The digestive tract of humans starts from the mouth to the anus, where the rectum is a chamber that begins just at the end of the descending colon. The rectum is a hollow chamber where stool is stored for a certain amount of time and then the defecation reflex is elicited by pelvic nerves which causes defecation. The name “rectum” is derived from the Latin word ‘rectus’ which means straight. The term, however, was coined first to describe the distal (end) portion of the colon during its studies on animals.
In this article, we review the anatomy of the rectum and discuss its role and most common diseases.
The rectum is part of the digestive tract. Proximally, it is continuous with the sigmoid colon and distally with the anal canal. The rectosigmoid junction lies anterior to the third sacral vertebrae (S3). This is the point where teniae coli of the sigmoid colon (the end part of the colon or large intestine) form a continuous longitudinal layer of smooth muscles.
The anorectal flexure of the anal canal is generally maintained in a resting state by the tone of puborectalis muscles and is very important for fecal continence.
Anteriorly, there are three sharp flexures called the lateral flexures of the rectum. The dilated terminal part of the rectum is called the ampulla of the rectum. This is the place where fecal mass is stored until it is expelled during the process of defecation.
The peritoneum is present on the anterior and lateral surfaces of the superior one-third of the rectum only, on the anterior third of the middle third only. In males, the reflection of the peritoneum over the rectum forms floor of the rectovesical pouch and in females it reflects over the rectum to form a recto-uterine pouch.
The rectum lies anterior to three inferior sacral vertebrae and is related anteriorly fundus of the urinary bladder, ureters, ductus deferens, seminal glands and prostate in males. In females, it is related anteriorly to the vagina.
The role of the rectum is to hold the stool for a certain amount of time until a message from the brain gives the green light for its release (evacuation). When this happens, the sphincters muscle relax, giving access to the opening (anus) through which the stool can be expelled. If the release is not possible at that moment, the sphincters muscle contracts and the rectum retain the faeces until further notice.
Arterial Supply and Venous Drainage
The proximal part of the rectum is supplied by the superior rectal artery. The Middle and inferior parts of the rectum are supplied by the middle rectal arteries. The anorectal junction and anal canal are supplied by inferior rectal arteries. Anastomoses between superior and inferior rectal arteries provide potential collateral circulation.
Blood from the rectum drains to superior, middle and inferior rectal veins.
Portocaval anastomosis is formed because the superior rectal vein drains into the portal venous system and the middle and inferior rectal veins drain into the systemic system. There is a submucosal rectal venous plexus which consists of two parts: internal rectal venous plexus and external rectal venous plexus.
Nerve Supply of the Rectum
The nerve supply comes from the sympathetic and parasympathetic systems. The sympathetic supply is from the lumbar spinal cord. The parasympathetic supply is from the S2-S4 spinal cord which passes via pelvic splanchnic nerves and inferior hypogastric plexus. Visceral afferents follow parasympathetic fibres to the S2-S4 spinal sensory ganglia.
Clinical Relevance and Associated Diseases
Colorectal carcinoma is the third most common cancer in the United States and overall, is the second major cause of cancer-related deaths. Cancer is the abnormal growth of otherwise regularly working cells of the body with their own vascular supply and ability to resist cell death. Colorectal carcinoma can occur in the colon or in the rectum or in both. They are however often termed together because of very common features.
Colon is usually divided into 4
- The ascending colon
- Transverse colon
- The descending colon
- The last part of colon is called the sigmoid colon right after which the rectum starts.
Most colorectal carcinomas start with the growth of polyps. Not all polyps are cancerous but some of them can change into cancer over a span of many years. Removal of polyps by colonoscopy reduces the chances of turning into cancer.
The different types of polyps can be classified as follow.
Adenomatous Polyps (Adenomas)
2/3rd of all the polyps are adenomatous which makes it the most common type of colon polyp. Screening for those with risk factors is necessary and when these polyps are found they are tested as a potential pre-cancerous lesion. The process of turning these into cancer usually takes years, and although only a small percentage turns malignant, a screening reduces its risk significantly. The adenomatous polyps are of 3 types tubular, villous and tubulovillous.
These polyps are also common but they usually do not turn into malignant carcinoma. They are usually small in size. If during screening hyperplastic polyps are found, they are removed and then tested to confirm for cancerous potential.
These have the potential to turn into cancer depending on their size and location in the colon. Large serrated polyps which are typically flat, located mostly in the upper portion of the colon are usually pre-cancerous.
As the name suggests they are associated with inflammatory diseases especially those with inflammatory bowel disease. They are called pseudopolyps due to the fact that they are not actual polyps rather develop due to ongoing chronic inflammation in the colon. They are usually benign and do not carry the risk of malignancy.
Following features of the polyps suggest likely chances of being malignant or increased risk of changing into malignancy in future.
- If size of the polyp is > 1cm
- If 3 or > 3 polyps are found
- If removed polyp shows dysplastic changes.
American cancer society (ACS) recommends that all those who are asymptomatic should start screening for adenomatous polyp and colorectal cancer starting at the age of 50. These individuals with the risk factors should undergo colonoscopy at an earlier age and more frequently than those without any associated risk factors.
When an abnormal cell mass forms, it can grow in the wall of the colon or rectum or both together over time. There are layers of smooth muscles in the walls of the rectum and colon. Initially, colorectal cancer starts in the innermost layer which is called mucosa and then can grow outward breaching the layers one by one and in the last stages reaches up to the abdominal cavity. Once cancer is in the smooth muscles walls it can invade the blood vasculature or lymphatic vessels. This is how they travel and metastasize (migration of tumour cells from the actual site to other sites of the body either through blood or lymph vessels).
Clinically Oriented Anatomy by Keith L. Moore, Arthur F. Dalley II