descends on the anterior abdominal wall and reflects over the superior surface of the bladder. Here, it loosely attaches to the superior surface of the bladder, creating a small triangular space called the supravesical fossa.
Then, the peritoneum continues over the superior surface of the bladder, and extends laterally to cover the lateral walls of the pelvic cavity. This creates a space on each side of the bladder, called the paravesical fossa. To remember this, “para-” means beside and “vesical” means bladder, so these fossae are beside the bladder!
The peritoneum continues posteriorly to cover the posterior wall of the urinary bladder. Here, the peritoneum crosses the distal parts of the ureters and the ductus deferens on each side, creating paired peritoneal folds called the ureteric folds.
Next, due to the bladder being positioned anterior to the rectum, the peritoneum leaves the posterior surface of the bladder and reflects over the anterior surface of the rectum, which creates a sac between the bladder and the rectum, called the rectovesical pouch.
This pouch extends laterally and posteriorly, forming a fossa on each side of the rectum, called the pararectal fossa. Finally, the peritoneum ascends to cover the anterior surface of the rectum, before engulfing the sigmoid colon
All right, now in females, the peritoneum travels in a similar fashion as it does in males, with the main differences coming near the posterior surface of the urinary bladder as the bladder sits anterior to the uterus.
Here, the peritoneum reflects over the anterior surface of the uterus, reaching the uterine fundus. This creates a space between the bladder and the uterus, called the vesicouterine pouch. Now, the peritoneum covering the anterior surface and the fundus of the uterus extends laterally, forming a double peritoneal fold called the broad ligament of the uterus, which keeps the uterus in position. This ligament also covers the uterine tubes and suspends the ovaries.
Okay, now let’s take a deep breath and have a quick quiz! Can you name these structures?
The apex of the urinary bladder points anteriorly towards the pubic symphysis, and it’s connected to the umbilicus by a ligament called the median umbilical ligament. The fundus, on the other hand, lies opposite to the apex, and forms the convex posterior surface of the bladder.
Next is the body, which lies between the apex and the fundus and makes up most of the urinary bladder. Finally, the fundus and the two inferolateral surfaces meet below to form the neck of the bladder, which continues inferiorly with the urethra.
Now, the bladder lies mostly free within the lesser pelvis, except for its neck, which is fixed by ligaments. These ligaments are the lateral ligaments of the bladder, the puboprostatic ligament in males, and the pubovesical ligament in females.
The lateral ligaments of the bladder extend from the neck of the bladder and blend with the pelvic fascia. In males, the puboprostatic ligament extends from the prostate to the pubic bones. In females, the pubovesical ligament extends from the neck of the bladder to the inferior part of pubic bones.
Now, let’s zoom into the wall of the urinary bladder, which is mainly formed by a muscle called the detrusor muscle. This muscle contracts to increase the pressure inside the bladder, which leads to urination. The muscle fibers of the bladder wall thicken as they reach the neck, forming an involuntary circular muscle called the internal urethral sphincter.
This muscle surrounds the opening between the neck of the bladder and the urethra, called the internal urethral orifice. The internal urethral sphincter contracts and constricts the orifice, which helps maintain urinary continence. In males, it also contracts during ejaculation to prevent retrograde ejaculation of semen into the bladder.
Now, let’s look inside the urinary bladder, which has irregular folds of mucous membrane called rugae. These rugae are best seen when the bladder is empty, because as the bladder fills, they begin to stretch and fade away.
Inside, the posterior wall of the urinary bladder has a smooth triangular area that doesn’t contain rugae -even if the bladder is empty- called the trigone of the urinary bladder. The base of this triangle is formed by an imaginary line that connects the openings of the ureters, called the ureteric orifices. The apex of the trigone points downwards, and is formed by the internal urethral orifice.
All right, now let’s look at the blood supply of the urinary bladder. First, the arterial blood supply of the bladder comes mainly from branches of the internal iliac artery. In both male and female individuals, the superior vesical arteries supply the anterior and superior parts of the urinary bladder.
In males, the inferior vesical arteries supply the lower parts of the urinary bladder, such as the neck and the fundus. In females, the vaginal artery supplies the posteroinferior parts of the urinary bladder instead of the inferior vesical arteries. Finally, the obturator and inferior gluteal arteries send small branches to supply the urinary bladder.
The venous blood of the bladder drains into a network of veins called the vesical venous plexus. In males, this plexus connects with the prostatic venous plexus, and these two plexuses surround the fundus of the bladder, the prostate, the seminal glands, the ductus deferens, and the lower part of the ureters.
In females, the vesical venous plexus communicates with the uterovaginal venous plexus, and surrounds the neck of the bladder and the upper part of the urethra. The vesical plexus often drains into the inferior vesical veins, which drain into the internal iliac veins. The vesical venous plexus may also drain into the sacral veins, which drain into the internal vertebral venous plexus.
The lymphatic drainage of the superior and lateral surfaces of the bladder drain to the external iliac lymph nodes. The drainage of the neck and the fundus of the bladder drain to the internal iliac, sacral, and common iliac lymph nodes.
So, we know that our bladder stores urine, but what happens when it’s full? Typically, we have voluntary control of our bladder, however our bladder is also controlled by autonomic pathways, which can prevent or cause urination through the micturition reflex, where autonomic control is a balance between parasympathetic innervation and sympathetic innervation. The urinary bladder receives input from the sympathetic and parasympathetic divisions of the autonomic nervous system.