1.What is the physiological function of the anus and rectum?
The anus and rectum are the final channel of the human digestive system, from the ingestion of food to the discharge of residues out of the body, an extremely complex process of digestion and absorption. This process is almost entirely completed in the upper gastrointestinal tract. The rectum has no digestive role and can only absorb small amounts of water, glucose and amino acids, etc., and store the residues that have already been absorbed. Therefore, the physiological function of the anorectum is mainly defecation. Defecation is a complex and comprehensive action, which includes not only the involuntary low-level reflexes and arbitrary high-level reflexes, but also requires the cooperation of the abdominal muscles, diaphragm, lungs, anal levator muscle and anal sphincter.
2.What is the clinical importance of rectal column, anal flap and anal gland?
The rectal column, also known as the anal column, is the main structure of the lower rectum. The mucosa of the lower rectum forms six to eight longitudinal striated folds due to sphincter contraction, which is clinically known as the rectal column. The rectal column contains the suprarectal venous plexus formed by the terminal branches of the superior rectal artery and the eponymous vein, which are varicose and expand to form internal hemorrhoids.
Anal flap: There are semilunar mucosal folds called anal flaps between the lower ends of the adjacent muscular columns, which are remnants of the original anorectal mucosa, the upper edge of which is free, the lower edge is continuous with the epithelium of the anal canal, and both sides are continuous with the surface of the adjacent anal columns, so that between the anal columns, after the anal flap, above the dentate line, 8 to 10 small fossae are formed, which are funnel-shaped and called anal saphenous fossae. In fact, the anal flap is the anterior wall of the anal fossa, making the fossa a pocket with the mouth facing up and the bottom facing down, with anal glands at the bottom of the fossa. The anal flap is easily damaged by the friction of feces, which can lead to anal fissure and saphenous infection, and further development of saphenous infection can lead to perianal abscess.
The anal glands are located at the bottom of the anal fossa, which is a cup-shaped depression on the mucosal surface between the rectal columns of the dentate line. The fossa is about 3-5 mm deep, and most of them have anal glands at the bottom of the fossa, which secrete mucous fluid to lubricate the stool. The distribution of the anal glands varies greatly among individuals, with some being completely submucosal, some penetrating into the sphincter, and some branches crossing the internal sphincter into the joint longitudinal layer. Because of the upward opening of the anal fossa, it is easily infected by fecal contamination and injury, causing infection of the anal glands and the formation of perianorectal abscesses. Therefore, 90% of anal fistulas have an internal opening in the anal gland.
3.What is the dentate line? What is the difference between the top and bottom of the dentate line?
The dentate line is the junction line between the rectum and the anal canal, and is named because the lower end of the rectum has structures such as the rectal column, anal flap, anal sinus, and anal papilla, and the edge of the lower end of the rectum becomes serrated. (Also known as the comb line) is an important anatomical landmark. There is a big difference between the upper and lower dentate line, so the clinical work is more important.
4.Why does the dentate line have special significance in the diagnosis and treatment of anorectal diseases?
The dentate line is not only an anatomical line of demarcation, but is also very important in clinical work, and its significance is summarized as follows.
The dentate line is the dividing line between the skin and the mucosa. The dentate line is the mucosa of the rectum, and most of the malignant tumors formed on the dentate line are adenocarcinoma and mucinous carcinoma. Below the dentate line is the metastatic epithelium, and the carcinoma formed is squamous carcinoma.
Since the blood near the dentate line passes partly through the portal vein system and partly through the inferior vena cava system, and there is a traffic branch between the two in the dentate line, when liver and spleen diseases cause obstruction of blood flow in the portal vein system, it can cause varicose veins near the dentate line in the lower rectum and form a collateral circulation, and because of the dilatation of the veins in the lower rectum, it can easily lead to venous rupture and produce serious symptoms of blood in the stool.
The dentate line is also used to distinguish the types of hemorrhoids, those that occur above the dentate line are called internal hemorrhoids, those below the dentate line are called external hemorrhoids, and those above and below the dentate line are fused as one and are mixed hemorrhoids.
The nerves in the rectum above the tooth line are vegetative nerves, and these nerves are not sensitive to pain, so diseases above the tooth line, such as internal hemorrhoids, proctitis, rectal polyps, and early rectal cancer, often do not feel significant pain. The anal canal below the dentate line is innervated by the spinal nerve, which is a sensory nerve and is quite sensitive to pain. For example, anal fissures, inflammatory external hemorrhoids and anal canal cancer are often characterized by pain.
The lymphatic fluid of the rectum above the dentate line flows upward into the pelvic lymph nodes of the visceral lymphatic system; if rectal cancer has lymphatic metastasis, enlargement of lymph nodes in the pelvis will occur first. The lymphatic fluid of the anal canal below the dentate line flows to the inguinal lymph nodes which belong to the body lymphatic system; when cancer of the anal canal has lymphatic metastasis, enlargement of inguinal lymph nodes will occur.
The dentate line is the place where the 2 primitive tissues, endoderm and ectoderm, fuse on embryonic development, so almost all congenital anorectal malformations occur near the dentate line.
5.What is the anorectal ring? What is the physiological function?
The anorectal ring is the general name of the sphincter muscle group at the connection between the anal canal and the rectum, which plays a key role in maintaining the self-control of the anal canal. This ring is composed of the puborectalis muscle, the deep external sphincter, the anal levator, the joint longitudinal muscle and the pubococcygeal muscle. During rectal palpation, a clear edge can be suddenly touched backward from the intersphincteric groove up to the upper end of the anal canal, which is the location of this ring. If the patient is asked to contract the anus, there is a clear sensation of finger strangulation. The rectal ring plays an important role in maintaining the self-control of the anal canal. If the ring is completely cut off during surgery, it is bound to cause anal incontinence; if the ring is preserved, even if the sphincter is cut off, the self-control of the anal canal will not be significantly affected, so care should be taken to protect the ring during surgery.
If the anal canal rectal muscle ring must be cut during surgery, it is best to follow the posterior midline of the anal canal. This is because the superficial and deep external anal sphincter and the puborectalis muscle have some muscle fibers attached to the anal caudal ligament, and some fibers of the puborectalis muscle also intersect with the pubococcygealis muscle. Therefore, this incision method allows the muscle fibers to be attached to the caudal ligament and does not lead to significant retraction, and the integrity of the postoperative rectal ring is better, which does not lead to severe anal closure or anal incontinence.
If the ring has to be cut in other parts, wire therapy can be used, i.e., wire instead of knife, and while the ring is slowly cut in about 15 days, some of the muscle fibers are re-adhered to the surrounding tissues after hanging off, so that when the ring is hung off, a considerable part of the muscle fibers are re-adhered to maintain the normal function of the anal canal.
At present, the anatomy and physiology of the anorectal ring are not completely consistent, and some issues need to be studied and discussed in depth.