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The rectum initiates the terminal section of the large intestine at the level of the S3 vertebra, just after the sigmoid colon. Approximately 15 cm long, it is characterized by the cessation of the omental appendices and the absence of teniae coli and haustra. It is distally continuous with the anal canal as it protrudes through the pelvic diaphragm/levator ani. The rectum plays a significant role in fecal continence and the storage of feces via the ampulla. The anal canal is the final portion of the large intestine and is approximately 3 cm long. The anal canal also contributes to the maintenance of fecal continence and has great clinical significance due to the different embryological origins of the superior and inferior sections of the canal. Disruption in the development of the hindgut either environmentally or genetically can cause many disorders of the rectum and anal canal.[1][2]

The pectinate or dentate line is the junction between the superior and inferior anal canal. There are many differences between these two regions, including their embryological origins, innervation, venous and arterial supply, and lymphatic supply. Above the pectinate line, the anal canal has an endodermal origin and is lined by simple columnar epithelia. Blood supply is from the superior rectal artery, which originates from the inferior mesenteric artery and returns via the superior rectal veins into the inferior mesenteric vein.[17] Due to the venous anastomoses that occurs in the anal canal and the backup of blood flow into the rectal veins, hemorrhoids may be present in patients with portal hypertension. The lymphatic drainage of the superior anal canal is via internal iliac lymph nodes. The superior anal canal receives innervation from the inferior hypogastric plexus, which is a visceral innervation. It has both sympathetic and parasympathetic functions that control the tonicity of the internal anal sphincter, thereby contributing to the rectal ampulla reflex. The rectal ampulla senses the distension created by the buildup of feces and causes an inhibitory reflex of the internal anal sphincter, facilitating fecal continence. Below the pectinate line, the anal canal has an ectodermal origin and is predominantly lined by stratified squamous epithelium. The inferior rectal canal obtains its blood supply from the inferior rectal artery, which originates from the internal iliac artery. Blood returns via the inferior rectal vein, which ultimately drains into the inferior vena cava.[17] The lymphatic drainage of the inferior anal canal is the superficial inguinal lymph nodes. The inferior anal canal receives somatic innervation via the branches of the pudendal nerve, specifically the inferior anal/rectal nerve. Its efferent somatic innervation controls the voluntary actions of the external anal sphincter. Due to its somatic innervation, it can sense pain, temperature, and touch. This difference in innervation is implicated in the clinical presentation of hemorrhoids and anal fissures. Patients with lesions below the pectinate line usually complain of significant pain with any contact near the lesions, and pain is often unbearable, whereas lesions above the pectinate line often go unnoticed because of its visceral innervation and the lack of pain sensation.[2]