Abstract
The rectum and anal canal form the terminal regions of the gastrointestinal tract. They are essential for fecal storage and defecation. Situated within the pelvic cavity, the rectum transitions into the anal canal in the perineum, housing critical neurovascular structures vital for continence. Extensive knowledge of these structures is vital for surgical precision. This chapter examines the anatomy, histology, embryology, and clinical relevance of these structures. Key anatomical features such as rectal folds, curvatures, and the anal sphincter complex are described. Histological adaptations, including goblet cells for lubrication and M cells for immune defense, are reviewed alongside the structural transitions in the epithelium of the anal canal, which predispose certain zones to specific pathologies. The vascular and lymphatic networks, crucial for rectal cancer staging and hemorrhoidal disease, are detailed, alongside somatic and autonomic innervation critical to anorectal function. This overview emphasizes the anatomical and histological complexity of the anorectal region, its clinical significance in disease, and the implications for effective diagnosis, treatment, and surgical interventions.
Keywords
- anatomy
- histology
- embryology
- surgical anatomy
- rectum
- anal anatomy
1. Introduction
The rectum, anal canal, and anus contribute to the most distal end of the gastrointestinal tract. The rectum is situated within the pelvic cavity, while the anal canal and anus project into the perineum. When comparing quadrupedal and bipedal organisms, several evolutionary adaptations of the rectum and anal canal are noteworthy. In quadrupedal organisms, the rectum is more horizontally aligned with the gastrointestinal tract, functioning primarily as a conduit for fecal matter with minimal storage capacity or capacity for voluntary defecation [1]. On the other hand, bipedal species, including humans, have evolved a rectal ampulla, allowing for fecal retention and voluntary defecation control. This adaptation has been crucial in accommodating the pressures exerted by an upright posture, influencing factors such as continence, influence of gravity, pressure distribution, and defecatory mechanisms [1]. Thus, the anatomical evolution of the rectum and anal canal in humans reflects broader adaptations to locomotion, pelvic stability, and social behaviors surrounding appropriate settings for defecation.
Furthermore, many structures are intimately related to the rectum and the anus. Among these structures are delicate neurovascular networks which can present subtle clinical signs of rectal or anal involvement. In the same token, the proximity to such structures has severe implications during rectal or anal surgery if injured. Thus, knowledge of the anatomy of the rectum and the anus, including their relations is paramount for the effective management of rectal and anal pathology and disease.
There are several gaping holes in the literature that reflect our understanding of the anatomy of the rectum and anal canal relative to other regions and structures of the body. One of the compounding factors contributing to this is the cultural taboo surrounding discussions of defecation and anorectal function [2, 3]. Another factor is the mere complexity of studying these structures due to their deep pelvic location, intricate neurovascular supply, and involvement in both autonomic and voluntary control mechanisms [3, 4, 5, 6, 7]. The lack of detailed investigation has implications for clinical practice, as many rectal and anal conditions, including functional disorders and malignancies, remain poorly understood or are diagnosed late due to inadequate screening tools, accessibility to such tools, and public education [5, 6, 7, 8].
The aim of this chapter is to outline the clinically relevant anatomy, relations, histology, and embryology of the rectum and anal canal, highlighting the complexity and importance of this underappreciated region of the human body. It will first cover the embryology and histology of the rectum and anal canal, followed by the extent, relations, and neurovascular supply of the rectum and anal canal.
2. Embryology
The rectum and the upper part of the anal canal are derived from the primitive gut tube; specifically, the hindgut. The hindgut is suspended dorsally by the mesorectum [9]. The mesorectum is a specialized layer of connective tissue that is similar to the peritoneum but differs slightly as the mesorectum itself is encased within the mesorectal fascia. The rectal epithelium is derived from the endoderm of the hindgut, while the surrounding tissue is derived from the splanchnic mesoderm [10]. The lower end of the anal canal is derived from the cloaca [9]. The cloaca around the 4th to 7th weeks of development partitions into the urorectal septum that keeps the anal canal and the urogenital sinus as separate structures. Failure to separate results in anorectal malformations such as imperforate anus or rectovaginal/rectourethral fistulas [10].
3. Histology of the rectum
The histology of the rectum is continuous with the histology of the hindgut with slight differences.
3.1 Mucosa
The mucosal layer of the colon is the most metabolically and immunologically active. The mucosa consists of epithelium, lamina propria, and the muscularis mucosae.
3.1.1 Epithelium
Rectal mucosa is covered with glycocalyx which contributes to the microbial ecosystem and acts as a protective barrier. Deep to the glycocalyx is simple columnar epithelium lining regularly spaced crypts (similar to a test-tube rack). The crypts are aligned perpendicular to and extend to the muscularis mucosae. Within the epithelium several types of cells are found many of which include absorptive enterocytes (colonocytes), goblet cells, and M cells [11].
Since the function of the rectal is for storage of feces the number of absorptive enterocytes is significantly less than that of the right side of colon. These cells where present in the rectal epithelium characterized by rigid, tightly packed apical microvilli with a lightly eosinophilic cytoplasm [11]. Relating the structure to function of the rectum again, the rectal mucosa is prone to abrasion from the friction pushing feces into the anal canal has potential to cause, thus a large number of goblet cells here would make sense. Goblet cell nuclei, appear hyperchromatic, dense, and irregular [11]. M cells in the rectum are integral to the gut’s immune defense system, particularly in sampling and presenting luminal antigens to immune cells to trigger appropriate immune responses. Their role during inflammation may also reflect a protective mechanism or contribute to chronic inflammation, as seen in inflammatory bowel diseases [11, 12]. On histology, M cells appear with a reduced number of irregular microvilli, and have a basolateral membrane forms an intraepithelial pocket which provides a docking site for intraepithelial B and T cells [11, 12].
3.1.2 Lamina propria
The lamina propria of the colon, located between the crypts and muscularis mucosae, plays a critical role in immunologic, metabolic, proliferative, and motility functions. It is a highly organized structure housing inflammatory and mesenchymal cells within a loose connective tissue matrix, facilitating immune surveillance and response [11, 13].
3.1.3 Muscularis mucosae
The muscularis mucosae is external to the lamina propria. The muscularis mucosae is a thin layer of smooth muscle physically tethered to the mucosa. It receives innervation from the inner part of the submucosal plexus. Assessment of the integrity, invasion, and injury of the muscularis mucosae has diagnostic significance [14].
3.2 Submucosa
The submucosa of the rectum is composed of smooth muscle fibers, connective tissue, vasculature, lymphatic channels, and various neural networks. The submucosal neural plexuses are the inner plexus (Meissner’s) which innervates the muscularis mucosae and the second is the outer submucosal plexus (Henle’s) which contributes to the innervation of the muscularis layer.
Clear distinction between the mucosa and submucosa are crucial for rectal cancer staging. Tumor invasion into the submucosa (T1 stage in TNM staging) indicates an early stage of cancer spread beyond the mucosal layer, specifically beyond the muscularis mucosae [13]. Tumor invasion into the submucosa increases the likelihood of metastasis through lymphatic spread to regional lymph nodes or distant metastases [13].
3.3 Muscular externa
The muscularis externa layer of the rectum consists of two smooth muscle layers. The first is the inner circular layer which facilitates constriction and segmentation movements of the rectum. The next is the outer longitudinal layer which aids in shortening and propelling colonic contents from rectum into the anal canal. Between these two layers exists a myenteric plexus (Auerbach’s) which innervates the two layers. Additionally, interstitial cells (of Cajal) which are specialized pacemaker cells that drive peristalsis are found throughout the muscularis propria. As mentioned above, distinction between the histological layers of the rectum is crucial for rectal cancer staging [13]. If the tumor is confined to the muscularis externa, it is classified as T2 in the TNM staging system. When the tumor breaches the muscularis externa and extends into the perirectal fat or mesorectal tissue, it is classified as T3. Further extension into adjacent organs or structures (such as the bladder, prostate, or peritoneum) is classified as T4 [13].
4. Anal canal
The anal canal is defined both anatomically and surgically. The anatomical anal canal extends from the anal verge to the dentate (pectinate) line. While the surgical anal canal extends from the anal verge to the anorectal line. The anatomical delineation aligns with its embryological origin from the cloaca, where the dentate line serves as a landmark where there is a change in the neurovasculature [15]. The anal canal has several important structures part of it and related to it as outlined below.
4.1 Internal anatomy and histology of the anal canal
The histology of the anal canal is continuous with the histological arrangement of the gastrointestinal tract i.e. mucosal, submucosal, muscularis, and serosa/tunica adventitia layers. The contents and composition of each of these layers corresponds to the location and function of the anal canal.
4.1.1 Mucosa of the anal canal
The mucous membrane of the anal canal has 10–15 permanent longitudinal folds called anal columns, joined at the bases to form anal valves at the level of the pectinate line. Histologically, there are four important transition zones in the epithelium of the anal canal – colorectal, anal transitional, squamous, and perianal zones (Figure 1) [11, 16].
The colorectal zone is approximately at the anorectal junction where a change in epithelium is seen from simple to stratified columnar epithelium. This zone is most susceptible to colorectal like adenocarcinomas (Figure 2) [11].
The anal transitional zone (ATZ) features stratified columnar or stratified cuboidal epithelium. Here the epithelium begins to transition towards a stratified squamous appearance (Figure 2). The ATZ extends from the dentate line to the new squamous columnar junction at the pectineal line. On anoscopy, the SCJ is the most important and readily identifiable landmark, typically visible as a clear line of tissue that is paler than the adjacent proximal rectal epithelium – this corresponds to the pectinate line. The boundaries between the colorectal and ATZ are not well demarcated. This zone, because of the volatile nature of transitioning cells, is susceptible to anal intraepithelial neoplasia and squamous cell carcinomas in the presence of oncogenic strains of HPV [11, 16, 17].
The squamous zone is composed of uninterrupted, non-keratinized stratified squamous epithelium. This extends towards the white line (of Hilton) (Figure 2) [11, 16, 17].
The perianal skin zone mirrors the histology of the skin - i.e., keratinized stratified squamous epithelium (Figure 2), with hair follicles, sebaceous glands, sweat glands, and apocrine glands (circumanal glands). Melanocytes are also seen in this zone. This extends from the white line towards the anal margin (i.e., perianal skin). Due to the nature of the histology, this zone is susceptible to hidradenitis suppurativa [18], melanomas [19], lichen planus [20], and benign fibroepithelial growths.

Figure 1.
Histological zones of the anal canal. Colorectal zone (I); Anal transitional zone (II), Squamous Junction (III), Perianal Zone (IV). Illustration by Joyce El-Haddad.

Figure 2.
Histological zones of the anal canal. Colorectal zone (I); Anal transitional zone (II), Squamous Junction (III), Perianal Zone (IV) and their respective epithelium. Illustration by Joyce El-Haddad.
4.1.2 Submucosa of the anal canal
The submucosal layer of the anal canal houses the internal and external haemorrhoidal plexuses. These plexuses contain both tributaries of veins (superior and inferior rectal veins) and arterioles (superior and inferior rectal arteries) [15, 21]. The internal rectal (haemorrhoidal) plexus within the submucosa layer in the upper anal canal pushes the subepithelial tissue towards the mucosa and lumen of the anal canal. These expansions are known as the anal cushions and are most prominent in the 3, 7, 9 o’clock positions [15, 21].
Under normal physiological and mechanical conditions, the anal cushions help seal the anal canal and contribute to fecal and flatus continence [7]. The external rectal (haemorrhoidal) plexus is found in the submucosal layer of the perianal space beneath the perianal skin. The anatomy of the haemorrhoidal plexuses and anal cushions are contentious. Stelzner [22], describes their anatomy to be akin to erectile tissue (corpus cavernosum recti) [13], in that they possess deep vascular spaces that are engorged with arteriovenous blood to seal the anal canal. While some earlier studies suggested a connection between haemorrhoidal disease and portal hypertension, more recent evidence distinguishes anal varices secondary to portal hypertension as a separate entity from hemorrhoids [23, 24, 25].
It is important to reiterate, that that the term ‘hemorrhoids’ does not indicate pathology, but rather the prolapse and transformation of the anal cushions into anal nodules secondary to the engorgement of the respective plexus does. Consistent and accurate use of the terminology has potential to improve interdisciplinary communication and research, and patient education.
4.2 Muscularis layer of anal canal (anal sphincter complex)
The anal sphincter complex consists of both the internal and external anal sphincters which are separated by a conjoint longitudinal muscle layer. The anal sphincter complex plays a critical role in defecation and maintaining continence. This complex corresponds histologically to the muscularis layer of the anal canal.
4.2.1 Internal anal sphincter
The internal anal sphincter (IAS) is a continuation and thickening of the inner circular layer of the rectum. The IAS is smooth muscle and thus innervated by the autonomic nervous system. The IAS commences at the anorectal junction and ends above the anal verge [26]. At the intersphincteric groove, the lower margin of the IAS is palpable. Under normal, non-pathological conditions, the IAS is contracted and provides the resting anal tone felt during a digital anal examination. This function ensures that continence is maintained at rest. Cali et al. [16] reported normal variations of resting anal tone and noted that due to larger and thicker sphincteric muscle mass, males generally exhibit greater squeeze and resting pressures [27]. They also reported that females who have undergone vaginal deliveries had a decrease in maximum resting pressures which was exacerbated in multiparous females [27].
4.2.2 External anal sphincter
The external anal sphincter (EAS) is a skeletal muscle under somatic innervation. The EAS historically has been divided into 3 parts (deep, superficial and subcutaneous parts) however functionally these are irrelevant as the EAS along with the levator ani and puborectalis muscles function as a collective unit. The EAS and the levator ani work in concert during voluntary contraction to enhance continence [26, 27]. For example, during coughing or sneezing (i.e., increased intrabdominal pressure), the levator ani contracts reflexively to provide additional support, while the EAS tightens to prevent leakage. Anatomically and with respects to attachments, the EAS can be divided into an upper and lower part. The upper part of the EAS attaches posteriorly to the anococcygeal ligament and anteriorly to the perineal body [26, 28].
The lower portion of the EAS extends beneath the IAS and is penetrated by the terminal fibers of the conjoint longitudinal muscle (see below). Anteriorly, it connects to the bulbospongiosus and bulbocavernosus muscles [28] – this forms an ‘8’ shaped figure that connects the urogenital and anal triangles.
4.2.3 Conjoint longitudinal muscle
The conjoint longitudinal muscle (CLM) is the continuation of the outer longitudinal muscle of the rectal wall. It originates at the level the rectal muscularis layer and is situated between the IAS and EAS and plays a role in anorectal stability. The CLM inserts into inserts into the perineal body anteriorly and the anococcygeal raphe posteriorly. The CLM is thought to be thicker in fetal life, and as IAS develops the CLM regresses eventually into connective tissue later in life [26]. The CLM fibers transverse the submucosa and connect to the mucosa of the anal canal to anchor the anal cushions. The anal cushions, composed of vascularized tissue, are crucial for maintaining continence. If the CLM weakens, the anchoring support diminishes, allowing the cushions to prolapse and form hemorrhoids [26]. Consequently, the downward movement of the anal cushions due to weakened CLM tension can disrupt venous return in the hemorrhoidal plexus, leading to engorgement of the arteriovenous plexuses, inflammation, and potentially symptomatic/prolapsed hemorrhoids [29].
The CLM and its extensions can also provide potential pathways for the spread of infection. The extensions of CLM between IAS and EAS create potential anatomical planes and pathways through which infections can spread between the rectum, anal canal, and adjacent spaces, such as the perineum or ischioanal fossae. Its insertion into the perineal body and anococcygeal raphe also provides a direct anatomical bridge for infections to travel in spaces such as the ischioanal fossae and/or presacral spaces. However, it should be noted that during early stages of infection and before the anatomical barriers are breached, the CLM plays a role in compartmentalizing infections in the abovementioned spaces [16, 26, 30].
5. Extent of the rectum and the anal canal
Knowledge and orientation of the extent of the rectum is essential to effective surgical precision, imaging interpretation, preoperative planning, patient positioning during clinical examinations, and education [15, 28]. However, as basic as defining the extent of the rectum and anal canal may seem, it remains controversial [13]. This section will provide an overview of the consensus in the literature regarding the extent of the anorectal area.
5.1 The upper limit of the rectum
The upper limit of the rectum begins at the rectosigmoid junction (RSJ). The RSJ serves as an important landmark for procedures such as total mesorectal excision (TME), resection margins in rectal cancer surgeries, and colostomy planning. This junction is characterized by the absence of taeniae coli and omental appendages (epiploic appendages). The RSJ is found at the level of the third sacral vertebrae. The RSJ is said to be 2.8 cm long in adults and 0.7 cm in neonates [30]. Histologically, the RSJ also marks the transition from sigmoid colon mucosa to rectal mucosa. The RSJ can be identified via an array of modalities. Some examples include:
5.1.1 Identifying the RSJ on CT and MRI
The sigmoid take off (STO) has been suggested as an imaging landmark to identify the RSJ on CT and MRI [31]. The STO is defined as the junction between the mesorectum and sigmoid mesocolon [13]. The STO has also been defined as the region where the fixed mesorectum ends and no longer tethers the rectum to the sacrum, and thus the rectum becomes mobile [28, 32]. The STO is seen on a sagittal view where the sigmoid colon moves horizontally away from the sacrum (Figure 2). Axially, the sigmoid colon moves ventrally (Figure 3). Identification of the STO allows for accurate classification of sigmoid or rectal tumors, above and below the STO, respectively [31].

Figure 3.
Mid sagittal view of the female pelvis. A represents the sigmoid colon and B represents the rectum. Sigmoid colon from this view is seen to move horizontally away from the sacrum. The asterisk represents the sigmoid take off (STO) at the level of S3. Illustration by Joyce El-Haddad.
While the STO is a promising landmark for defining the RSJ and improving the sensitivity of rectal cancer diagnoses, it is not without its limitations. For example, Hazen et al. [31], aimed to evaluate the use of the STO in clinical practice and found that in the presence of complex anatomy and pathology, and/or suboptimal and inconsistent imaging conditions identification of STO was not feasible and requires extensive training and expertise. The authors also highlighted that without adequately addressing the abovementioned challenges, the risk of misclassification of tumors would have a downstream effect on treatment plans and ultimately prognosis. Similar observations have been corroborated by other studies [13].
5.1.2 Identifying the RSJ on sigmoidoscopy
On sigmoidoscopy, various landmarks of the RSJ have been proposed [33]. The RSJ is said to be 12–17 cm proximal from the anal verge [34, 35]. However, it should be noted that the range of lengths seem to be based on a limited and narrow demographic group and likely do not consider age, sex, ancestral heritage, nor the presence or absence of various pathologies.
5.1.3 Identifying the RSJ on barium enema studies
To identify the RSJ reliably, Rubesin et al. [35] highlight that the patient should be positioned laterally to avoid sigmoid loops from obstructing the RSJ view. Rubesin et al. recommend that the overhead view obtained with the tube angled about 30° caudad and with the patient in the prone position is optimal to observe the rectosigmoid junction [35]. To the author’s knowledge, no literature to date outlines landmarks to identify the RSJ that are specific to barium enema studies (Figure 4).

Figure 4.
Axial view of the pelvis at the level of S3. The sigmoid colon from this view is seen to move anteriorly away from the sacrum. Asterisks represents the sigmoid take off (STO). Illustration by Joyce El-Haddad.
5.2 The lower limit of the rectum and upper limit of the anal canal
The lower limit of the rectum is even more controversial than the upper limit. The main classifications are anatomical and clinical (Figure 5).

Figure 5.
Surgical vs. anatomical classification of anal canal. The lower limit of the rectum is clinically found at the anorectal line (1) which aligns with the anorectal muscle sling. Anatomical lower limit of the rectum is the dentate line (2). Lower limit of the anal verge (margin) (3) Illustration by Joyce El-Haddad.
5.2.1 Anatomical
The dentate (pectinate) line remains the most commonly referred to landmark to distinguish the end of the rectum and the start of the anal canal (Figure 5). This known as the start of the “anatomical anal canal”. The dentate line used as the demarcation of the end of the rectum also aligns with its histology, as the dentate line is where the transition in the histology between columnar rectal mucosa to squamous anal mucosa occurs.
5.2.2 Surgical anal canal
Surgically, the lower limit of the rectum is said to be at the anorectal junction (ARJ). The ARJ is approximately 1 cm above the dentate line (Figure 5). The ARJ is in line with the level of the muscular anorectal ring and thus palpable on a digital rectal exam. The ARJ is also used as a surgical landmark to separate the rectum from the anus during low anterior resections or abdominoperineal resections [21]. Identification of the ARJ is important to use as a landmark to measure the height of a suspected tumor. From a sagittal view, the anorectal junction is situated at the level of an imaginary line between the lower margin of the sacral and pubic bone [14]. From a coronal view the ARJ aligns with an imaginary line along the upper edge of the puborectal sling [14].
5.3 Lower limit of the anal canal
The lower limit of the surgical anal canal is around the mucocutaneous junction of the non-keratinised squamous mucosa (anal verge) and the keratinised perianal skin in line with the intersphincteric groove [36]. The intersphincteric groove is approximately 2 cm below the pectinate line in adults. The anal margin, also referred to as the perianal skin extends radially for about 5 cm distal to the anal verge. This is the skin around the anus proper and is what is seen on clinical examination (Figure 5) [37].
6. Internal anatomy and relations of the rectum
When the rectum is empty, it possesses several temporary longitudinal folds that stretch when the rectum is full. The folds likely exist as a way to accommodate for volume changes and reduce risk of perforation. The rectum typically has 3 permanent semilunar folds which become more prominent when the rectum is distended [26]. The three folds (although the number can vary) are known as superior, middle and inferior rectal folds. The middle rectal fold in particular serves as a landmark during sigmoidoscopy or rectal surgery as it is lies immediately above the rectal ampulla. In addition, tumors below this valve are palpable on a digital rectal examination [38]. Externally, the rectum typically has two curvatures. The first is the sacral flexure – an anteroposterior curve with anterior concavity. The second is the anorectal flexure - an anteroposterior curve with anterior convexity that is maintained by the puborectalis muscle. Additionally, externally the rectum has several important relations that are noteworthy (Figure 5).
6.1 Peritoneal relations of the rectum
The rectum is both intra and extraperitoneal. The upper two thirds of the rectum is classified as intraperitoneal given that its anterior and lateral aspects are covered by peritoneum. Meanwhile, the lower third is classified as extraperitoneal. It should be noted that in the middle third of the rectum, only the anterior aspect of the rectum is covered by peritoneum [26] It is important to understand the peritoneal relations of the rectum, particularly in the context of rectal perforations. Any perforations through a part of the rectal wall that is covered by peritoneum means that the perforated matter has access to the peritoneum, potentially leading to sepsis [30].
6.2 Fascial relations of the rectum
The mesorectum covers the rectum and its surrounding fat. The mesorectum transmits the superior rectal artery (and its branches) and vein (and its tributaries), and lymphatics. Anterolaterally in the mesorectum, are branches of the inferior hypogastric plexus and the middle rectal arteries (where present). Dissection around the inferior hypogastric plexus must be carefully done to preserve urinary bladder and sexual functions [26]. There are suggestions that branches of the inferior hypogastric plexus and the middle rectal arteries are enclosed by the lateral ligaments, however, not much is known about the morphology, extent, and prevalence of these ligaments.
The mesorectum, with its fatty tissue predominantly deposited posteriorly, provides not only a protective role but also supplies vascular, lymphatic, and neural elements crucial for rectal function. The mesorectum itself is encased within the mesorectal fascia (fascia propria of the rectum). The mesorectal fascia covers the rectum down to the level of levator ani muscle [26] Surgically, the mesorectal fascia provides the resection plane for total mesorectal excision (TME). Posterior to the mesorectal fascia is the presacral space (retrorectal space). This space itself is anterior to the presacral fascia which some texts suggest is synonymous with Waldeyer’s fascia. The exact definition of Waldeyer’s fascia remains debated, with two predominant interpretations: (1) it is considered the presacral fascia, which forms the base of the retrorectal space and condenses into the rectosacral ligament; or (2) it is described as the rectosacral fascia, a dense connective tissue layer extending from the periosteum of the fourth sacral vertebra to the posterior rectal wall (Figure 6) [39]. Waldeyer’s fascia, however, remains a critical landmark during posterior rectal mobilization. It is suggested that improper blunt dissection may compromise the mesorectum, increasing the risk of cancer metastasis. Additionally, tearing of Waldeyer’s fascia from the sacrum can lead to significant bleeding from the presacral veins [39].

Figure 6.
Fascial relations of the rectum in the female pelvis. Illustration by Joyce El-Haddad.
Anteriorly, in males the mesorectal fascia becomes coalesced with the rectoprostatic (rectovesical) fascia (Denonvilliers fascia), which is anterior to the rectovesical pouch. In females, the mesorectal fascia anteriorly combines with rectovaginal fascia (Denonvilliers fascia) to form the rectovaginal septum which is anterior to the rectouterine pouch (of Douglas). It has been suggested that the merging of the respective fascia potentially limits the spread of malignancy from one adjacent organ to the other (Figure 7) [40].

Figure 7.
Fascial relations of the rectum in the male pelvis. Illustration by Joyce El-Haddad.
7. Arterial supply of the rectum and anal canal
The rectum is supplied by three arteries - the superior, middle (bilateral), and inferior rectal (bilateral) (hemorrhoidal arteries). Sometimes, the median sacral artery from the abdominal aorta can contribute to the supply of the posterior wall of the rectum and the anal canal.
7.1 Superior rectal artery
The inferior mesenteric artery (IMA) branches from the aorta at the level of L3 anterolaterally to the left. It gives off branches to the distal segments of the colon and its pelvic continuation is the superior rectal artery (SRA). The SRA commences at the level of the pelvic brim and crosses the left common iliac vessels with the inferior hypogastric nerves on both sides. It pierces the mesorectum and gives off branches to the rectosigmoid junction, an upper rectal branch which then divides into right and left terminal branches (Figure 8) [41]. At the level of S3 the SRA enters the upper mesorectum and the right and left terminal branches that descend posterolaterally [1]. The right terminal branch of SRA is usually larger than the left and is considered the true continuation of the SRA (Figure 8) [41]. The left terminal branch of SRA is considered a collateral branch of IMA and supplies the anterior surface of the rectum [41]. The perforating arteries of the SRA branches enter the rectal submucosa and anastomose with branches of the middle and inferior rectal arteries. The SRA is the main artery that supplies the rectum and contributes significantly to the arterial supply of the internal hemorrhoids [26] The branches of SRA terminate usually above the anal valves [41]. The SRA also supplies two muscles of the anal sphincter complex i.e., the IAS and CLM. The SRA is closely associated with lymph nodes serving as the primary pathway for lymphatic drainage from the rectum and upper anal canal.

Figure 8.
Blood supply of the rectum. Illustration by Joyce El-Haddad.
7.2 Middle rectal artery
The prevalence rate of the middle rectal artery (MRA) ranges from 12 to 97% [42]. The MRA is a direct branch off the anterior division of the internal iliac artery (Figure 8). The MRA is thought to provide the anatomical basis for lateral spread of rectal cancer [42]. The blood supply territories of the MRA are not clear.
The MRA is thought to contribute to the supply of the lower rectum and anal canal and thought to provide a collateral pathway with SRA and IRA. This thought process comes from the fact that the MRA perfuses the rectal stump significantly post ligation of the SRA [43]. Careful consideration should be taken when near the vicinity of the MRA to reduce risk of bleeding and consequently impairment of the surgical field [43]. The MRA perforates the inferior hypogastric plexus and the mesorectal fascia laterally.
Heinze et al., aimed to evaluate the anatomy of the middle rectal artery to facilitate surgical management. Out of 22 body donor dissections, they found the MRA to have originated from the internal pudendal artery in 45.5%, the gluteal artery in 22.7%, a common gluteal-pudendal trunk in 22.7%, or formed a trifurcation with these arteries in 9.1% of cases [42]. Where present, the MRA also gives off branches to the surrounding pelvic organs. The MRA perforating branches enter the rectal submucosa and anastomose with branches of the superior and inferior rectal arteries.
7.3 Inferior rectal artery
The inferior rectal artery (IRA) is the terminal branch of the internal pudendal artery. The internal pudendal arteries on the left and right sides enter the pudendal (Alcock’s canal) (Figure 8). The IRA then exits medially into the ischioanal fossae to supply the distal third of the rectum to supply the internal and external anal sphincters, anal canal, and perianal skin [26] There is bilateral communication between the pair of IRAs which leaves a watershed area in the dorso-caudal area of the rectal ampulla [26, 44, 45]. The watershed area potentially provides an anatomical rationale as to why anastomotic leaks are observed in the dorso-caudal rectal ampulla post low anterior resection of the rectum [44, 45].
Similar to the SRA and MRA, the IRA perforating branches enter the rectal submucosa and anastomose with branches of the middle and superior rectal arteries. Klosterhalfen et al. [45], outlined two main anatomical variants of the IRA. The first variant of the IRA gives no branches to the posterior commissure of the anal canal, whilst the second variant which is less common, supplies the posterior commissure. The authors concluded that where there is less perfusion to the posterior commissure (as seen in the first type of variant), makes the region more prone to ischemia and slower at healing if an injury such as anal fissure occurs [45].
8. Venous drainage of the rectum and anal canal
The rectum and anal canal are drained by two main veins – the inferior mesenteric vein and the internal iliac vein.
8.1 Rectal venous plexus
The rectal venous plexus surrounds the rectum and has direct connections to local venous plexuses such as the vesical venous plexus in males and uterovaginal venous plexus in females [26]. The rectal venous plexus is divided into 2 parts based on the orientation relative to the histological layers of the rectum. The internal veins are found deep to the mucosa of the rectum and upper anal canal [26]. The external veins are found outside the muscularis layer.
8.2 Superior rectal vein
The unilateral superior rectal vein (SRV) drains the upper two-thirds of the rectum and the upper anal. The SRV eventually drains into the portal system via the inferior mesenteric vein. The superior rectal vein forms from the rectal plexus. The SRV runs along the upper mesorectum and the root of the sigmoid mesocolon. The SRV runs to the left of the SRA [26]. The SRV continues as the inferior mesenteric vein at the pelvic brim once the SRV crosses the left common iliac vessels. The pelvic brim is also the site where the abdominal ureter becomes the pelvic ureter. This close relationship is clinically significant because procedures involving the ligation or manipulation of the SRV, may pose a risk of inadvertent injury to the ureter.
8.3 Middle and inferior rectal veins
The lower third of the rectum and lower anal canal are drained by the middle and inferior rectal veins which enter the systemic venous circulation via the internal iliac veins. The venous drainage of the rectum provides several areas of hematogenous metastasis of cancer. For example, although rare, tumors in the lower rectum and anal canal can first metastasize to the lungs via systemic circulation without the involvement of hepatic metastases [44, 46]. In addition, the connection between systemic veins and the portal venous system provides a site for development of varicosities due to portal hypertension secondary to advanced liver disease.
9. Innervation of the rectum and anal canal
The rectum, the upper anal canal, along with the rest of the hindgut receives autonomic innervation.
9.1 Visceral afferents
The visceral afferent pathway for most of the hindgut corresponds to the levels of the sympathetic motor division. While most of the hindgut afferents follow the sympathetic pathway, the rectum’s location below the peritoneal reflection (i.e., pelvic pain line), means that majority of its visceral afferents may travel via parasympathetic routes typically associated with pelvic viscera below the pelvic pain line. However, although minor, the upper aspect of the rectum which straddles the pelvic pain line, may have its visceral afferents follow the levels of sympathetic motor division. This may account for the blended sensory input seen in rectal distension and pain, where both noxious and innocuous stimuli are processed across different spinal levels [47, 48]. The convergence of these afferents within the spinal cord may explain the broad and poorly localized nature of rectal pain, as well as its tendency to mimic somatic or pelvic pathologies. Thus, afferent signals from the hindgut are received by afferent neurons from L1–L3 (via lumbar splanchnic nerves) and S2–S4 (via pelvic splanchnic nerves) dorsal root ganglia (Figure 9) [47, 48].

Figure 9.
Innervation of the rectum and the anal canal. On the left-hand side is a schematic representation of visceral and somatic afferent innervation. (A) represents visceral afferents from L1–L3 (via lumbar splanchnic nerves) and S2–S4 (via pelvic splanchnic nerves), which convey pain and distension from the rectum to their respective dorsal root ganglia. (B) represents somatic afferent fibers of the pudendal nerve (S2–S4), which transmit fine touch, pain, and temperature from the perianal skin and anal canal below the pectinate line. On the right-hand side is a schematic representation of the motor innervation to the rectum and anal canal. (C) represents preganglionic sympathetic fibers from L1–L3 spinal cord levels projecting via lumbar splanchnic nerves to the (D) inferior mesenteric ganglion, where they synapse. (E) shows the postganglionic sympathetic fibers that travel via the inferior hypogastric plexus to the rectal wall. Preganglionic fibers may also descend through the sympathetic chain to the sacral levels and exit via (F) sacral splanchnic nerves, which also contribute postganglionic fibers to the superior and inferior hypogastric plexuses. (G) and (H) represent the preganglionic and postganglionic parasympathetic fibers, respectively, arising from the pelvic splanchnic nerves (S2–S4), which synapse in ganglia near or within the target organ. (I) represents the somatic motor fibers of the pudendal nerve (S2–S4) that innervate the external anal sphincter and perianal muscles, facilitating voluntary control. Note: Both afferent and efferent innervation to the rectum is bilateral. Illustration by Joyce El-Haddad.
Visceral afferents in the rectum convey the sensation of rectal filling and are involved in reflex propulsive activity - a coordinated effort between the rectum, relaxation of pelvic floor, and anal sphincter complex that pushes feces through the rectum to the anal canal, into the external environment [26]. Feng and Guo [49] suggested that the lumbar splanchnic nerves predominantly carry high-threshold nociceptive signals, likely involved in detecting significant mechanical stress (e.g., distension) in the rectum and adjacent areas. They also suggested that the sacral nerves, in contrast, encode both nociceptive and innocuous sensations, which can contribute to a more complex pain presentation [48]. The diffuse nature of visceral and referred pain makes it challenging to pinpoint its exact source without additional diagnostic methods. Furthermore, the complex referral pattern of pain from the rectum (proctalgia fugax) and the overlap with innervation of adjacent structures in the pelvis might mimic conditions like lower back pain, pelvic pain syndromes, gynecological conditions, and/or biopsychosocial factors and should be considered accordingly.
9.2 Sympathetic motor innervation
The sympathetic preganglionic motor portion is conveyed from the lumbar splanchnic nerves (L1-L2/3) via the sacral splanchnic nerves (S1-S2). The cell bodies of the preganglionic sympathetic motor neurons originate from the intermediolateral columns L1-L2/3 and synapse in the inferior mesenteric plexuses. The preganglionic neurons also reach the sacral splanchnic nerves which then go onto to contributing to the superior (aka presacral nerve) and inferior hypogastric plexuses (Figure 9). The postganglionic neurons reach the rectal wall via neural networks that are intertwined along the corresponding arteries. The postganglionic sympathetic neurons from both sources release noradrenaline (norepinephrine) and thus, sympathetic innervation of the rectum results in inhibition of peristalsis by acting on the muscularis layer of the rectum [26].
9.3 Parasympathetic motor innervation
The hindgut receives parasympathetic motor innervation from the pelvic splanchnic nerves (S2-S4). The cell bodies of the preganglionic neurons originate from the lateral intermediate substance, found in S2, S3 and S4 segments of the spinal cord (i.e., the sacral parasympathetic nucleus). The preganglionic axons enter the inferior hypogastric plexus where they will either go directly to the rectum or ascend via the right and left hypogastric nerves to the superior hypogastric plexus to innervate some proximal segments of the hindgut or innervate adjacent pelvic organs (Figure 9). Parasympathetic stimulation via the release of acetylcholine causes colonic propulsion and the relaxation of the internal anal sphincter, both functions essential in defecation [26].
9.4 Somatic innervation of the anal canal
Of significance to pain perception and patient presentations of anorectal pathologies, is the change at the pectinate line in innervation from visceral afferent via superior rectal nerve (proximally) to somatic afferents via the pudendal (pelvic) nerve (distally) (Figure 9). The pudendal nerve also provides somatic motor innervation to the EAS.
10. Lymphatic drainage of the rectum and anal canal
The lymphatic drainage of the rectum and anal canal corresponds to the arterial supply. However, it should be noted that the lymphatic drainage of the rectum and anal canal is complex, not fully clear, and not fully agreed upon [44, 50, 51]. Population, anatomical variation, and imaging-based studies are likely to prove useful here.
The rectum and the upper anal canal (above pectinate line) eventually drains into the preaortic nodes (inferior mesenteric nodes) (Figure 10). The lower anal and EAS drains via the superficial inguinal nodes to the paraaortic nodes (via the common iliac nodes) (Figure 9). The lymphatic drainage of the rectum ultimately drains to cisterna chyli, and eventually to the thoracic duct.

Figure 10.
Lymphatic drainage of the rectum. Illustration by Joyce El-Haddad.
Lymphatic drainage of the rectum and upper anal canal can be divided via two intertwined groups: intramural, intermediate, and extramural lymphatic groups. The intramural nodes are found within the submucosal layer of the rectum and upper anal canal and drain into the extramural group. These nodes are significant as the extramural nodes are often the first site of metastatic spread of rectal cancers. The extramural group i.e., superior, middle, and inferior rectal nodes are located outside the rectal wall and correspond to the arteries [50].
In terms of the intermediate group – from the literature, it is not clear what layer of the rectal wall they are confined to [26, 50]. However, from the name it likely suggests that it would be the muscularis layer. Understanding the role of the intermediate nodes is important as tumors or other space occupying lesions can obstruct proximal pathways and redirect lymphatic drainage/spread laterally or distally. For example, some rectal cancers may spread laterally to internal iliac nodes, bypassing proximal pathways [50].
11. Conclusion
The anatomy of the rectum and the anal canal is often overlooked or shied away from. More often than not, the rectum and anal canal are reduced to pipes ‘down there’. However, the importance of their normal function is quintessential to life. Only those with dysfunction of the rectum and/or anal canal, and those who treat such patients can truly appreciate the profound significance of the aforementioned sentiment.
The anatomy of the rectum and anal canal is complex and of significant clinical relevance, particularly in colorectal surgery, oncology, and functional disorders of the lower gastrointestinal tract. Understanding the intricate anatomical and histological features of the rectum and anal canal is crucial for accurate diagnosis, surgical precision, and effective treatment of conditions such as colorectal cancer, haemorrhoidal disease, and congenital anorectal malformations. Despite extensive research, there remains several gaps and controversies in the understanding of the anatomy of the rectum and anal canal. A significant challenge lies in the variability of anatomical structures, and the inconsistent identification of key landmarks such as the rectosigmoid junction and anorectal junction, both of which influence and potentially complicate surgical planning and diagnostic accuracy. The precise role of the middle rectal artery in the lateral spread of rectal cancer is also still debated, and variations in lymphatic drainage patterns add to the complexity, with differing interpretations of how tumor cells seed and spread through these networks.
Moreover, the terminology surrounding conditions like hemorrhoids and anal cushions can be misleading, often conflating normal anatomical features with pathological states. Further, the mesorectal fascia’s role in rectal cancer surgery, particularly in total mesorectal excision, remains an area of ongoing investigation. These unresolved anatomical questions highlight the need for further research and refined imaging techniques to better clarify these structures and their clinical significance. Future research, including imaging-based and body donor-based studies, may help clarify these ambiguities and contribute to improved clinical outcomes.
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