The anal canal is the final segment of the gastrointestinal tract and plays an important role in defecation and maintaining faecal continence. In this article, we shall look at the anatomy of the anal canal – its position, structure, relations and neurovascular supply.
The anal canal is located within the anal triangle of the perineum between the right and left ischioanal fossae. It is the final segment of the gastrointestinal tract, with a length of 4cm. Starting as a continuation of the rectum, the canal passes inferoposteriorly to terminate at the anus. During defecation, the anal canal is opened up and faecal material is released; otherwise, it is collapsed by the internal and external anal sphincters to prevent the passage of faecal material.
The anal canal is surrounded by two types of sphincters: internal and external. The internal anal sphincter surrounds the upper 2/3 of the anal canal and is formed from a thickening of the involuntary circular smooth muscle in the bowel wall, while the external anal sphincter is a voluntary muscle and covers the lower 2/3 of the anal canal, overlapping with the internal sphincter. Above, the internal and external sphincters blend superiorly with the puborectalis muscle of the pelvic floor. At the junction of the rectum and the anal canal, the anorectal ring is found, a muscular ring formed by the fusion of the internal and external sphincters, as well as the puborectalis muscle. It is palpable on digital rectal examination.
The superior aspect of the anal canal has the same epithelial lining as the rectum (columnar epithelium). However, in the anal canal, the mucosa is organised into longitudinal folds, known as anal columns. These join their inferior ends at the anal valves, while above the valves are small pouches referred to as anal sinuses, which contain glands that secrete mucus. The anal valves together form an irregular circle, called the pectinate line, also known as the dentate line, and divides the canal into two parts which differ in structure and neurovascular supply.
Above the pectinate line is derived from the embryonic hindgut, while below the line the anal canal is lined by non-keratinised stratified squamous epithelium, known as the anal pecten, a smooth and pale surface which transitions at the level of the intersphincteric groove to true skin (keratinised stratified squamous). The anal canal is in close proximity to several other important structures in the pelvis and perineum, including the coccyx and sacrum, the anococcygeal ligament, the perineal body, the urogenital diaphragm, the urethra and the bulb of the penis.
The pectinate line divides the anal canal into two parts – which have a different arterial supply, venous drainage, innervation and lymphatic drainage.
Above the Pectinate Line – visceral innervation via the inferior hypogastric plexus.
The two parts of the anal canal have different arterial supply, venous drainage, innervation and lymphatic drainage. Above the pectinate line, blood supply is provided by the superior rectal artery, which is a branch of the inferior mesenteric artery, along with anastomosing branches from the middle rectal artery. Similarly, the superior rectal vein, which empties into the inferior mesenteric vein, provides the venous drainage for this part of the anal canal. Finally, visceral innervation is provided via the inferior hypogastric plexus.
Below the pectinate line, blood supply is provided by the inferior rectal artery, a branch of the internal pudendal artery, along with anastomosing branches from the middle rectal artery. Venous drainage is provided by the inferior rectal vein, which empties into the internal pudendal vein. The lower part of the anal canal does not receive visceral innervation as this area is derived from the ectoderm of the proctodeum, and so is not considered part of the gastrointestinal tract.
In conclusion,the anal canal has a crucial role in the maintenance of faecal continence. It is located within the anal triangle of the perineum between the right and left ischioanal fossae and is the final segment of the gastrointestinal tract. It is surrounded by two types of sphincters, the internal and external, and divided between its upper and lower parts by the pectinate line, which differences in both structure and neurovascular supply.
Except during defecation, the anal canal is collapsed by the internal and external anal sphincters in order to prevent the inadvertent passage of faecal material. This important muscle group is responsible for the maintenance of faecal continence, by ensuring that the anal canal remains closed when not specifically intended to be opened.
The anal canal is divided into two parts - the upper and the lower - which are separated by the pectinate line (or dentate line). This line runs in an irregular circle around the canal, and serves to distinguish the two parts, both in terms of structure and function. This is because they have different embryological origins - with the upper part being derived from the embryonic hindgut, and the lower from the ectoderm of the proctodeum.
The superior aspect of the anal canal is lined by the same columnar epithelium as that found in the rectum. However, in the anal canal, the mucosa is organised into longitudinal folds - known as anal columns. These columns are joined at their inferior ends by anal valves - small protrusions which are also sometimes referred to as anal sinuses, due to the presence of glands which secrete mucus within them.
Below the pectinate line, the anal canal is lined by non-keratinised stratified squamous epithelium (known as the anal pecten). It is a smooth and pale surface, which transitions at the level of the intersphincteric groove to true skin (keratinised stratified squamous).
The anal canal lies in close proximity to a variety of important structures in the pelvis and perineum - such as the coccyx and sacrum, the anococcygeal ligament, the ischioanal fossae, the perineal body, the urogenital diaphragm, the urethra, the bulb of the penis, and the vagina.
The anus has a particular arterial supply, venous drainage, nerve supply and lymphatic drainage, depending on the level at which it is located in relation to the pectinate line. These characteristics are as follows:
Haemorrhoids are vascular cushions which are found within the anal canal of healthy individuals, and which play a role in the maintenance of faecal continence. In people suffering from constipation, excessive straining during defecation, and/or raised intra-abdominal pressure (for example, due to pregnancy or ascites) these haemorrhoids may become swollen and distended, a phenomenon which is known as pathological haemorrhoids.
Pathological haemorrhoids are typically located at the 3, 7 and 11 o’clock positions, when the patient is in the lithotomy position. In addition to causing unpleasant sensations such as itching and bleeding, these haemorrhoids may also require medical attention depending upon the severity of the symptoms, and may be managed conservatively or surgically.
