Hello everyone! This is Joao from Kenhub and today, we're going to be talking about the urinary system.
The organs of the urinary system are located in the abdominal cavity which we see here. Now looking deeper, we see that this system includes the kidneys like we see here highlighted on this image of the abdomen, the ureters which leave the kidneys and travel inferiorly within the abdomen, the urinary bladder which we see in the lower pelvic region, and the urethra which is located inferior to the bladder and we can see if we change views to this cross-section of the pelvis – this image right here. These organs work together as a group to filter out excess fluid, the by-products of metabolism and other waste materials from your bloodstream not to mention that they also help store, transport and excrete urine.
Now, let's take a closer look at the organs of the urinary system starting with the kidneys. Before we do that, I'm just going to move here to this image again where we're going to highlight the kidneys. The kidneys are two bean-shaped organs located in the abdominal cavity. They are located in the retroperitoneum underneath the intraperitoneal organs which have been removed in this illustration right here. Each kidney is invested in a thin connective tissue capsule known as – as you would expect – the renal fibrous capsule which we see here. This protects the kidneys from damage or injury.
Also seen from this view at the superior pole of each kidney is an endocrine gland enclosed in a fibrous capsule and a cushion of perirenal fat called the suprarenal or adrenal gland which we highlighted here. Now, these glands are not part of the urinary system and function instead as part of the endocrine system, don't forget that.
At the medial border of each kidney is the renal hilum where the renal arteries, renal veins and the renal pelvis enter and leave the kidneys. Note that the right kidney sits slightly lower than the left kidney since it sits posteroinferior to the liver. By posteroinferior, I mean basically underneath.
Now that we looked at the anatomy of the kidneys, let's explore the function. The main function of the kidney is to eliminate excess bodily fluid, salts and the by-products of protein metabolism. This results in the production of urine that is transported out of the kidneys through the ureters which happened to be the next structures that we're going to be talking about – the ureters.
The ureters emerge from the kidneys posterior to the renal vessels. They are actually a continuation of the renal pelvis of the kidneys which we just looked at in the renal hilum image. Upon leaving the kidneys, both the ureters pass inferiorly over the abdominal surface of the psoas major muscles which you see here with the gonadal vessels crossing them anteriorly shown here in a circle and the genitofemoral nerve posteriorly. On the right side, the right ureter continues to travel to the bladder posterior to the duodenum. It is crossed by the branches of the superior mesenteric vessels which is circled here though it has been cut in this illustration view – so keep that in mind.
On the left side, the left ureter continues lateral to the inferior mesenteric vessels which have also been cut but are circled right here. The left ureter is subsequently crossed by its branches. Both ureters continue inferiorly to enter then the true pelvis. They both cross the common iliac arteries bifurcate on each side on this path which is circled right here. To show this better, we will change the views in this illustration. We are now looking down into the pelvis with the anterior plane represented at the top of the page and the posterior at the bottom.
We see that once in the pelvis, the ureters pierce through the wall of the bladder, shown here with an arrow, opening into its posterior aspect. Anatomically speaking, the ureters can be divided into three parts – the abdominal part which extends from the renal pelvis to the linea terminalis of the bony pelvis, the pelvic part which extends from the linea terminalis to the wall of the urinary bladder, and the intramural part which is the portion of the ureter that passes through the wall of the bladder right here.
Well, as you may have guessed from the path the ureters take, these muscular tubular structures are responsible for transporting urine from your kidneys all the way to the urinary bladder which happened to be the next topic that we're going to be talking about next – the urinary bladder.
The urinary bladder which we are now highlighting on this image is a hollow organ with strong muscular walls. It is quite distensible and varies in shape and size – generally has an average capacity of between four hundred to six hundred milliliters. It is located in the lesser pelvis on the muscular sheath of the pelvic diaphragm and lies mainly on the levator ani muscle and its fascia which we can see if we change views. Here are the levator ani muscle and its fascia pointed out with arrows on the right and left.
Now due to anatomical differences in the male and female pelvis, the relations of the urinary bladder differs somewhat between the sexes. As you can see here in this sagittal cross-section of the male pelvis, the urinary bladder is bordered posteriorly by the rectovesical pouch and the rectum. We have circled both structures right here and pointed out the triangular rectovesical pouch space. In the female pelvis, the bladder is bordered by different structures than the male. The urinary bladder is bordered posteriorly by the uterus which I am pointing out here and their vesicouterine pouch which is located here deep to the uterus.
We now have some visuals here that can help you understand why pregnant women feel the need to urinate so frequently. It is important to note that a healthy bladder is only enlarged and rounded when it contains urine and is more full. The bladder is never empty and also enlarged like this. We show you this view to clearly understand the anatomy.
When the bladder is empty, it is flatter and smaller like a deflated balloon. This anatomical shape is due directly to the function of the bladder. The bladder serves as a temporary reservoir of urine that comes from the ureters. Just think of it of a holding tank. So, it is only ever as big as it is full up to its limit of around four hundred to six hundred milliliters and without this important organ, we would all be constantly urinating.
Now, let's move on to the last structure of the urinary system – the urethra. The urethra is a muscular tube connecting the urinary bladder to the outside. Again, there are differences in the structure of the urethra between the sexes. Here, we are looking at the female urethra.
The female urethra seen here in a coronal section is shorter than the male urethra and it extends from the internal urethral orifice of the urinary bladder which is circled in the image to the external urethral orifice also circled here located between the labia minora in the vestibule anterior to the vagina. A quick clinical note here – since the female urethra is shorter than the male urethra, there is an increased susceptibility to urinary tract infections in females.
Now we're going to move on to the male urethra which extends from the internal urethral orifice of the urinary bladder to the external orifice located at the tip of the glans penis and both are circled here. The male urethra can be divided anatomically into four parts which we will highlight and zoom in on – the intramural part which is the short most proximal part that runs from the bladder to where the urethra enters the prostate, the prostatic urethra which is the part of the male urethra that passes through the prostate gland, the intermediate or membranous part which is the shortest part of the male urethra between the prostate and the corpus spongiosum of the penis, and the spongy urethra which is the longest part of the male urethra and is contained in the corpus spongiosum of the penis.
I want to briefly do a quick note on the function of the urethra. You can probably guess this from the anatomy but the urethra conducts urine from the urinary bladder to the outside. The female urethra which is what we see now highlighted in green acts exclusively as a urinary passage. The male urethra which is now highlighted serves not only as a channel for the evacuation of urine from the urinary bladder but also as a passageway for semen during ejaculation. Therefore, it functions as both part of the urinary system and also the reproductive system. This is the last structure urine passes through before it leaves your body.
Before we finish, I would like to do a quick summary of what we have learned.
We first went over the organs of the urinary system then we went over the details by splitting the tutorial into the anatomy and the function of each organ within the system. We learned about the bean-shaped kidneys in the retroperitoneum filtering excess fluids, salts and by-products of protein metabolism to produce urine. We then followed the flow of this urine through the muscular tubular ureters which conduct urine from the kidneys into the urinary bladder. We learned that the urinary bladder can hold between four hundred and six hundred milliliters of urine and it stores it in a reservoir until it is time to urinate. Last, we looked at the urethra in both males and females which conducts urine from the urinary bladder to the outside as it is excreted from your body. Altogether, we learned that the organs of the urinary system work to filter out excess fluid, the by-products of metabolism, and other waste materials from the bloodstream, and also to store, transport and excrete urine out of your body.