Video: Blood vessels of the rectum and anal canal.

Hello everyone! It's Nicole from Kenhub, and in today's tutorial, we'll be talking about the blood vessels of the rectum. So as you know before we're going to talk about the blood vessels of the rectum, I just want to introduce you to the image we're going to be looking at today and as you can see, we're mainly going to be looking at this image which is a dorsal view of the blood vessels of the pelvis. And in addition to the blood vessels in this image, you can also see the distal end of the descending colon which I've drawn for you in green just here, the sigmoid colon over here, our main structure which is the rectum just here, the pelvic floor muscles surrounding it here and the anus down the bottom just here. We also have the pelvis which I'm pointing out with my arrow just here and the obturator internus muscle over here. But, of course, let's first begin this tutorial by talking about what the rectum is and what it does.

The rectum is, of course, a part of the large intestine and is about thirteen centimeters in length. Interestingly, the rectum does not contain teniae coli – the longitudinal bands of smooth muscle on the external surface of the rest of the large intestine which you can see on the bottom left – or appendices epiploicae which are small sacs of fat also found along the large intestine and which you can see on the second image down the bottom just here. Additionally, the rectum is notable in that the anterior aspect of the first two-thirds of the rectum are covered with peritoneum which we haven't drawn in on this image but if you can imagine is on the other side – the anterior side of the rectum – which is not visible to us while the last third is devoid of peritoneum.

Now let's take a couple of minutes to talk about the rectum's function. So, functionally, the rectum is essentially a temporary storage site for feces before they leave the body by the anus which if you recall is down here where my little drawing is in green. And, of course, feces are formed in more proximal parts of the large intestine by absorption of most of the water and electrolytes from food particles. And as the feces accumulate in the rectum, they can cause the rectum to expand stimulating nerve receptors which results in the contraction of the smooth muscle within the rectum. This results in an urge to expel the feces, however, this is only carried out with conscious relaxation of the external anal sphincter – so basically after you felt the urge to go to the toilet.

The rectum which we can see again highlighted by my lovely drawing skills in green is located at the distal end of the large intestine between the sigmoid colon and the anus or in other words beginning at the level of S3 which is represented by my rectangle and ending at the tip of the coccyx which is represented by the rectangle down the bottom just here.

Since there are a lot of blood vessels in this region, we're going to break them down a little bit for you starting with talking about the arteries of and around the rectum, the venous drainage of and nearby the rectum, and for each the arteries and veins, we're going to be breaking these down into further details so into major blood vessels, the branches associated with the rectal area and the vessels directly related to the rectum itself. This might seem a little bit complicated now but you'll be able to see it a lot more clearly once we get to it in the tutorial.

So let's begin by talking about the arteries of the rectum and before we talk about the blood vessels associated with the rectum, I just want to go over the blood vessels of the lower half of the body relevant to our tutorial in general beginning with the abdominal aorta. Like the rest of the gastrointestinal tract, the arterial supply to the rectum is of course derived from the abdominal aorta which we can see highlighted in green on our right. and the abdominal aorta normally begins when it enters the abdomen through the aortic hiatus of the diaphragm which in this image we have indicated by my little oval on the right there and that's about at the level of T12, and after passing through this aortic hiatus, the abdominal aorta continues inferiorly before bifurcating at the level of L4 into the right common iliac artery and the left common iliac artery. And just a note to mention at this point in time as the branches of the common iliac arteries are mostly reflective of one another, from now on we'll just be looking at left-sided images only. So just keep that in mind as we go along.

So having said that, the left common iliac artery which at around four centimeters is slightly shorter than the right common iliac artery continues inferiorly to split into a left external iliac artery which along with the right external iliac artery is the primary supply for the lower limb and a left internal iliac artery which constitutes the main blood supply to the pelvis, perineum and the gluteal region. And we'll be mainly looking at the branches of the internal iliac artery as we go through this tutorial.

As we mentioned, there are a lot of branches that are associated with the rectum and as you'll see the internal iliac artery is of course one of them, however, in addition to the internal iliac artery, two other branches which both arise directly from the abdominal aorta are of relevance to us in this tutorial giving as a total of three branches. And as we mentioned, the internal iliac artery which you can see on the right finds us with many of these branches but also so does the inferior mesenteric artery which arises from the anterior aspect of the abdominal aorta at the level of L3 which you can see on our right. And of course, we also have the median sacral artery which arises from the posterior aspect of the abdominal aorta just above the bifurcation of the aorta at the level of L4.

So let's of course begin with the internal iliac artery. So as you can see, the internal iliac artery gives off many branches that supply structures in the pelvis, however, for the purposes of this tutorial, we're going to focus on the following five branches which are listed in order of most superior origin to the most inferior origin. So, to begin, we're going to be looking at the superior gluteal artery which arises posteriorly, the obturator artery which arises anteriorly, the inferior gluteal artery which arises posteriorly, the middle rectal artery which arises anteriorly and is one of our arteries directly supplying the rectum, the internal pudendal artery which arises posteriorly and also gives rise to the inferior rectal artery which we'll be talking about today.

And let's of course begin by looking at the superior gluteal artery. So as you can see, the superior gluteal artery is the largest branch of the internal iliac artery and arises from the posterior aspect of the internal iliac artery at the level of S1 and you can see that circled by my green circle on the right. And as we mentioned, the superior gluteal artery exits the pelvis via the greater sciatic foramen. The superior gluteal artery also supplies the ileum as well as the piriformis, the obturator internus and the gluteus maximus. The superior gluteal artery also gives rise to the lateral sacral artery but that's not visible in this image.

The next branch of the internal iliac artery that we're interested in with regards to the blood supply of the rectum is the obturator artery. The obturator artery arises from the internal iliac artery in the pelvis which again you can see circled by my green circle before traveling towards the obturator foramen which I've outlined for you on both sides here in green. While still in the pelvis – so the part of the vein that is visible in the image – the obturator artery supplies the ileum and the iliacus muscle before it leaves the pelvis to enter the thigh via the obturator canal which is a small gap in the superior part of the obturator foramen not covered by the obturator fascia that allows the passage of the obturator vessels into the thigh. And in the breakout, you can see that I've highlighted the obturator canal and let's just draw in the obturator nerve in yellow and the obturator vein in blue just for completion's sake. Once in the thigh, it also supplies the pectineus muscle, the muscles of the medial compartment of the thigh and the femoral head.

The inferior gluteal artery arises within the pelvis before descending posteriorly and exiting the pelvis via the greater sciatic foramen below the piriformis and below the superior gemellus. Now in this image, you can't see the piriformis or the superior gemellus but if you can pretend that the shape I'm drawing in is the piriformis traveling from the sacrum to the greater trochanter of the femur and the shape that I'm drawing in is the superior gemellus traveling from the ischial spine to the middle part of the greater trochanter then you have a rough idea of how the inferior gluteal artery travels beneath that and you can see it wiggling in green just there. The inferior gluteal artery supplies the piriformis, the obturator internus, the gluteus maximus and the hamstrings which are the semitendinosus, semimembranosus and the biceps femoris.

The second last branch of the internal iliac artery that we want to look at is the middle rectal artery which is the first of the arteries that directly supplies the rectum and of course the middle rectal arteries arises in the pelvis before descending downwards and splitting into smaller branches. It then enters the rectum which you can see by my second green circle and provides blood supply to the middle third of the rectum which you can see in my green rectangle. The middle rectal artery can be quite variable and sometimes it arises as a branch of the inferior vesical artery in males or the vaginal artery in females but here were pretty firmly arising from the internal iliac artery.

The internal pudendal artery arises from the internal iliac artery just below the origin of the inferior gluteal artery which if you remember is this artery just here and descends laterally to exit the pelvis via the greater sciatic foramen. It then enters into the perineal region which is this whole region down here in my green oval. The internal pudendal artery then reenters the pelvis via the pudendal canal – a canal formed by the fascia of the obturator internus and along with the internal pudendal artery contains the pudendal nerve and the pudendal vein. So if you remember at the beginning of this tutorial, we mentioned the obturator internus muscle which I'll remind you is this muscle over here and over the top of the obturator internus muscle, you should be able to see a very fine layer of fascia as illustrated by our amazing illustration team and you can see it now highlighted by my green drawing. And, of course, you can see the inferior rectal artery running through the fascia accompanied by the inferior rectal vein in blue and on the pudendal nerve, the pudendal canal is therefore the part that's now highlighted in green. And this artery provides blood supply to the perineum and the genitalia which are not visible in this image.

One of the important branches of the internal pudendal artery that I want to pause to talk about briefly is the inferior rectal artery. And the inferior rectal artery is one of the arteries to directly supply the rectum. The inferior rectal artery is essentially a terminal branch of the internal pudendal artery that arises within the pudendal canal and if you remember, our pudendal canal is just here. The inferior rectal artery then enters the rectum and primarily supplies the distal third of the rectum which you can see in this green rectangle, the internal anal sphincter and the external anal sphincter. It also supplies the anal canal and the perianal skin.

And now that we finished talking about all the branches of the internal iliac artery, let's now take some time to look at the inferior mesenteric artery.

So before the abdominal aorta bifurcates into the common iliac arteries, the abdominal aorta gives off many branches that supply different parts of the gastrointestinal tract, and the most inferior of these branches is the inferior mesenteric artery which arises from the left anterolateral part of the abdominal aorta at the level of L3 before descending in the root of the left colic mesentery. And, of course, you can see the inferior mesenteric artery highlighted in green.

The inferior mesenteric artery primarily supplies the hindgut and gives rise to three branches – the left colic artery, the sigmoid arteries and the superior rectal artery – and together these arteries supply the large intestine from the distal transverse colon which is superior to our view here to the upper anal canal. And, in essence, it's all of these area that I've drawn for you in green.

So the first branch to arise from the inferior mesenteric artery just after its origin is the left colic artery. And let's now have a chat through all of these branches of course starting with the left colic artery, which then ascends within the left colic mesentery anterior to the left psoas major muscle. And of course if we draw some shapes to symbolize the psoas major muscle, we can see how the left colic artery runs in front of that. The left colic artery supplies the distal third of the transverse colon which you can't see in this image and the entire length of the descending colon, the lower part of which you can see just here.

The inferior mesenteric artery also gives off multiple sigmoid arteries usually between two and five in number, and these arteries descend in the sigmoid mesocolon anterior to the left ureter and the left psoas major muscle. So let's see here, the sigmoid arteries are anterior to the left ureter and the psoas major muscle is behind that in green and the sigmoid artery supply the distal part of the descending colon which you can see around about here as well as the sigmoid colon which if you remember is just here.

One brief structure I want to mention before moving on to the final branch of the inferior mesenteric artery is the marginal artery of the colon. And as you can see, both the left colic artery and the sigmoid artery form loops of arteries known as arcades and these help ensure that there is some crossover of the blood supply to different areas of the large intestine so if one artery becomes occluded, the other artery can then compensate thereby preventing ischemia. And these arcades occur along most of the length of the large intestine and they're collectively known as the marginal artery of the colon.

The final branch of the inferior mesenteric artery that we're going to talk about today is the superior rectal artery which is also considered to be its terminal branch. The superior rectal artery arises in the pelvis and descends within the sigmoid mesocolon before reaching the rectum, and that little pathway is represented here by my arrow. And the superior rectal artery supplies the upper two-thirds of the rectum.

Now that we finished talking about the branches of the inferior mesenteric artery, let's move on to talk about the median sacral artery.

The final artery associated with the blood supply of the rectum is the median sacral artery and unlike the other three arteries which arise from branches of the abdominal aorta, the median sacral artery arises from the posterior aspect of the aorta about 1.5 centimeters above its bifurcation and then descends over the last two lumbar vertebrae which are represented by these two rectangles. It also descends over the sacrum and the coccyx which we can see here as a triangle. The median sacral artery largely supplies the sacrum but also contributes a small blood supply to the rectum supplying the posterior wall of the anorectal junction which since we're looking at the posterior aspect of the rectum is around about here where the rectum meets the anus.

Now that we've talked through the major vessels associated with the rectum, let's just briefly go over how these all come together to supply the rectum.

So we discussed over the previous slides, there are four blood vessels which supply the rectum. The first artery is the superior rectal artery which is a branch of the inferior mesenteric artery and that supplies the upper two-thirds of the rectum, and the superior rectal artery overlaps some of the region of the middle rectal artery which you'll see up next. The middle rectal artery – a branch of the internal iliac artery – supplies the middle third of the rectum and the inferior rectal artery – a branch of the internal pudendal artery – supplies the lower third of the rectum which is down here in the green rectangle. And the final artery, the median sacral artery, branches directly from the aorta and contributes a small blood supply to the posterior wall of the anorectal junction.

Now let's move on to the second part of the tutorial and have a look at the venous drainage associated with the rectum. Unlike the arteries, we'll start at the top and work our way down to the bottom so you can follow the vein and its relationship to its corresponding artery if there is one, but remember the flow of blood is going from distal to proximal so the flow of blood is going the opposite way. And let's begin with a brief overview of the inferior vena cava.

The inferior vena cava is a large vein that carries deoxygenated blood from the middle lower parts of the body to the right atrium. And as you can see in the image of the thorax with the ribs cut away after receiving blood from the lower body, the inferior vena cava which is highlighted in green down here then travels superiorly to pierce the diaphragm which you can see is this muscle just here. This piercing of the diaphragm happens at the level of T8 and it eventually drains its blood all the way into the right atrium. And in this ventral view of the heart in isolation, we can, of course, see the inferior vena cava highlighted in green down here as it empties into the right atrium which you can see I've sketched out in green. And I just wanted to pause briefly to remind you that we're looking at images of the left side of the body today. So what we're mainly focusing on the tributaries of the left common iliac vein though of course the corresponding veins on the right side of the body mostly follow the same pathway as the left.

And so of course when we talk about veins that the inferior vena cava is formed by even though we're going to mention the right common iliac vein, we'll mainly be looking at the tributaries of the left common iliac vein. The left common iliac vein is therefore formed by the left external iliac vein which drains the lower limb and the left internal iliac vein which drains the gluteal region and the pelvic viscera.

Similar to the branches of the abdominal aorta, there are a couple of branches that are specifically related to the blood supply of the rectum that we want to look at today and the first one of these is the internal iliac vein. The other main vein we'll look at in this section is the inferior mesenteric vein which is a tributary of the portal vein but we'll have a look at that one a little bit later. So let's firstly look a bit more closely at the internal iliac vein.

The internal iliac vein is a major vein that receives most of the blood from the pelvis and some of the blood from the gluteal region. Like the internal iliac artery which gives off many branches, the internal iliac vein receives many tributaries, however, today we're going to mainly focus on the following four tributaries – the superior gluteal vein, the obturator vein, the middle rectal vein which is one of the veins directly supplying the rectum, and the internal pudendal vein which receives the inferior rectal vein, another one of our direct rectal veins. And so let's of course begin with the superior gluteal vein.

The superior gluteal vein arises in the thigh and enters the pelvis with the superior gluteal artery via the sciatic foramen. The superior gluteal vein drains blood from the ileum, the piriformis, the obturator internus and the gluteus maximus before draining into the internal iliac vein. An important thing to note about the superior gluteal vein is that it is one of several veins that are often but not always can be found as a venae comitantes, that is, two accompanying veins instead of one lying on either side of its corresponding artery. The venae comitantes are paired in this way as it allows for the stronger pulse of the artery to help out with venous return. So in this image of the superior gluteal vein and artery with the superior gluteal artery highlighted in green, you can see me drawing two veins. So the first one is already visible here and I'm just drawing over it and I'm drawing another vein in on the other side. And they're both of course running alongside the superior gluteal artery. The other veins to look out for its potential venae comitantes that are relevant for us in this tutorial are the obturator vein and the inferior gluteal vein.

The next branch of the internal iliac vein that is related to the blood vessels of the rectum is the obturator vein which also arises in the thigh and enters the pelvis via the obturator foramen before draining into the internal iliac vein. And, of course, the obturator foramen is this little foramen here in our green circle. The obturator vein drains the ileum, the iliacus, the pectineus, the medial compartment muscles and the femoral head which you can see here on the right. And as we mentioned on the previous slides, the obturator vein can sometimes be found as a venae comitantes.

The third tributary of the internal iliac vein and also one of the veins that drain the rectum directly is the middle rectal vein. As you can see on this image, the middle rectal vein arises from the rectal venous plexus which is this network over here and drains the middle part of the rectum before of course draining into the internal iliac vein.

The fourth branch of the internal iliac vein that we're interested in today is the internal pudendal vein which begins in the deep veins of the penis and accompanies the internal pudendal artery before draining into the internal iliac vein. In summary, the internal pudendal vein drains the perineum and also receives the inferior rectal vein which you can see in this image and what we'll be looking at on our next slide.

The final tributary of the internal iliac vein of interest to us is the inferior rectal vein, and the inferior rectal vein as we mentioned is one of the veins that directly drains the rectum and very specifically drains the inferior part of the rectum which you can see highlighted in our rectangle just here. And as we mentioned before on the previous slide, the inferior rectal veins drain into the internal pudendal vein which in turn drains into the internal iliac vein.

Now that we've spoken about the tributaries of the internal iliac vein, let's talk about the inferior mesenteric vein. The inferior mesenteric vein is a large vein that drains blood from the rectum which I've drawn for you in the middle here, the sigmoid and descending colons which now highlighted by my drawing up here, and as well as some of the transverse colon which is not visible in this image. Unlike the inferior mesenteric artery which we saw in the first section branching directly from the abdominal aorta itself, the inferior mesenteric vein which we can see in this ventral view of the abdomen with the liver pulled back, does not empty directly into the inferior vena cava and let's just point out some structures here. So we mentioned the liver which is up here and over here we have the most superior part of the stomach while over here we have the spleen and the diaphragm back here with the aorta running through it and back here we have the inferior vena cava. So as we were saying instead of the inferior mesenteric vein emptying directly into the inferior vena cava, it instead empties into the splenic vein which together with the superior mesenteric vein empties into the portal vein which is now highlighted in green. And are therefore plays an important role in providing blood from the large intestine for detoxification in the liver.

So now that we've explained that, let's just go back to our original image and have a look at how the inferior mesenteric vein is formed. The inferior mesenteric vein is formed by the confluence of these tributaries – the superior rectal vein and this image on our right over here is the left superior rectal vein. And in addition to the superior rectal vein in this tutorial, we'll also have a look at the sigmoid veins. But let's of course begin by looking at the superior rectal vein.

As we mentioned in the previous slide, the left superior rectal vein which you can see highlighted in green and the right superior rectal vein which you can see highlighted in green now also converge to form the inferior mesenteric vein. And the superior rectal vein is the last of the veins that directly drains the rectum draining the superior part of the rectum. The inferior mesenteric vein also receives tributaries from the sigmoid veins which of course drain the sigmoid colon which is this part of the colon just here that I've drawn out for you in green before draining into the inferior mesenteric vein.

Now that we've had a look at the major veins associated with the venous drainage of the rectum, I just want to go over the veins that directly drain the rectum so you can have a clearer idea of how it all comes together. So as you may have noted, there are three main veins that directly drain the rectum and these veins are the superior rectal vein which is a branch of the inferior mesenteric vein and which drains the upper portion of the rectum, the middle rectal vein a branch of the internal iliac vein and which drains the middle portion of the rectum, and the inferior rectal vein, a branch of the internal pudendal vein which itself is a branch of the internal iliac vein and drains the inferior portion of the rectum.

One important thing to note about the venous drainage of the rectum is that direct drainage of the rectum actually arises from three venous plexuses which in turn form the three rectal veins. And these three plexuses are the perimuscular venous plexus which is formed by the superior and middle rectal veins and which mainly drains the muscular wall of the rectum which is highlighted in my green rectangle on the right and the middle rectal vein receives the primary venous drainage of the plexus though the superior portion of the rectum is often drained by the superior rectal vein.

The external venous plexus which is formed by the inferior rectal veins and which drains the mucosal, submucosal and perianal tissues is our next plexus and do note that this venous plexus is often confused for the perimuscular venous plexus because of its name but just try to remember that it's actually this little plexus down here in my circle.

A third plexus, the internal venous plexus, also drains the mucosal, submucosal and perianal tissues but as it's not visible on this image we won't discuss it in great detail. And the external and internal venous plexuses together are sometimes called the submucosal plexuses.

To finish up this tutorial, let's go over some clinical notes related to the blood supply of the rectum and the main clinical topic I want to talk about today is haemorrhoids. And haemorrhoids, otherwise known as piles, is a common clinical condition often referred to as swollen veins. This condition can be caused by pressure in the anal or rectal region such as straining while making a bowel movement which results in increased pressure within the rectum. This in turn creates the dilation of the veins resulting in swollen veins, bleeding, itching and pain and possibly prolapse. So haemorrhoids can usually be classified as internal or external. So let's first have a look at the internal haemorrhoids.

Internal haemorrhoids are caused by the dilation of the internal venous plexus and so if you can imagine us cutting this image along the coronal plane with my little saw just here – there we go – then here we have an image of the rectum and the anal canal with the anterior portion cut away. And in case you can't see it, we have our internal venous plexus cut in coronal plane just here highlighted in green and pointed out by my arrow. And so when the pressure increases in the rectum, the internal venous plexus dilates to look a little bit like this with our black outline being the inner mucosal wall and the blue lines representing the venous plexus. As for symptoms of internal haemorrhoids, they're usually painless and may only be indicated by bleeding.

Now moving on to look at external haemorrhoids. External haemorrhoids are caused by the dilation or thrombosis of the external venous plexus and like in the image before, we have the external venous plexus highlighted in green just here as pointed out by my arrow and when the external venous plexus is put under pressure within the rectum, we usually get a haemorrhoid like this one just here. Now, external haemorrhoids are usually more painful than internal haemorrhoids and a thrombosis or strangulation of the external venous plexus can result in the skin in the region turning purple or blue as we can see in the image as well as causing significant pain and the risk of gangrene which like in this image can turn the affected skin black.

While most cases of haemorrhoids can be treated with simple home remedies including over-the-counter creams and preventative measures such as encouraging a patient to eat more fiber, more severe cases of haemorrhoids may require more involvement. Persistent bleeding, for example, can result in the use of rubber band therapy, sclerotherapy or coagulation techniques by using infrared light – and I have a little image of an infrared light up at the top – whereas severe cases may require a surgical procedure known as haemorrhoidectomy or haemorrhoid stapling. If we go back to our image of the internal venous plexus with the internal haemorrhoids protruding into the anal canal, we can see what's called a circular stapler entering the anus to cut off an internal haemorrhoid with the triangular-shaped anvil in green coming down into the stapler and cutting the haemorrhoid off – and you can see the haemorrhoid just falling away into the stapler just there.

And that's it for our clinical note. So now that we've gone through everything in this tutorial, let's summarize what we've learned.

So coming back to our image of the rectum and the blood vessels from the posterior aspect, let's remind ourselves that the rectum is a storage site for feces before they are expelled from the body and the rectum is located within the pelvic region. Its blood supply involves three major arteries. The internal iliac artery which constitutes the main blood supply to the pelvis, perineum and gluteal region and the internal iliac artery has five branches – the superior gluteal artery which is the largest of the branches arising from the internal iliac artery, the obturator artery which travels through the obturator foramen, the inferior gluteal artery which is involved in supplying the hamstrings, the middle rectal artery which is one of the arteries that directly supplies the rectum and the internal pudendal artery which mostly supplies the perineum and the genitalia and also gives rise to the inferior rectal artery, another direct branch to the rectum.

The second major branch that supplies the rectum is the inferior mesenteric artery and if you remember back to the through the tutorial, the inferior mesenteric artery arises from the aorta around the L3 and gives rise to three branches – the left colic artery which supplies most of the colon on the left side of the body, the sigmoid arteries which supply the sigmoid colon and the superior rectal artery, the third of our direct branches to the rectum.

The median sacral artery is our last major artery that's involved in the supply of the rectum and it's involved in supplying the posterior wall of the anorectal junction. It's also one of our direct branches through the rectum. And of course the four major arteries that directly supply the rectum are the superior rectal artery supplying the upper two-thirds of the rectum, the middle rectal artery supplying the middle portion of the rectum, the inferior rectal artery supplying the inferior portion of the rectum and the median sacral artery supplying the posterior wall of the anorectal junction.

We also looked at the venous drainage of the rectum which includes the two major veins. The internal iliac vein, a major vein that receives most of the blood of the pelvis and the internal iliac vein has four tributaries – the superior gluteal vein which if we may remember can be a venae comitantes, the obturator vein which travels with the obturator artery to travel through the obturator foramen, the middle rectal vein which directly drains the middle part of the rectum and the internal pudendal vein which travels with the internal pudendal artery and which receives the inferior rectal vein which itself directly drains the inferior portion of the rectum.

The second major vein involved with the venous drainage of the rectum is the inferior mesenteric vein and the inferior mesenteric vein has two major tributaries – the superior rectal vein which drains the superior part of the rectum and the sigmoid veins which drain the sigmoid colon. And of course there are three veins that directly receive blood drainage from the rectum and these are the superior rectal vein which drains the upper portion of the rectum, the middle rectal vein which drains the middle portion of the rectum and the inferior rectal vein which drains the inferior portion of the rectum.

And of course let's not forget about our venous plexuses. So, firstly, the perimuscular venous plexus and it's important to remember here that the perimuscular venous plexus is not the same as the external venous plexus. The external venous plexus is this little plexus down here below. And the rectum also has the internal venous plexus. And of course sometimes the external venous plexus and the internal venous plexus are collectively called the submucosal plexuses.

And lastly in our clinical notes, we looked at the haemorrhoids which are dilations of rectal venous plexuses and haemorrhoids can be classified into internal haemorrhoids which is the dilation of the internal venous plexus and external haemorrhoids which are the dilations of the external venous plexus.

So that brings us to the end of this tutorial and thanks again for watching.