The role of the oral cavity in digestion. Digestion, its types and functions

Digestion of food is a rather complex process, which boils down to the breakdown of large molecules of proteins, fats and carbons into monomers that are easily absorbed by the cells of the body. AT different departments digestive tract various compounds break down, which are then absorbed by the mucous membrane of the small intestine and are carried throughout the body. Digestion begins in oral cavity.

Before considering how digestion occurs in, it is necessary to at least briefly familiarize yourself with its structure.

The structure of the oral cavity

In anatomy, it is customary to divide into two departments:

• The vestibule of the mouth (the space between the lips and teeth);
• The oral cavity itself (limited by the teeth, the bony palate and the diaphragm of the mouth);

Each element of the oral cavity has its own function and is responsible for a specific food processing process.

The teeth are responsible for the mechanical processing of solid foods. With the help of fangs and incisors, a person bites off food, then crushes it with small ones. The function of large molars is to grind food.

The tongue is a large muscular organ that attaches to the floor of the mouth. The tongue is involved not only in the processing of food, but also in the processes of speech. Moving, this muscular organ mixes the crushed food with saliva and forms a food bolus. In addition, it is in the tissues of the tongue that taste, temperature, pain and mechanical receptors are located.

The salivary glands are parotid, sublingual and enter the oral cavity with the help of a duct. Their main function is the production and excretion of saliva, which is of great importance for the digestive process. The functions of saliva are as follows:

• Digestive (saliva contains enzymes that break down carbons);
• Protective (saliva contains lysozyme, which has strong bactericidal properties. In addition, saliva contains immunoglobulins and blood clotting factors. Saliva protects the oral cavity from drying out);
• Excretory (substances such as urea, salts, alcohol, some medicinal substances are excreted with saliva);

Digestion in the oral cavity: mechanical phase

A wide variety of food can enter the oral cavity and, depending on its consistency, it either immediately passes into the esophagus during the act of swallowing (drinks, liquid food), or undergoes mechanical processing, which facilitates further digestion processes.

As already mentioned, with the help of teeth, food is crushed. The movements of the tongue are needed in order to mix the chewed foods with saliva. Under the influence of saliva, food softens and is enveloped in mucus. Mucin, which is contained in saliva, takes part in the formation food bolus, which subsequently passes into the esophagus.

Digestion in the oral cavity: enzymatic phase

It also includes some enzymes that are involved in the breakdown of polymers. In the oral cavity, the splitting of carbons occurs, which continues already in small intestine.

Saliva contains an enzyme complex called ptyalin. Under their influence, the breakdown of polysaccharides to disaccharides (mainly maltose) occurs. In the future, maltose, under the influence of another enzyme, is broken down to glucose monosaccharide.

The longer the food is in the oral cavity and is amenable to enzymatic action, the easier it is to be digested in all other parts of the herbal tract. This is why doctors always recommend chewing food as long as possible.

This completes digestion in the oral cavity. The food bolus passes further and, falling on the root of the tongue, starts the reflex process of swallowing, in which food passes into the esophagus and then enters the stomach.

To summarize, processes such as grinding food, analyzing its taste, wetting with saliva, mixing and primary decomposition of carbohydrates take place in the oral cavity.

The organs of the oral cavity include lips, cheeks, gums, teeth, hard and soft palate, tongue and salivary glands. The tongue, lips and teeth are used to grip and grind food.

Cattle grab grass, hay and other food with their tongues.

Sheep seize forage forked upper lip and tongue, and the grass is cut with incisors. Horses grab grass and hay with moving lips. In pigs, food capture occurs with the help of the tongue and lips. Carnivores use fangs and incisors to bite off food. Birds usually either peck at food or grab it with their beaks.

Chewing is carried out due to the joint activity of the upper and lower jaws, teeth, chewing muscles and tongue. During chewing, the food is crushed and moistened with saliva, which makes it easier to swallow.

The act of chewing is regulated by nuclei located in the medulla oblongata cranial nerves, innervating the masticatory muscles, tongue and pharynx.

Cows grind feed less thoroughly than other animals, since most of their chewing occurs during burping and chewing.

According to the nature of the secretion secreted, the salivary glands are divided into serous, mucous and mixed. The mucous glands secrete saliva containing a mucous substance - mucin. These include small glands and individual goblet cells. Serous glands (parotid and small glands of the tongue) separate the secret, which includes proteins. The submandibular, sublingual and buccal glands form a serous-mucous secret.

The ducts of three pairs of large salivary glands flow into the oral cavity: parotid, submandibular and sublingual. In addition, in the oral cavity there are also small parietal glands - labial, lingual, palatine, buccal (Fig. 16.2).

Saliva, wetting the food, facilitates the process of chewing. In addition, it liquefies the food mass and extracts flavoring substances from it.

Rice. 16.2. Salivary glands: a- cows; b- pigs; in- horses:

1 - parotid gland; 2 - labial glands; 3 - sublingual gland long duct; 4 - sublingual gland short duct; 5 - submandibular gland; 6 - buccal glands; 7 - duct of the submandibular gland

[Pismenskaya V.N., Boev V.I. Workshop on anatomy and histology of farm animals. M.: KolosS, 2010. S. 165]

Animals different types salivation has its own characteristics. In pigs, salivation is characterized by the fact that the submandibular and small glands of the oral cavity secrete saliva continuously, and the sublingual and parotid glands - only during the intake of food. Pig saliva contains amylolytic enzymes a-amylase and a-glucosidase, which break down starch in an alkaline environment.

Of all the salivary glands in horses, only the small glands of the oral cavity continuously secrete. In normal feeding, the saliva of horses contains very few enzymes that hydrolyze starch.

In ruminants, the parotid glands secrete constantly, both during feeding and chewing, and during periods of rest, while other glands secrete saliva only during feeding. The high alkalinity of saliva in ruminants, due to the increased concentration of urea, phosphate and bicarbonate, helps to neutralize the acidic products formed during the fermentation of feed in the rumen and maintains a certain pH value of the ruminal environment, which is necessary for the development of various bacteria.

The regulation of salivation is a complex process consisting of unconditional and conditioned reflexes. When food is captured and enters the oral cavity, the receptor apparatuses of the mucous membrane of the lips and tongue are excited. The food causes irritation of the nerve endings of the fibers of the trigeminal, facial, glossopharyngeal and vagus nerves. Through these afferent nerves, impulses from the oral cavity enter the center of salivation, located in the medulla oblongata, as well as to the lateral horns of the upper thoracic segments. spinal cord. From there, impulses along the efferent parasympathetic and sympathetic nerve fibers are sent to the salivary glands.

Parasympathetic fibers from the nuclei of the center of salivation go to the parotid gland as part of the glossopharyngeal nerve, and to the submandibular and sublingual - through the branch facial nerve(drum string). Sympathetic nerve fibers exit the spinal cord at the level of the II-IV thoracic segments as part of its ventral roots, go to the superior cervical ganglion, where they switch to postganglionic sympathetic neurons that innervate the salivary glands.

Saliva contains about 99% water and 1% inorganic and organic substances.

Per day, the parotid salivary glands in cattle secrete 30-65 liters of saliva, lower buccal - 7-16, posterior and upper buccal (palatine, buccal and pharyngeal glands) - 20-50, submandibular - 4-7, sublingual - 1 l . The total volume of saliva secreted per day can reach 90-190 liters. About 50% of the total volume of saliva is formed in the parotid glands, 40% in the buccal, 7% in the submandibular and about 3% in the sublingual glands. Salivation decreases as the pH of the ruminal fluid increases.

After chewing the food and moistening it with saliva, a food lump is formed in the oral cavity, which is pushed into the lower parts of the pharynx and then into the esophagus by coordinated contractions of the muscles of the oral cavity, pharynx, larynx and esophagus. The swallowed lump moves through the esophagus due to peristaltic movements.

Saliva performs a number of important functions in animals:

• digestive function - saliva dissolves food substances, contributes to the formation of taste sensations and affects appetite. In addition, the saliva enzyme a-amylase breaks down polysaccharides (starch and glycogen) to maltose, and the second enzyme (maltase) breaks down maltose to glucose;
• promotes softening of the feed when it is chewed and facilitates the formation of a food coma and its ingestion;
• protective function - saliva contains the enzyme lysozyme, which has a bacteriostatic property and takes part in the processes of regeneration of the oral mucosa;
• has a hemostatic effect, since it contains blood clotting factors;
• excretory function - saliva removes some metabolic products and toxic substances from the blood.

For normal digestion, chewing is of great importance - the mechanical process of crushing and grinding food. The upper jaw is immobile during chewing. Through the facial muscles and tongue, food moves in the oral cavity. Actually chewing muscles, temporal and external and internal pterygoid raise and put forward the lower jaw, and the muscles of the bottom of the oral cavity lower it. Reflex contraction of the masticatory muscles is caused by food irritation of the mucosal receptors.

Centripetal impulses are transmitted along the 2nd and 3rd branches trigeminal nerves, facial and glossopharyngeal, and centrifugal - along the motor nerves of the masticatory muscles, facial and sublingual. The nature and number of chewing movements are strictly natural when food of different consistency is introduced into the mouth. In athletes, compared with non-athletes, the tension of the masticatory muscles increases at rest and when teeth are closed. Food crushed during chewing is mixed with saliva.

Salivation and salivation are produced by the salivary glands, which are divided into protein (serous), mucous and mixed. The mucous glands are located on the root of the tongue, hard and soft palate and in the pharynx. They secrete a mucous liquid of an alkaline reaction, containing, in addition to salts and a small amount of protein, a lot of mucin. The serous glands of the tongue and parotid glands form saliva containing protein and salts, and the mixed (submandibular and sublingual glands) produce saliva rich in mucin and containing protein and salts. Water makes up 98.5-99.5% of all saliva. Up to 1.5 dm3 of saliva is secreted per day in an adult. It wets dry substances and dissolves or lubricates solids, which makes it easier for them to slip into the stomach during swallowing, as well as neutralizes harmful liquids, dilutes them and washes away harmful substances. The saliva enzyme ptyalin hydrolyzes boiled starch and breaks it down with the subsequent participation of the maltase enzyme to glucose. Ptyalin acts in alkaline, neutral and slightly acidic environments. Saliva also contains lysozyme, an antibiotic produced in the salivary glands that dissolves microbes.

Saliva is separated reflexively when food irritates the receptors of the oral mucosa. Of these, centripetal impulses are transmitted mainly along the lingual and glossopharyngeal nerves, while centrifugal impulses go to the parotid gland along the glossopharyngeal and sympathetic nerves, to the submandibular and sublingual glands - along the branch of the facial nerve (drum string) and along the sympathetic. The center of salivation is located in the medulla oblongata. In humans, salivation is strongly stimulated by water and acids. Chewing increases salivation; after saturation, the amount of saliva decreases. Salty food reduces salivation, and the restriction of water intake and the introduction of large amounts of water do not affect salivation. Sunbathing almost does not change the secretion of saliva.

swallowing. It is carried out reflexively and consists of three phases: 1) voluntary movement of food in the oral cavity behind the anterior palatine arches, 2) involuntary, very rapid passage of the food bolus through the pharynx into the esophagus, and 3) involuntary slow movement of the food bolus through the esophagus.

Voluntary swallowing is caused by irritation of the receptors of the pharynx when the tongue touches the surface of the pharynx or the introduction of a certain amount of saliva or food into the pharynx. Swallowing in the absence of food or saliva in the oral cavity is impossible. When there is no food or liquid in the oral cavity, it is impossible to produce more than 5-6 consecutive swallows, as there will not be enough saliva. From the pharyngeal receptors, centripetal impulses enter the medulla oblongata along the fibers of the trigeminal, glossopharyngeal, and superior laryngeal nerves, while centrifugal impulses are sent to the muscles involved in swallowing along the motor branches of the trigeminal, glossopharyngeal, hypoglossal, and vagus nerves. Swallowing is related to breathing. Each swallow along the centripetal fibers of the glossopharyngeal nerve reflexively inhibits breathing. The slightest irritation of the mucous membrane of the larynx with a crumb of food or a lump of mucus along the centripetal fibers of the superior laryngeal nerve holds the breath. Swallowing reflexively accelerates the pulse due to inhibition of the tone of the vagus nerves.

From the pharynx, when swallowed, food enters the esophagus, which is its continuation. The esophagus through the chest cavity and the opening in the diaphragm passes into the stomach. It has several narrowings, the largest - at the point of passage through the diaphragm. The wall of the esophagus consists of three membranes: mucous, muscular and connective tissue.

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The oral cavity is the initial section of the digestive tract where:

1. Analysis of the taste properties of substances;
2. Separation of substances into food and rejected;
3. Protection of the digestive tract from the ingress of low-quality nutrients and exogenous microflora;
4. Grinding, wetting food with saliva, initial hydrolysis of carbohydrates and the formation of a food lump;
5. Irritation of mechano-, chemo-, thermoreceptors, causing excitation of the activity of not only their own, but also the digestive glands of the stomach, pancreas, liver, duodenum.

The oral cavity performs the role of an external barrier to protect the body from pathogenic microflora due to the presence of the bactericidal substance lysozyme (muromidase) in saliva, the antiviral effect of saliva nuclease, the ability of saliva immunoglobulin A to bind exotoxins, and also as a result of phagocytosis of leukocytes (4000 in 1 cm 3 of saliva) and oppression pathogenic microflora normal oral flora.

Salivation

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salivary glands hormone-like substances are produced that are involved in the regulation of phosphorus-calcium metabolism of bones and teeth, in the regeneration of the epithelium of the mucous membrane of the oral cavity, esophagus, stomach, and in the regeneration of sympathetic fibers when they are damaged.

Food stays in the oral cavity for 16-18 seconds, and during this time, the saliva secreted by the glands into the oral cavity wets dry substances, dissolves soluble and envelops solids, neutralizes irritating liquids or reduces their concentration, facilitates the removal of inedible (rejected) substances, washing them off with mucous membrane of the oral cavity.

The mechanism of saliva formation

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Saliva is produced both in the acini and in the ducts of the salivary glands. The cytoplasm of glandular cells contains secretory granules located mainly in the perinuclear and apical parts of the cells, near the Golgi apparatus. In mucous and serous cells, granules differ both in size and in chemical nature. In the course of secretion, the size, number and location of the granules change, the Golgi apparatus becomes more distinct. As the secretory granules mature, they move from the Golgi apparatus to the top of the cell. In the granules, the synthesis of organic substances is carried out, which move with water through the cell along the endoplasmic reticulum. During secretion, the amount of colloidal material in the form of secretory granules gradually decreases and is renewed during the rest period.

In the acini of the glands, the first stage of the formation of saliva is carried out - primary secret, containing alpha amylase and mucin. The content of ions in the primary secret differs slightly from their concentration in extracellular fluids. In the salivary ducts, the composition of the secret changes significantly: sodium ions are actively reabsorbed, and potassium ions are actively secreted, but at a slower rate than sodium ions are absorbed. As a result, the concentration of sodium in saliva decreases, while the concentration of potassium ions increases. A significant predominance of sodium ion reabsorption over potassium ion secretion increases the electronegativity in the salivary ducts (up to 70 mV), which causes passive reabsorption of chloride ions, a significant decrease in the concentration of which at the same time is associated with a decrease in the concentration of sodium ions. At the same time, the secretion of bicarbonate ions by the epithelium of the ducts into the lumen of the ducts increases.

Secretory function of the salivary glands

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Humans have three pairs of major salivary glands: parotid, sublingual, submandibular and besides, a large number of small glands scattered in the oral mucosa. The salivary glands are composed of mucous and serous cells. The former secrete a mucoid secret of a thick consistency, the latter - liquid, serous or proteinaceous. The parotid salivary glands contain only serous cells. The same cells are found on the lateral surfaces of the tongue. Submandibular and sublingual - mixed glands, contain both serous and mucous cells. Similar glands are also located in the mucous membrane of the lips, cheeks, and on the tip of the tongue. The sublingual and small glands of the mucosa secrete a secret constantly, and the parotid and submandibular glands - when they are stimulated.

Daily produced from 0.5 to 2.0 liters of saliva. Its pH ranges from 5.25 to 8.0. An important factor, affecting the composition of saliva, is the rate of its secretion, which in humans in the "quiet" state of the salivary glands is 0.24 ml / min. However, the rate of secretion can fluctuate even at rest from 0.01 to 18.0 ml/min and increase when chewing food up to 200 ml/min.

The secret of the various salivary glands is not the same and varies depending on the nature of the stimulus. Human saliva is a viscous, opalescent, slightly turbid (due to the presence of cellular elements) liquid with a specific gravity of 1.001-1.017 and a viscosity of 1.10-1.33.

Mixed human saliva contains 99.4-99.5% water and 0.5-0.6% solid residue, which consists of inorganic and organic substances. Inorganic components are represented by ions of potassium, sodium, calcium, magnesium, iron, chlorine, fluorine, rhodanium compounds, phosphate, chloride, sulfate, bicarbonate and make up approximately 1/3 of the dense residue.

The organic substances of the dense residue are proteins (albumins, globulins), free amino acids, nitrogen-containing compounds of a non-protein nature (urea, ammonia, creatine), bactericidal substances - lysozyme (muramidase) and enzymes: alpha-amylase and maltase.
Alpha-amylase is a hydrolytic enzyme and cleaves 1,4-glucosidic bonds in starch and glycogen molecules to form dextrins and then maltose and sucrose.
Maltose (glucosidase) breaks down maltose and sucrose into monosaccharides. In saliva, there are also other enzymes in small quantities - proteases, peptidases, lipase, alkaline and acid phosphatase, RNases, etc. The viscosity and mucilaginous properties of saliva are due to the presence of mucopolysaccharides (mucin).

Salivation regulation

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The separation of saliva is a complex reflex act, which is carried out due to irritation of the receptors of the oral cavity with food or other substances ( unconditioned reflex stimuli), as well as irritation of visual and olfactory receptors appearance and the smell of food, the type of environment in which eating takes place (conditioned reflex irritants).

Excitation arising from stimulation of the mechano-, chemo- and thermoreceptors of the oral cavity reaches the center of salivation in the medulla oblongata along the afferent fibers of the V, VII, IX, X pairs of cranial nerves. Efferent influences to the salivary glands come through the parasympathetic and sympathetic nerve fibers. Preganglionic parasympathetic fibers to the sublingual and submandibular salivary glands go as part of the drum string (branch of the VII pair) to the sublingual and submandibular ganglia located in the body of the corresponding glands, postganglionic - from these ganglia to the secretory cells and vessels of the glands. To the parotid glands, preganglionic parasympathetic fibers come from the lower salivary nucleus of the medulla oblongata as part of the IX pair of cranial nerves. From the ear node, postganglionic fibers are directed to secretory cells and vessels.

The preganglionic sympathetic fibers innervating the salivary glands are the axons of the neurons of the lateral horns of the II-VI thoracic segments of the spinal cord and end in the superior cervical ganglion. From here postganglionic fibers are sent to the salivary glands. Irritation of the parasympathetic nerves is accompanied by copious secretion of liquid saliva containing small amounts of organic substances. When sympathetic nerves are stimulated, a small amount of saliva is released, which contains mucin, making it thick and viscous. For this reason, the parasympathetic nerves are called secretory, and sympathetic trophic. With "food" secretion, the parasympathetic influences on the salivary glands are usually stronger than the sympathetic ones.

The regulation of the volume of water and the content of organic substances in saliva is carried outsalivary center. In response to irritation of the mechano-, chemo- and thermoreceptors of the oral cavity by various food or rejected substances, bursts of impulses differing in frequency are formed in the afferent nerves of the salivary reflex arc.

The variety of afferent impulses, in turn, is accompanied by the appearance of a mosaic of excitation in the salivary center, corresponding to the frequency of impulses, and different efferent impulses to the salivary glands. Reflex influences inhibit salivation until it stops. Inhibition can be caused by pain irritation, negative emotions, etc.

The occurrence of salivation at the sight and (or) smell of food is associated with the participation in the process of the corresponding cortical zones hemispheres the brain, as well as the anterior and posterior groups of the nuclei of the hypothalamus (see Chapter 15).

The reflex mechanism is the main, but not the only mechanism for excitation of salivation.. The secretion of saliva is influenced by the hormones of the pituitary gland, pancreas and thyroid gland, sex hormones. Abundant separation of saliva is observed during asphyxia due to irritation of the salivary center with carbonic acid. Salivation can be stimulated by vegetotropic pharmacological substances(pilocarpine, prozerin, atropine).

Chewing

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Chewing- a complex physiological act, which consists in grinding food substances, wetting them with saliva and forming a food lump. Chewing provides the quality of mechanical and chemical processing of food and determines the time of its stay in the oral cavity, has a reflex effect on the secretory and motor activity of the digestive tract. Chewing involves the upper and lower jaws, chewing and mimic muscles of the face, tongue, soft palate and salivary glands.

chewing regulation

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chewing is regulated reflexively. Excitation from the receptors of the oral mucosa (mechano-, chemo- and thermoreceptors) is transmitted along the afferent fibers of the II, III branches of the trigeminal, glossopharyngeal, superior laryngeal nerve and the tympanic string to the center of chewing, which is located in the medulla oblongata. Excitation from center to chewing muscles transmitted along the efferent fibers of the trigeminal, facial and hypoglossal nerve. The ability to arbitrarily regulate the chewing function suggests that there is a cortical regulation of the chewing process. In this case, the excitation from the sensory nuclei of the brainstem along the afferent pathway through the specific nuclei of the thalamus switches to the cortical section of the taste analyzer (see Chapter 16), where, as a result of the analysis of the information received and the synthesis of the image of the stimulus, the question of the edibility or inedibility of the substance that entered the oral cavity is decided. cavity, which affects the nature of the movements of the chewing apparatus.

AT infancy the process of chewing corresponds to sucking, which is provided by reflex contraction of the muscles of the mouth and tongue, creating a rarefaction in the oral cavity in the range of 100-150 mm of water.

swallowing

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swallowing- a complex reflex act by which food is transferred from the oral cavity to the stomach. The act of swallowing is a chain of successive interrelated stages, which can be divided into three phases:

(1) oral(arbitrary),
(2) pharyngeal(involuntary, fast),
(3) esophageal(involuntary, slow).

First phase of swallowing

The food bolus (volume 5-15 cm 3) with coordinated movements of the cheeks and tongue moves to the root of the tongue, behind the anterior arches of the pharyngeal ring. From this moment on, the act of swallowing becomes involuntary (Fig. 9.1).

Fig.9.1. Swallowing process.

Irritation of the receptors of the mucous membrane of the soft palate and pharynx by the food bolus is transmitted along the glossopharyngeal nerves to the swallowing center in the medulla oblongata, efferent impulses from which go to the muscles of the oral cavity, pharynx, larynx and esophagus along the fibers of the hypoglossal, trigeminal, glossopharyngeal and vagus nerves, which is ensured the occurrence of a coordinated contraction of the muscles of the tongue and muscles that lift the soft palate.

Due to this, the entrance to the nasal cavity from the side of the pharynx is closed. soft palate and the tongue moves the food bolus down the throat.

At the same time, the hyoid bone is displaced, the larynx rises, and as a result, the entrance to the larynx is closed by the epiglottis. This prevents food from entering the respiratory tract.

Second phase of swallowing

At the same time, the upper esophageal sphincter opens - a thickening of the muscular membrane of the esophagus, formed by circular fibers in the upper half of the cervical part of the esophagus, and the food bolus enters the esophagus. The upper esophageal sphincter contracts after the passage of the food bolus into the esophagus, preventing the esophago-pharyngeal reflex.

Third phase of swallowing

The third phase of swallowing is the passage of food through the esophagus and its transfer to the stomach. The esophagus is powerful reflex zone. The receptor apparatus is represented here mainly by mechanoreceptors. Due to irritation of the latter by the food bolus, a reflex contraction of the muscles of the esophagus occurs. At the same time, the circular muscles are consistently contracted (with simultaneous relaxation of the underlying ones). Waves of contractions (called peristaltic) sequentially spread towards the stomach, moving the food bolus. The speed of propagation of the food wave is 2-5 cm / s. The contraction of the muscles of the esophagus is associated with the receipt of efferent impulses from the medulla oblongata along the fibers of the recurrent and vagus nerves.

Movement of food through the esophagus

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The movement of food through the esophagus is caused by a number of factors..

First of all, pressure drop between the pharyngeal cavity and the beginning of the esophagus - from 45 mm Hg. in the pharyngeal cavity (at the beginning of swallowing) up to 30 mm Hg. (in the esophagus).
Secondly, the presence of peristaltic contractions of the muscles of the esophagus,
Thirdly- muscle tone of the esophagus, which in the thoracic region is almost three times lower than in the cervical, and,
Fourth- gravity of the food bolus. The speed of passage of food through the esophagus depends on the consistency of food: dense passes in 3-9 s, liquid - in 1-2 s.

The center of swallowing through the reticular formation is connected with other centers of the medulla oblongata and spinal cord, the excitation of which at the time of swallowing causes inhibition of the activity of the respiratory center and a decrease in the tone of the vagus nerve. This is accompanied by respiratory arrest and increased heart rate.

In the absence of swallowing contractions, the entrance from the esophagus to the stomach is closed - the muscles of the cardial part of the stomach are in a state of tonic contraction. When the peristaltic wave and the food bolus reach the end of the esophagus, the tone of the muscles of the cardial part of the stomach decreases and the food bolus enters the stomach. When the stomach is filled with food, the tone of the cardiac muscles increases and prevents the reverse flow of gastric contents from the stomach into the esophagus.

Physiology of digestion.

Topic 6.5

Lecture No. 17 “Physiology of digestion. Metabolism and Energy.

Plan:

1. Physiology of digestion.

Digestion in the mouth

Digestion in the stomach

Digestion in the small intestine

Digestion in the large intestine

2. General concept on metabolism and energy.

3. Exchange of proteins, fats and carbohydrates.

4. Water-salt exchange. The value of vitamins.

Food in the form in which it enters the body cannot be absorbed into the blood and lymph and be used to perform various functions Therefore, it is subjected to mechanical and chemical processing.

Mechanical and chemical processing of food and its transformation into substances digestible by the body is called digestion.

Consider digestion in each section of the gastrointestinal tract.

Digestion in the mouth.

Food is retained in the oral cavity, no more than 15-20 seconds, but, despite this, its mechanical and chemical processing takes place.

Mechanical restoration carried out by chewing.

Careful grinding of food plays important role:

1) facilitates subsequent digestion and absorption.

2) stimulates salivation

3) affects the secretory and motor activity of the gastrointestinal tract.

4) ensures the formation of a digestive lump suitable for swallowing and digestion.

Chemical processing food is carried out with the help of salivary enzymes - amylase and maltase, which act on carbohydrates, exposing them to partial digestion.

0.5-2.0 liters of saliva is secreted per day, it consists of 95.5% water and 0.5% dry residue, has an alkaline reaction (pH = 5.8 - 7.4).

Dry residue consists of organic and inorganic substances. Among inorganic substances, saliva contains potassium, chlorine, sodium, calcium, etc.

Of the organic substances in saliva, there are:

1) enzymes: amylase and maltase, which begin to act on carbohydrates in the oral cavity;

2) mucin - a protein mucous substance that gives saliva viscosity, glues the food lump and makes it slippery, making it easier to swallow and pass the lump through the esophagus;

3) lysozyme - a bactericidal substance acts on microbes.

Digestion in the stomach.

The food bolus comes from the esophagus to the stomach, where it stays in it for 4-6 hours.

During the first 30-40 minutes after food enters the stomach, salivary enzymes amylase and maltase act on it, continuing to break down carbohydrates. As soon as the food bolus is saturated with acidic gastric juice, chemical treatment begins, under the influence of:

1) proteolytic enzymes (pepsinogen, gastrixin, chymosin), which break down proteins into simpler ones;

2) lipolytic enzymes - stomach lipases that break down fats into simpler ones.

In addition to chemical processing in the stomach, mechanical processing of food takes place, which is carried out by the muscular membrane.

Due to the contraction of the muscle membrane, the food bolus is impregnated with gastric juice.

The entire period of gastric secretion normally lasts 6-10 hours and is divided for 3 phases:

1 phase- complex reflex (brain) lasts 30-40 minutes, and is carried out on the meringue of conditioned and unconditioned reflexes.

branch gastric juice caused by the sight, smell of food, sound stimuli related to cooking i.e. irritation of the olfactory, visual and auditory receptors. Impulses from these receptors enter the brain - to the food center (in the medulla oblongata) and along the nerves to the glands of the stomach.

2 phase- gastric (chemical) lasts 6-8 hours, that is, while food is in the stomach.

3 phase- intestinal lasts from 1 to 3 hours.

Digestion in the small intestine.

The food mass in the form of gruel from the stomach enters in separate portions into the small intestine and is subjected to further mechanical and chemical processing.

Mechanical restoration consists in the pendulum movement of food gruel and mixing it with digestive juices.

Chemical processing- this is the action on the food slurry of enzymes of the pancreas, intestinal juices and bile.

Under the influence of pancreatic juice enzymes (trypsin and chymotrypsin), intestinal juice enzymes (catepsin and aminopeptidase), polypeptides are cleaved to amino acids.

Under the influence of the enzymes amylase and maltase of intestinal and pancreatic juices, complex carbohydrates (disaccharides) are broken down into simpler ones - glucose.

The breakdown of fats is under the influence of enzymes - lipase and phospholipase of intestinal and pancreatic juices to glycerol and fatty acids.

The most intensive chemical processing takes place in the duodenum, where food is affected by pancreatic juice and bile. In the remaining parts of the small intestine, the process of splitting nutrients ends under the influence of intestinal juice and the process of absorption begins.

In the small intestine depending on location digestive process distinguish:

abdominal digestion - in the lumen of the small intestine;

parietal digestion.

cavity digestion It is carried out due to digestive juices and enzymes that enter the cavity of the small intestine (pancreatic juice, bile, intestinal juice) and act on nutrients here. According to the type of cavity digestion, large molecular substances are broken down.

Parietal digestion provided by the microvilli of the intestinal epithelium and is final stage digestion of food, after which absorption begins.

Suction is the transfer of nutrients from the alimentary canal into the blood and lymph.

Absorption is carried out by villi on the mucous membrane of the small intestine.

Water, mineral salts, amino acids, monosaccharides are absorbed into the blood.

Glycerin is well absorbed into the lymph, and fatty acids, which are insoluble in water, cannot be absorbed in this form, so they first combine with alkalis and turn into soaps, which dissolve well and are absorbed into the lymph.

Digestion in the large intestine.

The main function of the large intestine is:

1) water suction

2) the formation of feces

Absorption of nutrients is negligible.

The secret of the mucous membrane of the large intestine has an alkaline reaction.

The secret is revealed significant amount rejected epithelial cells, lymphocytes, mucus, contains a small amount of enzymes (lipase, amylose, etc.). little undigested food masses enter this department.

An essential role in the process of digestion belongs to the microflora - Escherichia coli and bacteria of lactic acid fermentation.

Bacteria perform both beneficial and negative functions for the body.

The positive role of bacteria:

1. Lactic acid fermentation bacteria produce lactic acid, which has antiseptic properties.

2. Synthesize B vitamins and vitamin K.

3. Inactivate (suppress) the action of enzymes.

4. Suppress the reproduction of pathogenic microbes.

The negative role of bacteria:

1. Form endotoxins.

2. They cause fermentation and putrefactive processes with the formation of toxic substances.

3. When bacteria change in quantitative and species ratio, a disease can occur - dysbacteriosis.