The Rectum - structure of anatomy and physiology
The

Rectum

The rectum is the most terminal section of the digestive tube. It is a continuation of the large intestine, but it differs substantially from its anatomical and physiological features.

The length of the entire rectum is 13-15 cm, of which on the crotch and anal canal (the final part of the intestine that opens on the skin with the anus opening) is up to 3 cm, on the subperitoneal department - 7-8 cm, and on the intraperitoneal part - 3-4 cm.

The rectum consists of a mucous membrane, a submucosal layer and a muscular membrane. Outside, it is covered with a fairly powerful fascia, which is separated from the muscle shell by a thin layer of fat. This fascia surrounds not only the rectum, but in men as well, the prostate gland with seminal vesicles , and in women the cervix.

The mucosa of the rectum is covered with a cylindrical epithelium with a large number of intestinal cells. It contains in addition a lot of so-called Lieberkunov glands, consisting almost entirely of mucous cells. That is why in pathological processes from the rectum a copious amount of mucus is allocated.

The

At 2 cm above the anal opening, the mucosa forms a series of vertically arranged parallel elevations. These are the so-called columns of Morgagni. The number of them varies (from 6 to 14), they have the form of longitudinal ridges, rising above the level of the mucosa by 2-4 mm. The bars of Morganyi are formed by a fold of the mucous membrane. Between each two columns is a groove in the form of a groove, which ends with a blind pocket (crypt). Pockets play a big role in proctological practice. They often retain foreign bodies or feces that can cause inflammation and lead to the development of paraproctitis .

Disruption of the intestinal function of diarrhea , constipation ), various inflammatory processes ( proctitis , colitis ), contributing to prolonged irritation of the mucous membrane, lead to the appearance of crypt papillae in the bases, which are sometimes significantly increased. Hypertrophic papillae mistaken for polyps , while they represent only a simple elevation of normal mucosa.

Blood supply to the rectum is carried out by the upper, middle and lower hemorrhoidal arteries. Of these, the first artery is unpaired, and the remaining two paired, approaching the gut from the sides. Veins of the rectum go along with the arteries. The outflow of venous blood is carried out in two directions - through the portal system and through the system of the hollow vein. In the wall of the lower part of the intestine there are dense venous plexuses - submucosal and associated subfascial and subcutaneous, located in the area of ​​the sphincter and anal canal.

Before proceeding to the physiology of the rectum, let us dwell briefly on the mechanism of fecal matter formation. It is known that a person in a day from the small intestine to the thick passes on an average about 4 liters of food mush (chyme). In the large intestine (in the right part - in the blind and ascending colon), due to tonic contractions, peristaltic and anti-peristaltic movements, thickening, mixing of intestinal contents and the formation of stools occur. Of 4 l chyme in the large intestine, only 140-200 g of formed feces remain, which usually consists of remnants of digestible food (fibers of fiber, muscle and tendon fibers, grain covered with fiber, etc.), gut products (mucus sloughing cells of the mucous membrane, cholesterol , cholic acid, etc.), as well as from living and dead bacteria.

The left half of the colon performs an evacuation function, which is facilitated by the so-called large and small movements. Small movements - continuously occurring small contractions, mixing the contents of the intestine, large - intensive rapid contractions of entire departments, helping to promote intestinal contents. They occur 3-4 times a day.

The food from the stomach is evacuated an average of 2 to 2.5 hours. After 6 hours, the liquid intestinal contents, after passing 5-6 m of the small intestine, moves to the large intestine, through which it passes 12-18 hours. As already said, a day From the small intestine to the thick passes about 4 liters of semiliquid chyme. Over 3.7 liters of fluid during this time is absorbed just in the colon. Together with the liquid, toxic substances enter the bloodstream - food decay products and intestinal fermentation.

Venous blood, saturated with these products, flows through the portal vein system into the liver , where they are delayed, neutralized and ejected with bile . Thus, the colon also has a suction function.

Emptying the bowels - an act of defecation - occurs as a result of the complex interaction of a number of physiological mechanisms. By peristaltic movements, the stool masses gradually move into the sigmoid colon . Accumulation and retention of fecal matter occurs mainly due to contractions of the circular muscular gut layer.

When the stool masses are lowered into the ampulla of the rectum, new mechanisms are set in motion-reflex tonic contractions of the striated musculature of the external sphincter of the anus. The act of defecation consists of the following stages: filling the ampoule with calves, evacuating the peristalsis of the rectum and sigma during reflex relaxation of the sphincters, simultaneous activation of the auxiliary muscle group (abdominal press and others). The rectum remains empty for a long time after defecation.

It should be noted that the different in intensity of the action of the auxiliary muscle group are aimed at accelerating and strengthening the evacuation of the stool, especially in cases of its solid consistency or any pathological conditions (constipation, atony, spasms ).

The posterior passage and rectum have a rich receptive field, here, with stimulation, impulses are produced that are transmitted to the stomach and affect its functioning, salivation , and also bile secretion.

Emptying of the intestine is due to the influence not only of the unconditioned (stretching of the ampoule), but also to the action of conditioned stimuli, which create a habitual rhythm of defecation at a certain time of the day. The act of defecation is affected by the cerebral cortex, which is confirmed by the following fact: sudden mental or physical irritation can completely remove the already habitual chair and permanently delay the emptying of the intestine.

As you can see, the basic physiological function of the rectum - the act of defecation - is a complex process in which many mechanisms participate. Any violation of them leads to a disorder of this function.

The

The rectum is the terminal part of the intestine.

Anatomy
The rectum begins at level II-III of the sacral vertebrae and descends in front of the sacrum, having an S-shape with an extension in the middle part (color Figure 1). The upper bend of the rectum - the sacralis (flexura sacralis) - corresponds to the concavity of the sacrum, the lower - the perineal (flexura perinealis) - is turned back. Correspondingly, bends on the inner surface of the intestine produce transverse folds (plicae transversales recti) - more often two on the left, one on the right.

In the middle part, the rectum expands to form an ampulla (ampulla recti). The end section of the rectum - the anal canal (canalis analis) - is directed back and down and ends with an anus (anus). The length of the intestine is 13-16 cm, of which 10-13 cm fall on the pelvic region, and 2.5-3 cm on the perineal. Circumference of the ampullar part of the gut is 8-16 cm (with overflow or atony - 30-40 cm).

Clinicians distinguish 5 departments of the rectum: namdular (or recto-sigmoid), upper ampullar, medium-popular, low-ampullar and perineal.

The walls of the rectum consist of 3 layers: mucous, submucous and muscular. The upper part of the rectum is covered with a serous membrane in the front and sides, which surrounds it in the uppermost part of the intestine and from behind, passing into a short mesentery (mesorectum). The mucous membrane has a large number of longitudinally straightened folds.

rectum anatomy
Vessels and nerves of the rectum.
Fig. 1. Blood vessels and lymphatic vessels of the rectum (frontal crucifixion of the male pelvis, the peritoneum partially removed, the mucous membrane of the rectum in the lower part of it is removed).
Fig. 2. Blood vessels and nerves of the rectum (sagittal masculine pelvis).
1 - nodi lymphatici mesenterici inf .; 2 - a. et v. rectales sup .; 3 - colon sigraoldeum; 4 - plexus venosus rectalis; 5 - a. et v. rectales raedil sin .; 6 - plica transversa; 7 - nodus lymphaticus iliacus int .; 8 - ra. levator ani; 9 - tunica muscularis (stratum circulare); 10 - muscle tufts in the region of columnae anales; 11 - m. sphincter ani ext .; 12 - m. sphincter ani int .; 13 - anus; 14 - a. et v. rectales inf .; 15 - zona haemorrhoidalis (venous plexus); 16 - a. et v. rectales medii dext .; 17 - tunica mucosa recti; 18 - rectum; 19 - a. iliaca int .; 20 - v. iliaca int .; 21 - nodus lymphaticus sacralis; 22 - a. sacralis med .; 23 - plexus rectalis sup .; 24-plexus sacralis; 25 - plexus rectalis med .; 26 - columnae anales; 27 - prostata; 28 - vesica urinaria; 29 - plexus hypogastricus int .; 30 - mesorectum.
colon

In the anal canal, there are 8-10 permanent longitudinal folds (columnae anales) with depressions between them - sinus anales, which terminate in semilunar folds - valvulae anales. Slightly protruding zigzag line from the anal valves is called anorectal, dentate, or crestal, and is the boundary between the glandular epithelium of the ampulla and the flat epithelium of the anal canal of the rectum. The annular space between the anal sinuses and the anus is called the hemorrhoidal zone (zona hemorrhoidalis).

The submucosa consists of a loose connective tissue, which facilitates easy displacement and stretching of the mucosa. The muscular wall has two layers: inner - circular and outer - longitudinal. The first thickens in the upper part of the crotch to 5-6 mm, forming an internal sphincter (m. Sphincter ani int.). In the perineal region of the intestine, the longitudinal muscle fibers are intertwined with the fibers of the muscle lifting the anus (m. Levator ani), and partly with the outer pulp. The outer moss (m. Sphincter ani ext.), Unlike the internal one, consists of an arbitrary musculature, encompassing the crotch region and closing the rectum. It has a height of about 2 cm and a thickness of up to 8 mm.

The diaphragm of the pelvis is formed by the muscles lifting the anus, and the coccygeal muscle (m. Coccygeus), as well as the fascia covering them. The paired muscles that lift the anus are mainly composed of the ilio-coccygeal (M. iliococcygeus), the pubic coccygeal (M. pubococcygeus) and the pubic-rectum (m. Puborectalis) muscles and form a kind of funnel lowered into the small pelvis. The edges of it are attached to the upper sections of the inner walls of the small pelvis, and below the center of the funnel is inserted a rectum connected with the fibers of the muscle lifting the anus. The latter divides the cavity of the small pelvis into two sections: upper-inner (pelvic-rectal) and lower-external (sciatic-rectal). The upper-inner surface of the muscle lifting the anus is covered with the fascia diaphragmatis pelvis sup., Which connects to the rectum's own fascia.

The peritoneal cover extends only to the upper-redia section of the rectum, descending in front to the Douglas space and rising from the sides to the level of the III sacral vertebra, where both serous sheets connect to the initial part of the mesentery.

To the edges of this elongated downward oval of the peritoneal cover is attached its own fascia of the rectum, denser at the rear and comparatively less pronounced laterally, and in front of it it transforms into a tight prostate peritoneum aponeurosis (in men) or rectal-vaginal aponeurosis (in women). This aponeurosis is easily divided into two plates, one of which clothes the prostate gland with seminal vesicles, and the other - the front wall of the rectum; this facilitates the separation of these organs during surgery. Extrafascial removal of the rectum together with the lymphatic vessels, without disturbing their integrity, is considered the most important condition for a radical operation.

The blood supply to the rectum (color table, Figures 1 and 2) is through the unpaired upper rectal (a rectalis sup.) And through two paired - middle and lower - rectal arteries (rectal med. Et inf.). The upper rectal artery is the terminal and largest branch of the inferior mesenteric artery. A good vascular network of the sigmoid colon allows maintaining its full blood supply provided that the marginal vessel remains intact even after a high intersection of the upper rectal and one or three lower sigmoid arteries. The safety of crossing the artery above the "critical point of the Zudeck" can be ensured only if the integrity of the marginal vessel is preserved. Blood supply to the entire rectum to the anal part is mainly due to the upper rectal artery, which is divided into two, and sometimes more branches at the level of III-IV sacral vertebrae.

The middle rectal arteries emanating from the branches of the internal iliac artery are not always equally developed and are often completely absent. However, in a number of cases they play an important role in the blood supply of the rectum.

The lower rectal arteries emanating from the internal pudend arteries feed mainly the external sphincter and the skin of the anal region. There are good anastomoses between the branches of the systems of the upper, middle and lower rectal arteries, and the intersection of the upper rectal artery at different levels while maintaining the integrity of the middle and lower rectal arteries and their numerous anonymous branches in the anterior and lateral divisions of the rectum does not deprive the lower intestinal tract.

The venous plexus of the rectum (plexus venosi rectales) is located in different layers of the intestinal wall; distinguish submucosal, subfascial and subcutaneous plexus. The submucosal, or internal, plexus is located in the form of a ring of enlarged venous trunks and cavities in the submucosa. It is associated with the subfascial and subcutaneous plexuses. Venous blood flows into the portal vein system through the superior rectal vein (v. Rectalis sup.) And into the inferior vena cava via the middle and lower rectal veins (vv. Rectales med. Et inf.). There are many anastomoses between these systems. The absence of valves in the upper rectal vein, as in the entire portal system, plays an important role in the development of venous stasis and the expansion of the veins of the distal segment of the rectum.

Lymphatic system . Lymphatic vessels of the rectum are important, because they can spread tumors and infection.

In the rectal mucosa is a single-layered network of lymphatic capillaries connected to a similar network of the submucosal layer, which also forms the intertwining of lymphatic vessels of I, II and III orders. In the muscular shell of the rectum, a network of lymphatic capillaries is formed, composed of capillaries of circular and longitudinal layers of the rectum. In the serous membrane of the rectum, there are superficial (shallow) and deep (broadly pallid) networks of lymphatic capillaries and lymphatic vessels.

Absorbent lymph vessels mainly follow the course of blood vessels. There are three groups of extra-humeral lymphatic vessels: upper, middle and lower. The upper lymphatic vessels, collecting lymph from the walls of the rectum, are guided along the branches of the upper rectal artery and flow into the so-called Herrot's lymph nodes. The middle rectal lymph vessels go from the side walls of the intestine under the fascia, covering the muscle that raises the anus, towards the lymph nodes located on the walls of the pelvis. Lower rectal lymphatic vessels originate in the skin of the region of the anus and are associated with lymphatic vessels of the mucosa of the anal canal and ampulla. They go in the thickness of subcutaneous adipose tissue to the inguinal lymph nodes.

Outflow of lymph, and consequently, the transfer of tumor cells can go in many directions (see below).

The innervation of the recto-sigmoid and ampullar parts of the rectum is carried out mainly by the sympathetic and parasympathetic systems, the perineal - primarily by the branches of the spinal nerves (color Figure 2). This explains the relatively low sensitivity of the ampulla of the rectum to pain and the high pain sensitivity of the anal canal. The internal sphincter is innervated by sympathetic fibers, the outer one by the branches of the pudendi nerves (nn. Pudendi) accompanying the lower rectal arteries. The muscle lifting the anus is innervated by the branches, which mainly come from the III and IV sacral nerves, and sometimes from the rectum. This is important for resection of the lower sacral vertebrae for access to the rectum, since it indicates the need for the sacrum to cross below the third sacral orifices to avoid serious impairment of the functions of not only the anus muscle and the external sphincter, but also other pelvic organs.