Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Anatomy and Physiology of the Digestive System
Small Intestine

The intestine consists of two main parts: the Small Intestine and the Large Intestine (Fig. 7.12). The total length of the intestine is 6–8 m. The greater part of it (4–6 m) is occupied by the small intestine, or intestinum tenue (from the Greek enteren). It is formed by the duodenum, jejunum, and ileum.

Structure. The duodenum, duodenum, is the initial segment of the small intestine. It is relatively short (25–30 cm) and horseshoe-shaped. Its concave portion embraces the HEAD of the Pancreas. The duodenum is divided into superior, descending, horizontal, and ascending parts. The common Bile duct and the pancreatic duct open into the descending part.

The Significance of the duodenum for the Organism is exceptionally great. Within it, the chyme undergoes alkalinization and is exposed to bile, pancreatic juice, and intestinal juice. The duodenum continues into the jejunum.

The jejunum, jejunum, and ileum, ileum, form a continuous tube that folds repeatedly within the Abdominal cavity. There is no distinct anatomical boundary between them: approximately ²/₅ of its length is formed by the jejunum and ³/₅ by the ileum. The latter transitions into the large intestine (cecum) in the right iliac region.

The wall of the small intestine consists of mucous, muscular, and serous layers.

The mucous membrane is lined with a single-layered columnar epithelium. Its surface area is increased several-fold due to circular folds, villi, and microvilli. Circular folds are present throughout the entire length of the small intestine. They are covered with numerous villi (Fig. 7.13), which give the mucous membrane a velvety appearance. The villi are projections up to 1 mm in length, numbering 10–15 per 1 mm². The core of a villus is a connective-tissue stroma covered externally by epithelium. Situated within the stroma are Blood capillaries and a single central lymphatic capillary (central lacteal). Nutrients are absorbed through the intestinal epithelium into these vessels: Water, CARBOHYDRATES, and Amino Acids enter the blood capillaries, while fats enter the lymphatic capillary. Microvilli are cellular projections of the epithelium that significantly increase its surface area. Facing the intestinal lumen, microvilli are covered with the glycocalyx, a carbohydrate-protein (glycoprotein) complex located on the epithelial surface.

Class="center">

Fig. 7.12. Intestine, biliary tract, and pancreas:

1 — descending part of the duodenum; 2 — cystic duct; 3 — Gallbladder; 4 — right hepatic duct; 5 — left hepatic duct; 6 — common hepatic duct; 7 — common bile duct; 8 — superior part of the duodenum; 9 — Stomach; 10 — pancreas; 11 — horizontal part of the duodenum; 12 — duodenojejunal flexure; 13 — transverse colon; 14 — jejunum; 15 — descending colon; 16 — ileum; 17 — sigmoid colon; 18 — rectum; 19 — vermiform Appendix; 20 — cecum; 21 — ascending colon

On the mucous membrane of the descending part of the duodenum, alongside the circular folds, There is a single longitudinal fold that terminates in the major duodenal papilla (papilla of Vater). At its apex, the common bile duct (through which bile flows from the Liver) and the pancreatic excretory duct open. In most cases, these two ducts merge into a single channel.

The mucous membrane of the small intestine contains aggregations of lymphoepithelial tissue that perform an immune function in the body. These aggregations are represented by Solitary lymphoid nodules, which are located predominantly in the jejunum, and Aggregated lymphoid nodules (Peyer's patches), which are more commonly found in the ileum.

The muscular layer is formed by two layers (longitudinal and circular) of smooth Muscle Cells. They carry out several types of muscular contractions in the small intestine. Pendular movements are caused by the alternating contraction of the longitudinal muscle layer relative to the chyme, which facilitates the mixing of the chyme with digestive juices. Peristaltic contractions "squeeze" the chyme into the distal segments of the gastrointestinal tract. In the small intestine, contractions of the villi along their axis (shortening and lengthening) are also observed. This promotes the "agitation" of the chyme, accelerates nutrient absorption, and expels BLOOD AND Lymph containing absorbed substances from the villi into the submucosal vessels. The unabsorbed portion of food passes into the large intestine via peristaltic contractions of the small intestinal Muscles.

Fig. 7.13. Structure of a small intestinal villus:

1 — arteriole; 2 — capillary network; 3 — epithelium; 4 — venule; 5 — lacteal (lymphatic capillary); 6 — glands

The serous membrane covers the small intestine externally. The sole exception is the duodenum, where the serous coat is present only on the anterior wall, while its remaining walls are covered by adventitia. The jejunum and ileum are suspended by a mesentery attached to the posterior abdominal wall; hence, this segment of the small intestine is referred to as the mesenteric small intestine. Blood Vessels, Lymphatic vessels, and nerves run within the mesentery.

The Glands of the small intestinal mucosa produce intestinal juice, the volume of which reaches 2.5 L per day. Its pH ranges from 7.2 to 7.5, and rises to 8.5 during enhanced secretion. The juice is rich in digestive Enzymes (more than 20) that carry out The final stage of food molecule breakdown. The amylase, lactase, sucrase, and maltase contained within it break down carbohydrates. Lipase hydrolyzes fats emulsified by bile into glycerol and Fatty acids, while aminopeptidase breaks down Proteins by cleaving the terminal amino acid from peptide molecules. Enteropeptidase, present in the intestinal juice, facilitates The conversion of inactive trypsinogen from pancreatic juice into active Trypsin.

Both luminal and membrane (surface) Digestion can occur simultaneously in the small intestine. Luminal digestion results from the interaction of nutrients with enzymes that freely "float" in the lumen of the gastrointestinal tract, having arrived there as components of digestive juices. Membrane digestion takes place with the participation of enzymes anchored in the glycocalyx of the digestive tract epithelium. The concentration of enzymes is higher here, and their active sites face the intestinal lumen, allowing nutrients to contact them more frequently. Consequently, this type of digestion is more efficient. Membrane digestion was described in detail by the Russian scientist A. M. Ugolev.

The activation of intestinal juice secretion occurs reflexively upon contact of the chyme with the intestinal wall. The neural regulation of intestinal juice secretion is mediated by the sympathetic and parasympathetic systems. Parasympathetic nerve fibers deliver impulses to the small intestine that stimulate its secretion and peristalsis, whereas sympathetic fibers inhibit them. It should be noted that the muscular tissue in the wall of the small intestine possesses a certain degree of automatism, with the Autonomic Nervous system exerting merely a regulatory influence. Hormones such as adrenaline and noradrenaline suppress secretion and motility, whereas motilin and acetylcholine stimulate them.

The COMPOSITION OF THE juice depends on The chemical composition of the diet. For instance, a predominantly carbohydrate-rich diet is accompanied by an increased concentration of sugar-splitting enzymes, whereas fatty foods induce an increase in lipase activity.

The Significance of the small intestine for the organism is exceptionally great. It is the site where bile, pancreatic juice, and intestinal juice act upon the chyme. Here, the majority of nutrients are absorbed into the blood and lymph, while the undigested chyme passes into the large intestine.

Thus, the following processes take place in the small intestine:

1) mixing of the chyme;

2) emulsification of fats under the action of bile;

3) digestion of proteins, fats, and carbohydrates under METABOLISM/18.html">The Influence of enzymes contained in the intestinal and pancreatic juices;

4) absorption of water, nutrients, Vitamins, and mineral salts;

5) bactericidal Treatment of food by means of mucosal lymphoid structures;

6) evacuation of undigested residues into the large intestine.