Anatomy of the urinary organs of the pelvis

Chapters:

Watch video only

Introduction0:00–0:38

You're in 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. After exiting the kidney, urine travels through three main structures before leaving the body the ureters, the urinary bladder, and the urethra.

Ureters0:38–1:35

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.

Structures surrounding the ureters1:35–3:17

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 deference, which are paired tubes that carry sperm from the scrotum to the pelvic cavity.
The ductus deference 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 infraromedial 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.
OK, now the arterial blood supply of the ureters is segmental, meaning that each segment receives blood from different arteries.

Arterial supply of ureters3:17–3:45

Within the pelvis. The pelvic ureters are supplied by branches from the internal iliac artery, such as the superior vesicle artery, while the terminal parts of the ureters are supplied by the inferior vesicle arteries in males and the vaginal and uterine arteries in females.
Venous blood from the ureters is drained by veins that follow the arteries, and lymphatic vessels carry the lymph to the common iliac and internal iliac lymph nodes.

Venous and lymphatic drainage of ureters3:45–3:57

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.

Innervation of ureters3:57–4:23

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.

Urinary bladder4:23–5:13

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.

Structures surrounding the urinary bladder5:13–5:42

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.

Peritoneal folds in males5:42–7:15

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 a supra vesicle 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 paravesicle fossa. To remember this, para means beside and vesicle 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 recto vesicle 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.

Peritoneal folds in females7:15–7:58

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 vesico uterine 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. OK, now let's take a deep breath and have a quick quiz.

Quiz7:58–8:09

Can you name these structures? All right, now an empty bladder has an apex, body, fundus, and neck, as well as four surfaces superior, posterior, and two inferolateral surfaces.

Structure of urinary bladder8:09–11:10

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 pubo vesicle 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 pubo vesicle ligament extends from the neck of the bladder to the inferior part of the 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.

Arterial supply of urinary bladder11:10–11:54

The arterial blood supply of the bladder comes mainly from branches of the internal iliac artery. In both male and female individuals, the superior vesicle arteries supply the anterior and superior parts of the urinary bladder.
In males, the inferior vesicle arteries supply the lower parts of the urinary bladder, such as the neck and the fundus. In females, the vaginal artery supplies the postero inferior parts of the urinary bladder instead of the inferior vesicle arteries.
Finally, the obterator 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 vesicle venous plexus.

Venous drainage of urinary bladder11:54–12:45

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 deference, and the lower part of the ureters. In females, the vesicle venous plexus communicates with the utero vaginal venous plexus and surrounds the neck of the bladder and the upper part of the urethra.
The vesicle plexus often drains into the inferior vesicle veins, which drain into the internal iliac veins. The vesicle venous plexus may also drain into the sacral veins, which drain into the internal vertebral venous plexus.

Lymphatic drainage of urinary bladder12:45–13:02

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.

Innervation of urinary bladder13:02–16:29

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 micturian 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. The sympathetic fibers originate from the lower thoracic and upper lumbar regions, and these fibers travel through the superior and inferior hypogastric plexuses before reaching the bladder.
Sympathetic innervation causes relaxation of the detrusor muscle and. Contraction of the internal urethral sphincter to retain and store urine.
Remember, sympathetic innervation is our fight or flight response. So if we are running away from something, we don't want to be voiding our bladder, even though I'm sure this would scare away whatever is chasing us.
It is also important to note that in males, sympathetic innervation is responsible for ejaculation and to prevent semen from entering the urinary bladder during ejaculation, which is called retrograde ejaculation.
These fibers again cause contraction of the internal urethral sphincter. Next are the parasympathetic fibers which originate from the sacral segments of the spinal cord.
These fibers travel through the pelvic splanknik nerves, reaching a network of nerves called the inferior hypogastric plexus.
When the bladder fills, it begins to stretch, and if it reaches the threshold, it can activate the murian reflex by sending a signal along visceral afferent fibers.
This stimulates the parasympathetic fibers which causes contraction of the detrusor muscle and relaxation of the internal urethral sphincter facilitating urination.
Parasympathetic innervation is also responsible for erections in males, and the mnemonic point and shoot can help you remember that point or initiating an erection is parasympathetic, and shoot or ejaculation is sympathetic.
The final component of the micturian reflex is voluntary control of the external urethral sphincter. In males, the external urethral sphincter lies inferior to the prostate gland, and in females, it lies just inferior to the internal urethral sphincter.
We have voluntary control over this muscle through branches of the pudendal nerve, allowing us to choose when to void our bladder.
One last thing about the innervation of the bladder. The upper part of the urinary bladder lies above the pelvic pain line, which means that pain fibers that come from the upper part travel with the sympathetic fibers to the inferior thoracic and upper lumbar segments of the spinal cord and to the brain.
The lower part of the urinary bladder lies below the pelvic pain line, which means that pain fibers that come from the lower part travel with the parasympathetic fibers to the sacral segments of the spinal cord.

Quiz16:29–16:44

OK. Now let's take a deep breath and have a quick quiz.
Can you name this muscle? OK, can you name this ligament?

Urethra16:44–16:51

Now let's move on and talk about the urethra, which is what transports urine from the bladder to be excreted. The urethra differs between males and females, so let's start with the male urethra, which is a muscular tube that measures roughly between 18 and 22 centimeters.

Male urethra16:51–17:30

Meters long, this tube begins at the internal urethral orifice in the bladder and ends at the external urethral orifice in the gland's penis and forms a common passage for urine and semen to leave the body.
The male urethra actually has four parts an intramural or preprostatic part, a prostatic part, an intermediate or membranous part, and lastly, the spongy urethra.
First is the intramural or pre-prostatic part of the urethra, which extends through the internal urethral sphincter before entering the prostate gland.

Preprostatic and prostatic part of male urethra17:30–18:58

Next is the prostatic part of the urethra, which is the widest and most expandable part of the male urethra and travels through the prostate gland.
Now the prostatic part of the urethra is unique from the others, so let's take a closer look inside. The posterior wall of the prostatic urethra has the urethral crest, which is a longitudinal elevation that lies in the midline.
This crest has grooves on each side called the prostatic sinuses. These sinuses have many small openings for the prostatic ducts which carry secretions from the prostate gland into the urethra.
The urethral crest extends inferiorly, forming a rounded eminence called a seminal colliculus. This eminence has a small slit-like opening that leads into a small empty sack called the prostatic utericle.
Finally, on each side of the prostatic utericle are the openings of the ejaculatory ducts, which are paired structures, each formed by the union of the ductus deference and the duct of the seminal gland.
So in this part of the prostatic urethra, the urinary and the reproductive tracts merge. Next is the intermediate or membranous part of the urethra, which is the narrowest and the least expandable part of the urethra and penetrates the perineal membrane, which is a thin horizontal membrane in the perineum.

Membranous part of male urethra18:58–19:26

This part is surrounded by circular folds of voluntary muscle fibers called the external urethral sphincter, which, if you remember, is the sphincter under voluntary control.

Spongy part of male urethra19:26–19:58

Last is the spongy urethra, which is the longest and most mobile part of the urethra. This part passes through the penis and measures about 15 centimeters in length.
Two P-shaped glands called the bulbourethral glands open into the bulbar part of the spongy urethra through the bulbourethral ducts.
These glands contribute to the seminal fluid by secreting a lubricating fluid during sexual arousal prior to ejaculation.
Like the ureters, the blood supply of the male urethra is also segmental. The intramural and prostatic parts get their blood supply from branches of the inferior vesicle and middle rectal arteries, which are branches of the internal iliac artery.

Blood supply and innervation of male urethra19:58–20:47

Next, the membranous and spongy urethra are supplied by the dorsal artery of the penis, a branch of the internal pudendal artery.
The venous drainage of the urethra mirrors the arterial supply. Lastly, the male urethra receives autonomic sympathetic and parasympathetic innervation from branches of the prostatic nerve plexus, which is derived from the inferior hypogastric plexus, in addition to somatic innervation provided by the pudendal nerve.

Female urethra20:47–21:38

Now let's switch to the female urethra, which measures roughly 4 centimeters in length. This tube starts at the internal urethral orifice of the urinary bladder and travels anteroinferiorly, passing posterior and inferior to the pubic symphysis.
From here it travels through the pelvic diaphragm, external urethral sphincter, and perineal membrane. Finally, it ends at an opening called the external urethral orifice.
This orifice lies anterior to the vaginal opening in the vestibule of the vagina, which is the space between the two labia minora.
Finally, in the walls of the distal urethra are paired glands that secrete mucus during sexual intercourse called the paraurethral glands.
The blood supply of the female urethra comes from the internal pudendal and vaginal arteries. The venous drainage mirrors this arterial blood supply.

Blood supply and innervation of female urethra21:38–21:56

Finally, the female urethra is innervated by branches of the vesicle plexus and the pudendal nerve. All right, as a quick recap, let's follow the path of urine and the organs it passes through along its way.

Review21:56–25:02

First, the urine exits the kidneys into the ureters, which carries it to the urinary bladder. The ureters descend retroperitoneally in the abdomen and eventually enter the pelvis, where they drain into the posterior surface of the urinary bladder, forming a one-way valve that prevents urine from flowing back into the ureters during micturation.
The pelvic portion of each ureter is mainly supplied by the inferior vesicle artery in males and the uterine artery in females.
The venous blood of the ureter drains into the vesicle venous plexus and the internal iliac veins. Urine then enters the urinary bladder, which is a hollow organ that stores the urine before micturition.
It lies in the lesser pelvis, posterior, and above the pubic bones and can extend up to the umbilicus when full. The wall of the bladder is mainly composed of the detrusor muscle.
The bladder and surrounding structures are covered by parietal peritoneum, which extends inferiorly from the abdominal cavity to cover the superior and lateral surfaces of the urinary bladder and pelvis, forming different folds and fossae.
The bladder lies free within the lesser pelvis except for the neck, which is fixed by the lateral ligament of the bladder, the puboprostatic ligament in males, and the pubal vesicle ligament in females.
The upper part of the urinary bladder is supplied by the superior vesicle arteries, and the lower part is served by the inferior vesicle arteries in males and the vaginal artery in females.
Once the bladder is full enough, micturition is initiated through the autonomic pathways of the micturian reflex and the voluntary relaxation of the external urethral sphincter.
Urine is carried from the urinary bladder to the exterior in a tube called the urethra. The male urethra has 4 parts measuring up to 22 centimeters in length preprostatic or intramural, prostatic, membranous, and spongy parts.
The female urethra is much shorter than the male urethra, measuring about 4 centimeters long. It runs inferiorly through the pelvis, parallel to the vagina, and ends at the external urethral orifice.
The male urethra receives arterial blood from the inferior vesicle and middle rectal arteries and the dorsal artery of the penis.
It is innervated by branches of the prostatic nerve plexus and pudendal nerve. Finally, the female urethra receives arterial blood from the internal pudendal and vaginal arteries and is innervated by the vesicle plexus and the pudendal nerve.