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Structure of the penis and urethra seen on a longitudinal section.
Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial on the penis and the male urethra. So today we're going to be basing our tutorial on the two images you can now see in front of you. This caudal view here on the left shows the penis in the context of the urogenital triangle and its attachment to the bony pelvis. So we can see structures such as the corpus spongiosum and the glans penis which are separated out from the corpus cavernosum. We can also the ischium of the pelvis just here and some of the muscles of the perineum including the superficial transverse perineal muscles.
On our right, we can see a longitudinal cross-section of the penis. In addition to discussing the major features of the penis present in this image, we'll also be using this illustration to discuss the urethra in more detail as it travels from the neck of the bladder through the prostate and body of the penis to finally open up the urethral orifice. This image shows a more schematic representation of the anatomy which will help you get your head around the structures a little more easily. But before we explore these structures further, let's start by running through what we're going to talk about in this tutorial.
We're going to begin this tutorial by first discussing the penis primarily focusing on its formation as well as its anatomy which of course includes the urethra. Since the urethra has its own unique features, we're also going to have a closer at its anatomy. This will be followed by a brief discussion of some clinical notes which involve both structures. So firstly let's start by discussing the penis using this image of the penis in situ or in its natural position as external genitalia.
The penis is part of the male reproductive and urinary systems and is the largest part of the male genitalia. It's located on the external anterior surface of the pelvis slightly below the level of the pubic symphysis. The anatomical position of the penis is when it's erect therefore the dorsum, that is, the topside of the penis faces posteriorly. When flaccid – that is not erect – the dorsum of the penis faces anteriorly as in our image on the right. This is important to consider when discussing the penile anatomy and neurovasculature. So what exactly is the role of the penis?
The penis belongs to the genito-urinary system and therefore has functions that can be divided into reproductive functions and urinary functions. You may be wondering how specifically the penis fulfills these rolls. Well from a reproductive standpoint, the penis needs to become erect to facilitate sexual intercourse and acts as a conduit for the passage of semen. It also allows passage of urine from the bladder to the external environment via the urethra. This is required for the expulsion of urinary waste and the proper functioning of the urinary system.
So now that we've looked at the function of the penis, let's look at its formation.
The penis is formed by structures called erectile bodies. These structures contain large vascular spaces which have the potential to become engorged with blood causing them to become rigid in turn leading to the erection of the penis. The erectile bodies consist of two bilateral corpora cavernosa which we can see in this cross-sectional image and one median corpus spongiosum which lies between the two cavernous bodies. A layer of fibrous tissue called the tunica albuginea frames each erectile body and we can see it highlighted in green surrounding the corpora cavernosa. This tissue layer assists in the maintenance of the male erection.
We'll now move on to explore these erectile bodies in more detail. The bilateral corpora cavernosa known singularly as the cavernous body of the penis or the corpus cavernosum make up the largest portion of the penis. They come together with the corpus spongiosum to form the dorsal aspect of the penis. In this cross-sectional image, we can see the two corpora cavernosa on either side of the corpus spongiosum. The other erectile body – the corpus spongiosum also known as the spongious body of the penis – lies in the midline and makes up the ventral component of the penis. At the distal most point of the penis, the corpus spongiosum expands to form the bulbous glans of the penis. In the next illustration, we can see that the urethra passes through the corpus spongiosum and that the corpus spongiosum is continuous with the glans.
So now that we have a better idea of what the penis is made up of, let's focus on its anatomy. When considering its anatomy, the penis can be divided into three parts – the root which is the part that's attached to the external pelvis and associated tissues, the body which is the free pendulous part of the penis that is suspended from the pelvis, and finally the glans which is the bulbous expansion of the distal penis. Let's go on to look at each of these anatomical components in more detail.
So as you can see in our illustration on the right, the root of the penis has 3 components. It consists of two bilateral crura of the penis and the bulb of the penis. The crura of the penis are the two tapering processes of the corpora cavernosa which diverged laterally in the perineum to form an attachment with the bony pelvis specifically the ischial pubic rami. The word crura is the plural of crus which means leg and as we can see they act as legs anchoring the penis to the pelvis. We can see this a bit more clearly in the next image.
Located between the penile crura, we can see the bulb of the penis which is a single midline structure and is a proximal enlargement of the corpus spongiosum. The bulb of the penis is the point at which the urethra pierces and enters the penis. We can see the penile urethra a bit more clearly in this image as it travels through the corpus spongiosum towards the glans.
Between the root of the penis and the glans, we have the body of the penis. The body of the penis constitutes the majority of the penile length. It's composed of all three erectile bodies – the two corpora cavernosa which we can see here and the corpus spongiosum which we can see sitting between them. In the next illustration, we could see these structures in cross-section – the corpora cavernosa here and here and the corpus spongiosum here.
Next we'll move on to talk about the glans of the penis. The glans is the most distal aspect of the penis and as we've mentioned is an enlargement of the corpus spongiosum. We can see it in this image assuming a bulbous mushroom-like shape. There are two parts of the glans – the corona and the urethral orifice. Let's transition to a cross sectional image so we can see these structures a bit more clearly.
The corona is the flared rounded base of the glans and is considered the boundary between the glans and the body of the penis whereas the urethral orifice is the slit-like opening at the distal end of the glans. It is otherwise known as the external urethral orifice and marks the end of the urethra. The glans penis is homologue to the clitoral glans in females and in both genders the glans is highly innervated providing much of the sensation of the penis during sexual arousal. The penile glans is completely or partially covered by a layer of retractable skin called the foreskin which is also known as the prepuce. In uncircumcised males, the foreskin is wrapped around the fascia overlying the body of the penis and aids in the mechanics of sexual intercourse. We can see the foreskin highlighted in green in this caudal image of the penis.
Now that I've told you a bit about the penis, let's move on to discuss the urethra, which is a tubular structure and roughly 20 centimeters long. It runs from the internal urethral orifice of the bladder which we can see here to the external urethral orifice of the penile glans allowing for passage of urine and semen.
Anatomically, the urethra can be divided into 4 part – the pre-prostatic urethra, the prostatic urethra, the membranous urethra and the spongy urethra. The initial part of the urethra is referred to as the pre-prostatic urethra or the intramural part of the urethra. It passes through the muscular neck of the bladder running from the internal urethral orifice to the point where the bladder meets the prostate. It's typically about 0.5 to 1.5 centimeters in length but this can vary depending on whether the bladder is filled with urine or empty. Surrounding the pre-prostatic urethra is the internal urethral sphincter which serves to assist in the control of continence of the bladder and also to prevent the backflow of semen into the bladder during ejaculation.
As we move distally along the urethra, we come to the prostatic urethra. This is a part of the urethra that passes through the prostate and is about 3-4 centimeters in length. It extends from the base of the bladder just below the pre-prostatic urethra to the membranous part of the urethra. There are a few features found on the posterior aspect of the prostatic urethra that I want to point out. The first of which is an elevation called the urethral crest. This elevation is where the prostatic ductules open up on either side bringing prostatic fluid into the urethral lumen. Just inferior to this, there is another rounded elevation called the seminal colliculus which is the point where the ejaculatory ducts open up into the prostatic urethra. The ejaculatory ducts carry sperm from the testes and fluid from the seminal vesicles into the urethra, therefore, this is the point where the urinary and reproductive tracts merge.
One last interesting feature of the prostatic urethra is the prostatic utricle. This feature is the homologue of the vagina and uterus in females and doesn’t develop in males during fetal life.
Now let's move on to the membranous part of the urethra which is the second shortest part of the urethra and connects the prostatic urethra to the spongy urethra. It's about 1 to 1.5 centimeters long and is surrounded by the external urethral sphincter which we can now see highlighted in green. This urethral sphincter plays an important role in voluntary control of urinary flow.
The largest and most distal part of the urethra is the spongy urethra which is approximately 15 centimeters long and runs within the corpus spongiosum. The spongy urethra ends as it opens to the outside environment through the external urethral orifice. There are some additional features associated with the spongy urethra that I'd like to bring your attention to. The first of which are the bulbourethral glands. These glands secrete fluid into the urethra acting as lubrication. There's also the ampulla of the urethra, which is located within the bulb of the penis and the navicular fossa which is located within the glans. Both of these structures are dilations within the spongy urethra.
Finally, we'll finish off this tutorial with some clinical notes relating to the penis and urethra. Benign prostatic hyperplasia is a non-cancerous increase in the size of the prostate typically affecting men over the age of 60. It directly affects the prostatic part of the urethra as that’s what passes through the mass of the prostate. As the prostate increases in size, it encroaches on the lumen of the prostatic urethra compressing it, however, it doesn’t restrict flow. Rather it makes the bladder musculature work harder to push urine through therefore leading to the destruction of flow. Some of its clinical signs include involuntary urination, waking up in the night to urinate, urinary hesitancy and intermittent urine flow. It's often managed by watchful waiting as the symptoms of therapy can be worse than those of the condition itself. However, if the prostate enlarges too much then a prostatectomy can be performed. This condition differs from cancer both anatomically and clinically. Cancer typically starts in the outer zones of the prostate not affecting urine flow until its later stages whereas benign prostatic hyperplasia begins in the inner zones of the prostate affecting the urethra much earlier on.
So that concludes our tutorial on the penis and the male urethra. I hope you enjoyed it and thank you for listening!
Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.
Now, good luck everyone, and I will see you next time.