Topic 9. AGE DIGESTION

9.1. The structure of the alimentary canal

The alimentary canal consists of a system of organs that produce mechanical and chemical processing of food and its absorption. In humans, the alimentary canal looks like a tube 8-10 m long. The wall of the alimentary tube consists of three layers: the inner (mucous membrane), the middle (muscular membrane) and the outer (connective tissue, or serous, membrane). The smooth muscle tissue of the middle shell has two layers: inner - circular and outer - longitudinal. The following sections are distinguished in the alimentary canal:

a) oral cavity;

b) pharynx;

c) esophagus;

d) stomach;

e) small intestine; it includes three departments passing into each other: duodenum, jejunum and ileum;

f) large intestine - formed by the caecum, parts of the colon (ascending, transverse, descending and sigmoid colons) and the rectum.

The digestive juices produced by the glands enter the cavity of the alimentary canal. Part of the glands is located in the alimentary canal itself; large glands are located outside it, and the digestive juices produced by them enter its cavity through the excretory ducts.

Digestion of food begins in the oral cavity, where mechanical fragmentation and grinding of food occurs when it is chewed. The tongue and teeth are placed in the oral cavity. The tongue is a mobile muscular organ, covered with a mucous membrane, richly supplied with vessels and nerves.

The tongue moves food in the process of chewing, serves as an organ of taste and speech.

Teeth grind food; in addition, they take part in the formation of speech sounds. By function and shape, incisors, canines, small and large molars are distinguished. An adult has 32 teeth: 2 incisors, 1 canine, 2 small molars and 3 large molars develop in each half of the upper and lower jaws.

Teeth are laid in the uterine period and develop in the thickness of the jaw. In a child at 6-8 months of life, milk, or temporary, teeth begin to erupt. Teeth may appear earlier or later, depending on individual developmental characteristics. Most often, the middle incisors of the lower jaw erupt first, then the upper middle and upper lateral ones appear; at the end of the first year, usually 8 milk teeth erupt. During the second year of life, and sometimes at the beginning of the third, the eruption of all 20 milk teeth ends.

At the age of 6-7, milk teeth begin to fall out, and permanent teeth gradually grow to replace them. Before the change, the roots of the milk teeth dissolve, after which the teeth fall out. Small molars and third large molars, or wisdom teeth, grow without milk predecessors. The eruption of a permanent change of teeth ends by 14-15 years. The exception is wisdom teeth, the appearance of which is sometimes delayed up to 25-30 years; in 15% of cases they are absent on the upper jaw at all. The reason for the change of teeth is the growth of the jaws.

Mechanically crushed food in the mouth is mixed with saliva. The ducts of three pairs of large salivary glands open into the oral cavity: parotid, submandibular and sublingual. In addition, small salivary glands are located almost throughout the entire mucous membrane of the oral cavity and tongue. Intensive salivation begins with the appearance of milk teeth.

Saliva contains the enzyme amylase, which breaks down polysaccharides to dextrins and then to maltase and glucose. Mucin, a protein in saliva, makes saliva sticky. Thanks to mucin, food soaked in saliva is easier to swallow. Saliva contains a substance of a protein nature - lysozyme, which has a bactericidal effect.

With age, the amount of saliva secreted increases; the most significant jumps are observed in children from 9 to 12 months and from 9 to 11 years. In total, up to 800 cubic meters are separated from children per day. see saliva.

Esophagus. Food, crushed in the oral cavity and soaked in saliva, formed into food boluss, enters the pharynx through the pharynx, and from it into the esophagus. The esophagus is a muscular tube about 25 cm long in an adult. The inner lining of the esophagus is mucous, covered with stratified squamous epithelium with signs of keratinization in the upper layers. The epithelium protects the esophagus when a rough bolus of food moves through it. The mucous membrane forms deep longitudinal folds, which allows the esophagus to expand greatly as the bolus passes through.

In children, the mucous membrane of the esophagus is delicate, easily injured by coarse food, and rich in blood vessels. The length of the esophagus in newborns is about 10 cm, at the age of 5 years - 16 cm, at 15 years - 19 cm.

9.2. Digestion process

Features of digestion in the stomach. The stomach is the most expanded part of the digestive system. It looks like a curved bag that can hold up to 2 liters of food.

The stomach is located in the abdominal cavity asymmetrically: most of it is on the left, and the smaller part is on the right of the median plane of the body. The convex lower edge of the stomach is the greater curvature, the short concave edge is the lesser curvature. In the stomach, there is an entrance (cardiac part), a bottom (fundal part) and an exit (pyloric, or pyloric, part). The pylorus opens into the duodenum.

From the inside, the stomach is lined with a mucous membrane that forms many folds. In the thickness of the mucous membrane there are glands that produce gastric juice. There are three types of cells of the gastric glands: main (produce enzymes of gastric juice), parietal (produce hydrochloric acid), additional (produce mucus).

Human gastric juice is a colorless acidic liquid, which includes hydrochloric acid (0,5%), enzymes, minerals and mucus. The latter protect the gastric mucosa from mechanical and chemical damage. Hydrochloric acid kills bacteria in the stomach, softens fibrous foods, causes proteins to swell, and activates the digestive enzyme pepsin. During the day, an adult separates 1,2-2 liters of gastric juice.

Gastric juice contains two enzymes - pepsin and chymosin. Pepsin is produced by the gastric glands in an inactive form and is activated only in the acidic environment of the stomach. Pepsin breaks down proteins into albumose and peptones. Chymosin, or rennet, causes milk to curdle in the stomach. Finding chymosin in the gastric juice of children is especially easy during lactation. In older children, curdling occurs under the influence of pepsin and hydrochloric acid of gastric juice. Also in the gastric juice contains the enzyme lipase, which breaks down fats to glycerol and fatty acids. Gastric lipase acts on emulsified fats (milk fats).

In the stomach, food lingers from 4 to 11 hours and is subjected not only to chemical processing with the help of gastric juice, but also to mechanical action. In the thickness of the walls of the stomach there is a powerful muscle layer, consisting of smooth muscles, the muscle fibers of which run in the longitudinal, oblique and circular directions. Contractions of the muscles of the stomach contribute to better mixing of food with digestive juice, as well as the movement of food from the stomach to the intestines.

The stomach of infants has a rather horizontal position and is located almost entirely in the left hypochondrium. Only when the child begins to stand and walk does his stomach take a more upright position.

With age, the shape of the stomach also changes. In children under 1,5 years old, it is round, up to 2-3 years old it is pear-shaped, by the age of 7 the stomach has the shape of an adult.

The capacity of the stomach increases with age. If in a newborn it is 30-35 ml, then by the end of the first year of life it increases 10 times. At 10-12 years old, the capacity of the stomach reaches 1,5 liters.

The muscular layer of the stomach in children is poorly developed, especially in the bottom area. In newborns, the glandular epithelium of the stomach is poorly differentiated, the main cells are not yet mature enough. The differentiation of the cells of the glands of the stomach in children is completed by the age of seven, but they reach full development only at the end of the pubertal period.

The general acidity of gastric juice in children after birth is associated with the presence of lactic acid in its composition.

The function of hydrochloric acid synthesis develops in the period from 2,5 to 4 years. At the age of 4 to 7 years, the total acidity of gastric juice averages 35,4 units, in children from 7 to 12 years old it is 63. The relatively low content of hydrochloric acid in the gastric juice of children 4-6 years old leads to a decrease in its antimicrobial properties, which is manifested in the tendency of children to gastrointestinal diseases.

In a newborn child, the following enzymes and substances can be distinguished in the composition of gastric juice: pepsin, chymosin, lipase, lactic acid and associated hydrochloric acid. Pepsin, due to the low acidity of gastric juice, is able to break down only the proteins that make up milk. By the end of the first year of life, the activity of the enzyme chymosin rises to 256-512 units, although in the first month of a child's life it was only 16-32 units. The enzyme lipase, which is part of the gastric juice of infants, breaks down up to 25% of milk fat. However, one should take into account the fact that the fat of mother's milk is broken down not only by gastric lipase, but also by the lipase of mother's milk itself. This affects the rate of breakdown of fats in the stomach of artificially fed children. Their milk fats are always broken down more slowly than when breastfeeding. There is little lipase in cow's milk. As the child grows older, lipase activity increases from 10-12 to 35-40 units.

The amount of gastric juice, its acidity and digestive power, as well as in an adult, depend on food. For example, when feeding on women's milk, gastric juice is secreted with low acidity and digestive power; as gastric secretion develops, the most acidic juice is separated into meat, then into bread, and the juice into milk differs in the least acidity.

The secretory activity of the glands of the stomach is regulated by the vagus nerve. Gastric juice is released not only when the receptors of the oral cavity are irritated, but also by the smell, the type of food. It is also released at the time of the meal.

In an infant, the stomach is freed from food when breastfeeding after 2,5-3 hours, when fed with cow's milk - after 3-4 hours, food containing significant amounts of proteins and fats lingers in the stomach for 4,5-6,5 hours.

Digestion in the intestines. The contents of the stomach in the form of food gruel, soaked in acidic gastric juice, partially digested by muscle contractions of its walls, moves to its outlet (pyloric section) and passes from the stomach in doses to the initial section of the small intestine - the duodenum. The common bile duct of the liver and the pancreatic duct open into the duodenum.

In the duodenum, the most intensive and complete digestion of food slurry occurs. Under the influence of pancreatic juice, bile and intestinal juice, proteins, fats and carbohydrates are digested so that they become easily available for absorption and assimilation by the body.

Pure pancreatic juice is a colorless, transparent alkaline liquid. Intestinal juice contains the enzyme trypsin, which breaks down proteins into amino acids. Trypsin is produced by gland cells in an inactive form and is activated by intestinal juice. The lipase enzyme contained in the intestinal juice is activated by bile and, acting on fats, converts them into glycerol and fatty acids. The enzymes amylase and maltase convert complex carbohydrates into monosaccharides such as glucose. The separation of pancreatic juice lasts 6-14 hours and depends on the composition and properties of the food taken.

The bile produced by the liver cells enters the duodenum. And, although bile does not contain enzymes in its composition, its role in digestion is enormous. Bile activates the lipase produced by the cells of the pancreas; emulsifies fats, turning them into a suspension of small droplets (emulsified fats are easier to digest). In addition, bile actively influences the processes of absorption in the small intestine and enhances the secretion of pancreatic juice.

The duodenum continues into the jejunum of the small intestine, and the latter into the ileum. The length of the small intestine in an adult is 5-6 m. The inner lining of the small intestine is mucous and has many projections, or villi (about 4 million in an adult). Villi significantly increase the absorption surface of the small intestine. In addition to trypsin and lipases, intestinal juice contains over 20 enzymes that have a catalytic effect on the breakdown of nutrients.

In the walls of the small intestine there are longitudinal and circular muscles, the contractions of which cause pendulum-like and peristaltic movements, which improves the contact of food gruel with digestive juices and promotes the movement of the contents of the small intestine into the large intestine.

The length of the large intestine is 1,5-2 m. This is the widest section of the intestine. The large intestine is divided into the caecum with appendix (appendix), colon and rectum.

There is very little enzymatic processing of food in the large intestine. Here, the process of intensive absorption of water takes place, as a result of which feces are formed in its final sections, which is excreted from the body. Numerous symbiotic bacteria live in the large intestine. Some of them break down plant fiber, since human digestive juices do not contain enzymes for its digestion. Other bacteria synthesize vitamin K and some B vitamins, which are then absorbed by the human body.

In adults, the intestines are relatively shorter than in children: the length of the intestine in an adult exceeds the length of his body by 4-5 times, in an infant - 6 times. Especially intensively the intestine grows in length from 1 to 3 years due to the transition from dairy to mixed food and from 10 to 15 years.

The muscular layer of the intestine and its elastic fibers are less developed in children than in adults. In this regard, peristaltic movements in children are weaker. The digestive juices of the intestine already in the first days of a child's life contain all the main enzymes that ensure the process of digestion.

The growth and development of the pancreas continues up to 11 years, it grows most intensively at the age of 6 months to 2 years.

The liver in children is relatively larger than in adults. At 8-10 months, its mass doubles. The liver grows especially intensively at the age of 14-15, reaching a mass of 1300-1400 g. Bile secretion is already noted in a three-month-old fetus. With age, bile secretion increases.

Author: Antonova O.A.

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