The oral cavity is the beginning of the digestive tract. It is bounded in front by the lips, above by the hard and soft palate, below by the tongue and sublingual space, and on the sides by the cheeks. Through the pharynx (neck of the pharynx), the oral cavity communicates with the pharynx. The inner surface of the oral cavity, as well as other parts of the digestive tract, is covered with a mucous membrane, on the surface of which there are a large number of ducts of the salivary glands. The lower part of the soft palate and arches are formed mainly by muscles involved in the act of swallowing. The tongue is a mobile muscular organ located in the oral cavity and promotes the processes of chewing food, swallowing, and sucking. The body, tip, root and back are distinguished in the language. From above, from the sides and partially from below, the tongue is covered with a mucous membrane that fuses with its muscle fibers and contains glands and nerve endings that serve to sense taste and touch. On the back and body of the tongue, the mucous membrane is rough due to the large number of papillae of the tongue, which recognize the taste of food. Those located on the tip of the tongue are attuned to the perception of sweet taste, bitter taste at the root, and papillae in the middle and sides of the tongue recognize sour taste. A fold of the mucous membrane, called the frenulum, runs from the lower surface of the tongue to the gums of the lower front teeth. On both sides of it, at the bottom of the oral cavity, the ducts of the submandibular and sublingual salivary glands open. The excretory duct of the third, parotid salivary gland, opens in the vestibule of the mouth on the mucous membrane of the cheek, at the level of the upper second large molar. The pharynx is a muscular tube 12-15 centimeters long, connecting the oral cavity with the esophagus, located behind the larynx and consists of 3 parts: nasopharynx, oropharynx and laryngeal part, which is located from the upper border of the laryngeal cartilage (epiglottis), which closes the entrance to the respiratory tract during swallowing, to the entrance to the esophagus.

The esophagus connecting the pharynx to the stomach is located behind the trachea — the cervical region, behind the heart — the thoracic and behind the left lobe of the liver — the abdominal. The esophagus is a soft elastic tube about 25 centimeters long, with 3 constrictions: upper, middle (aortic) and lower, and ensures the movement of food from the mouth into the stomach. The esophagus begins at the level of the 6th cervical vertebra at the back (cricoid cartilage in front), passes through the esophageal opening of the diaphragm at the level of the 10th thoracic vertebra, and then passes into the stomach. The wall of the esophagus is able to stretch as a food lump passes through, and then contract, pushing it into the stomach. Good chewing saturates food with a large amount of saliva, it becomes more liquid, which facilitates and accelerates the passage of the food lump into the stomach, so food should be chewed as long as possible. Liquid food passes through the esophagus in 0.5-1.5 seconds, and solid food in 6-7 seconds. At the lower end of the esophagus there is a muscular constrictor (sphincter), which prevents the acidic contents of the stomach from being thrown back into the esophagus. The wall of the esophagus consists of 4 membranes: connective tissue, muscle, submucosa and mucosa. The mucous membrane of the esophagus consists of longitudinal folds of multilayer flat non-corneating epithelium, which provides protection from damage by solid food. The submucosa contains glands that secrete mucus, which improves the passage of the food lump. The muscular membrane consists of 2 layers: an inner (circular) and an outer (longitudinal) one, which makes it possible to ensure the movement of food through the esophagus. A feature of the movements of the muscles of the esophagus during swallowing is the suppression of the peristaltic wave of the previous sip by the next sip, if the previous sip did not pass into the stomach. Frequent repeated swallowing completely inhibits the motility of the esophagus and relaxes the lower esophageal sphincter. Only slow swallows and the release of the esophagus from the previous lump of food create conditions for normal peristalsis.

The stomach is designed for pretreatment of food lumps that have entered it, which consists in exposing it to chemicals (hydrochloric acid) and enzymes (pepsin, lipase), as well as mixing it. It has the appearance of a sac-like formation about 21-25 centimeters long and with a capacity of up to 3 liters, located under the diaphragm in the epigastric region of the abdomen (entrance to the stomach and the body of the stomach). In this case, the bottom of the stomach (upper section) is located under the left dome of the diaphragm, and the exit section (pyloric part) It opens into the duodenum in the right side of the abdominal cavity, partially passing under the liver. Directly in the pylorus, at the point where the stomach passes into the duodenum, there is a muscular constrictor (sphincter) that regulates the flow of food processed in the stomach into the duodenum, while preventing food from being thrown back into the stomach. In addition, the upper concave edge of the stomach is called the small curvature of the stomach (directed towards the lower surface of the liver), and the lower convex edge is called the large curvature of the stomach (directed towards the spleen). The absence of rigid fixation of the stomach along its entire length (attached only at the entrance of the esophagus and exit into the duodenum) makes its central part very mobile. This leads to the fact that the shape and size of the stomach can vary significantly depending on the amount of food contained in it, the tone of the stomach and abdominal muscles, and other factors. The walls of the stomach are in contact with the abdominal organs from all sides. Behind and to the left of the stomach is the spleen, behind it is the pancreas and the left kidney with the adrenal gland. The anterior wall is adjacent to the liver, diaphragm, and anterior abdominal wall. Therefore, the pain of some stomach diseases, in particular peptic ulcer disease, can be in different places depending on the location of the ulcer. The stomach wall has 4 main membranes — internal (mucous), submucosal, muscular (middle) and external (serous). The thickness of the gastric mucosa is 1.5-2 millimeters. The shell itself is covered with a single-layer prismatic epithelium containing gastric glands consisting of various cells, and forms a large number of gastric folds directed in different directions, located mainly on the back wall of the stomach. The mucous membrane is divided into gastric fields with a diameter of 1 to 6 millimeters, on which are located gastric dimples with a diameter of 0.2 millimeters, surrounded by villous folds. The excretory openings of the ducts of the gastric glands open into these dimples, producing hydrochloric acid and digestive enzymes, as well as mucus that protects the stomach from their aggressive effects. The submucosa, located between the mucous membrane and the muscular membranes, is rich in loose fibrous connective tissue, in which vascular and nerve plexuses are located. The muscular lining of the stomach consists of 3 layers. The outer longitudinal layer is a continuation of the eponymous layer of the esophagus. In small curvature, it reaches the greatest thickness, and in large curvature and the bottom of the stomach it becomes thinner, but occupies a large surface. The middle circular layer is also a continuation of the eponymous layer of the esophagus and completely covers the stomach. The third (deep) layer consists of oblique fibers, the bundles of which form separate groups. Contractions of 3 multidirectional muscle layers ensure high-quality mixing of food in the stomach and the movement of food from the stomach to the duodenum. The outer shell ensures the fixation of the stomach in the abdominal cavity and protects other shells from the penetration of microbes and from overgrowth.

The duodenum is the beginning of the small intestine, but it is so closely connected to the stomach that it even has a joint disease — peptic ulcer. This part of the intestine got its curious name after someone noticed that its length is on average the width of twelve fingers, that is, about 27-30 centimeters. The duodenum begins immediately behind the stomach, encompassing the horseshoe head of the pancreas. In this intestine, the upper (bulb), descending, horizontal and ascending parts are distinguished. In the descending part, at the top of the large (Vater’s) papilla of the duodenum, there is the mouth of the common bile duct and pancreatic duct. Inflammatory processes in the duodenum, and especially ulcers, can cause disorders in the gallbladder and pancreas, up to their inflammation. The wall of the duodenum consists of 3 membranes — serous (outer), muscular (middle), and mucous (inner) with a submucosal layer. With the help of the serous membrane, it is attached almost immobile to the posterior wall of the abdominal cavity. The muscular membrane of the duodenum consists of 2 layers of smooth muscles: external — longitudinal and internal — circular. The mucous membrane has a special structure that makes its cells resistant to both the aggressive environment of the stomach and concentrated bile and pancreatic enzymes. The mucous membrane forms circular folds, densely covered with finger—like outgrowths – intestinal villi. In the upper part of the intestine, there are complex duodenal glands in the submucosal layer. In the lower part, deep in the mucous membrane, there are tubular intestinal glands. Source: Vladislav Liflyandsky “Healthy stomach: Digestive system”.