Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Anatomy and Physiology of the Digestive System
Large Intestine

Structure. The Large Intestine, intestinum crassum (Greek colon), represents the terminal section of the digestive tract. It consists of the following parts: the cecum with the vermiform Appendix, the colon, and the rectum.

The length of the large intestine is 1.5–2.0 m. Its lumen is considerably wider than that of the Small Intestine. Furthermore, it differs from the latter by the presence of taeniae, haustra, and omental Appendices. Taeniae are localized thickenings of the longitudinal layer of Cytology/cytology/32.html">Smooth Muscle tissue. The large intestine has three taeniae running almost parallel to each other. Haustra are alternating dilations ("sacculations") of the lumen of the large intestine, which make it appear gathered into voluminous folds. Omental appendices are localized accumulations of yellow adipose tissue located beneath the Peritoneum covering the intestinal wall.

The cecum, caecum (Greek typhlon), is located in the right iliac fossa. It has the shape of a hemispherical pouch and is 6–12 cm long. The vermiform appendix (appendix), appendix vermiformis, extends from it. The length of the appendix averages 9 cm. Its shape and position vary, but most frequently it is located posterior and inferior to the end of the ileum. The lumen of the appendix is typically filled with mucus. The wall of the appendix contains A large number of lymphoid follicles that play a vital role in The Development of Immunity.

The ileum opens into the cecum. At this junction, There is a specialized valve consisting of two mucosal folds. This valve (the ileocecal valve, or valve of Bauhin) regulates the passage of the contents from the ileum into the large intestine and prevents their backflow.

The colon, colon, is the longest section of the large intestine. It consists of four parts: the ascending, transverse, descending, and sigmoid colon. The ascending colon, colon ascendens, extends vertically from the cecum and abuts the right lateral abdominal wall. The transverse colon, colon transversum, runs horizontally in the upper Abdominal cavity. It is completely covered by peritoneum and has its own mesentery, which makes it quite mobile. The descending colon, colon descendens, is located in the left half of the abdominal cavity, descending vertically. The ascending and descending colons are covered by peritoneum on only three sides, are fixed to the lateral abdominal wall, and are practically immobile. The sigmoid colon, colon sigmoideum (Greek romanum), is S-shaped and continues into the rectum. Just like the transverse colon, it has a mesentery and can alter its shape and position.

The rectum, rectum (Greek proctos), 15–20 cm in length, is located in the lesser pelvis cavity. In shape, it is not straight, as one might assume from its name, but forms two curves (Fig. 7.15) located in the sagittal plane: the sacral and perineal flexures. The lower part of the rectum is firmly fixed within the pelvic Diaphragm. Taeniae and haustra are not characteristic of this section of the large intestine. The rectum consists of the supra-ampullary part, the ampulla (dilated part), and the anal canal (narrowed part). It terminates at the anus. In the region of the anal canal, approximately 1.5 cm above the anal orifice, the mucosa forms 5–8 longitudinal folds known as anal columns. Inferiorly, they connect with one another to form anal sinuses. The latter are swallow-nest-shaped and play an important role in the continence of gas and feces. Well-defined transverse semilunar folds are located in the supra-ampullary part and the ampulla of the rectum.

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Fig. 7.15. The rectum:

1 — sigmoid colon; 2 — supra-ampullary part of the rectum; 3 — rectal ampulla; 4 — external sphincter; 5 — internal sphincter; 6 — anal sinuses; 7 — anal canal; 8 — perineal flexure; 9 — transverse fold; 10 — sacral flexure

The wall of the large intestine consists of mucous, muscular, and serous membranes. The mucous membrane of the large intestine features semilunar folds. It lacks villi (which are specific to the small intestine); however, it is characterized by the presence of deep and wide crypts (pits). Glands are located within the mucosal layer. Their secretion contains practically no Enzymes, but it does contain substances necessary for The formation of fecal masses. Over most of the large intestine, the epithelium is single-layered columnar. In the rectum (in the region of the anus), it first transitions to stratified non-keratinized squamous epithelium, and then to stratified keratinized squamous epithelium. Beneath the mucosa of the anal canal lies the hemorrhoidal venous plexus, the dilation of which leads to a condition known as hemorrhoids.

The smooth musculature in the large intestine is represented by two layers: circular and longitudinal. Sphincters are located at the exit of the rectum. The first is formed by smooth muscle, and its contractions occur involuntarily. The second sphincter, the external one, consists of striated muscle and is a muscle of the Perineum; its contractions are voluntary. The sphincters retain gas and fecal masses within the intestinal lumen.

The outer tunic of the large intestine is represented by the peritoneum and adventitia: the cecum, transverse colon, sigmoid colon, and the upper third of the rectum are covered by the peritoneum on all sides; the ascending and descending colons and the middle third of the rectum are covered by the peritoneum on three sides, and by adventitia on one side; the anal part of the rectum is covered by adventitia.

Functions. The large intestine performs A number of important functions. It is the primary habitat for intestinal Bacteria (in adults, Bifidus, Bacteroides, and Lactobacillus rods predominate). Bacteria synthesize certain Vitamins (K, B) and protect the host from pathogenic microorganisms by competing with them. They are capable of digesting substances not broken down by digestive juice enzymes, notably Cellulose, which they hydrolyze by approximately 50%. The remaining portion of cellulose participates in the formation of fecal masses. Bacteria also produce substances toxic to the Organism, such as hydrogen sulfide, indole, and skatole, which are detoxified in the Liver.

The final absorption of Water and mineral salts takes place in the large intestine. It is also the site where fecal masses, colored by Bile pigments, are formed. The rectum ensures their evacuation. Unabsorbed food particles, bacteria, exfoliated epithelium of the gastrointestinal tract, water (up to 150 ml), and the like are eliminated with the feces.

When the rectum becomes distended, the urge to defecate arises. Contraction of the pelvic diaphragm Muscles facilitates the evacuation of rectal contents. The abdominal wall muscles increase intra-abdominal pressure, which also AIDS in the expulsion of excrement. The rectal sphincters relax, and fecal masses are eliminated from the body. Defecation proceeds as a voluntary act. With significant distension of the rectum, it acquires an involuntary character. The reflex involuntary center of defecation is located in the sacral segments of the Spinal Cord.

The Regulation of the motor activity of the large intestine is carried out by neural and humoral mechanisms. The parasympathetic Nervous system exerts an activating effect on motility, whereas the sympathetic system has an inhibitory effect. Serotonin and adrenaline suppress contractions of the muscular coat of the large intestine, while acetylcholine enhances them.