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Anatomy and function of the urinary bladder.
Hey everyone! This is Nicole from Kenhub, and in this tutorial, we're going to look at the urinary bladder, its function, structure, location and neurovasculature.
So we’re going to start off by looking at some sagittal-ish sections of the infraperitoneal area with our female model on the left here and our male model on the right and of course our bladder is highlighted in both images in green. So the urinary bladder is a hollow, muscular distensible organ of the urogenital system and it’s located inferior to the peritoneum within the lesser pelvis and rests on the pelvic floor. And in this image of the female, we can see the bladder over here, the uterus here, the ovaries here and the rectum here. And over on our male model, we can see our bladder in green, the seminal ducts and the rectum just here.
And so the bladder’s main function is to collect urine that’s to be later voided by urination. And although the volume of the bladder varies from person to person, in general it can hold anywhere from 400 to 1000 milliliters of urine with the average capacity at around 400 to 600 milliliters, and so roughly that’s the same volume as a can of soft drink.
So the walls of the bladder are composed of thick smooth muscle called detrusor muscle and this muscle is lined with a transitional epithelium internally which in the bladder is called urothelium, and urothelium is a little bit special as it also lines the ureters and part of the ureter and is also known for its high elasticity.
So the body of the urinary bladder shown here in a male pelvis on top and in a female pelvis below is the portion of the bladder situated between the fundus and the apex, and the fundus being the inferior-posterior surface or base while the apex is the anterior aspect which is located towards the pubic symphysis. So this of course is a coronal section of the bladder but if we were to imagine a sagittal section then the apex with the upfront closest to us coming out of the screen near the pubic symphysis and the fundus would be at the back going into the screen. Urine that comes into the bladder from the kidneys via the ureters is collected in the body of the bladder and as the bladder fills with urine it takes an ovoid shape and extends towards the anterior abdominal wall within the greater pelvis.
The fundus of the bladder is the inferior part of the posterior wall or the base of the bladder and again we’re looking at a coronal section but just imagine the green section just pointing out into the computer out back, and in males, the fundus is closely related to the anterior wall of the rectum while in females, the fundus is closely related to the anterior wall of the vagina. And the fundus contains the trigone of the bladder which we're going to talk about in the next slide.
The trigone of the bladder is found on the internal surface of the fundus and it’s an upside down triangular area of smooth muscle where its superolateral angles formed by the ureteric orifices which we can see pointed out by these arrows, and these orifices are the openings of the two ureters in the bladder and between them lies a membranous fold called the interureteral fold. And the interureteral fold is a fold of mucous membrane which has several other names and these are Mercier’s bar, the interureteric fold or the interureteric crest.
So the neck of the bladder is situated at the base of the trigone and is therefore the most inferior aspect of the bladder. Inferiorly, the neck of the bladder is continuous with the proximal urethra. And the urethra which arises from the neck of the urinary bladder is a canal for the transportation of urine from the bladder to the external urethral orifice where urine exits the body, and here we have of course our cross section of the female bladder on the left and the male bladder and the urethra cross-section on the right.
The female urethra is approximately four centimeters long with its external opening situated between the clitoris and the vagina while the male urethra is much longer and can be divided into four parts: The pre-prostatic urethra which is the initial part of the urethra and is only about 0.5 to 1.5 centimeters long and is continuous with the neck of the bladder which is just here. It’s also surrounded by an involuntary muscle called the internal urethral sphincter muscle. The prostatic urethra which is about three to four centimeters long is continuous with the pre-prostatic urethra and as we can see it runs through the length of the prostate. The membranous urethra begins upon exiting the prostate and it’s about 1 to 1.5 centimeters long and it is surrounded by the external urethral sphincter muscle. And here we have this spongy urethra also referred as to the penile urethra, and the spongy urethra is the final portion of the male urethra and it's continuous with the membranous urethra and this part of the male urethra is the longest about 15 centimeters and contains the external urethral orifice.
And of course we can’t leave without talking about the blood supply. So the urinary bladder receives arterial blood supply from two branches of the internal iliac artery in both males and females. The superior vesical artery which supplies the anterosuperior aspect of the bladder and the inferior vesicle artery which supplies the fundus and the neck of the bladder. Venous blood from the bladder is drained via the vesical venous plexus to the internal iliac veins and the veins covering the bladder which are shown in blue in both images are part of the vesical venous plexus. Innervation of the urinary bladder is provided by the vesical nervous plexus highlighted in green in both images.
So it’s probably a good point in our tutorial here to mention the way urine is passed and which is otherwise known as the process of micturition as there are 2 ways that this can happen via a parasympathetic or reflex action called the micturition reflex or voluntarily through the sympathetic system. So in the case of the micturition reflex as we’ve mentioned, the bladder can fill up to around about 400 to 600 milliliters of liquid so when the bladder is full, it triggers stretch receptors located in the bladder wall and this occurs around 300 to 400 milliliters of liquid although at 150 mils it can cause an urge which is easily repressed.
Now once the bladder triggers these spinal nerves in the parasympathetic nervous system, the detrusor muscle is given the signal to contract and the bladder expels urine through the urethra. With regards to voluntary micturition, however, motor control is provided by fibers from both the parasympathetic and sympathetic nervous system but greater detail of this process will be covered in another tutorial. What will be useful to remember in this tutorial however is that the process of storing liquid and emptying the bladder are under the control of the sympathetic, parasympathetic and somatic nervous systems with storage controlled by the sympathetic system and micturition controlled by the parasympathetic system.
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