During the movement of chyme (significantly digested food products) through the small intestine, under the influence of intestinal juice, the intermediate compounds of the breakdown of proteins, fats and carbohydrates are digested to the final products.
Intestinal juice is a cloudy, rather viscous liquid, a product of the activity of the entire mucous membrane of the small intestine.
In the mucous membrane of the upper part of the duodenum, a large number of duodenal glands are laid. In structure and function, they are similar to the glands of the pyloric part of the stomach. The juice of the duodenal glands is a thick, colorless liquid of a slightly alkaline reaction, has a small enzymatic activity.
Intestinal glands are embedded in the mucosa of the duodenum and the entire small intestine.
In the intestinal juice there are more than 20 different enzymes involved in digestion: enterokinase, several peptidases, alkaline phosphatase, nuclease, lipase, amylase, lactase and sucrase, etc. Under natural conditions, they are fixed in the brush border zone and carry out parietal digestion.
The secretion of the intestinal glands increases during meals, with local mechanical and chemical irritation of the intestine and under the influence of certain intestinal hormones.
The leading role belongs to local mechanisms. Mechanical irritation of the mucous membrane of the small intestine dramatically increases the release of the liquid part of the juice. Chemical stimulants of the small intestine are products of digestion of protein, fat, pancreatic juice, hydrochloric acid (and other acids).
Motor function of the small intestine. The motility of the small intestine provides mixing of its contents (chyme) with digestive secretions, the promotion of chyme through the intestine, the change of its layer near the mucous membrane, an increase in intra-intestinal pressure, which contributes to the filtration of solutions from the intestinal cavity into the blood and lymph. Therefore, small bowel motility promotes hydrolysis and absorption of nutrients.
The movement of the small intestine occurs as a result of coordinated contractions of the longitudinal and circular layers of smooth muscles. It is customary to distinguish between several types of contractions of the small intestine:
- rhythmic segmentation;
- pendulum;
- peristaltic (very slow, slow, fast, rapid);
- antiperistaltic;
- tonic.
The first two types are rhythmic or segmental contractions.
Rhythmic segmentation It is provided mainly by contractions of the circular layer of the muscular membrane, while the contents of the intestine are divided into two parts. The next contraction forms a new segment of the intestine, the contents of which consist of the chyme of the two halves of the former segments. These contractions achieve mixing of the chyme and increasing the pressure in each segment.
pendulum contractions provided by the longitudinal muscles and participation in the contraction of the circular muscles. When this happens, the chyme moves back and forth and its weak forward movement in the caudal direction. In the upper sections of the human small intestine, the frequency of rhythmic contractions is 9-12, in the lower - 6-8 per minute.
peristaltic wave, consisting of the interception and expansion of the small intestine, promotes the chyme in the caudal direction. At the same time, several peristaltic waves move along the intestine. The peristaltic wave moves along the intestine at a speed of 0.1-0.3 cm / s, in the proximal sections it is greater than in the distal ones. The speed of the rapid (propulsive) wave is 7-21 cm/s.
At antiperistaltic contractions the wave moves in the opposite (oral) direction. Normally, the small intestine, like the stomach, does not contract antiperistaltically (this is typical for vomiting).
tonic contractions may have a local character or move at a very low speed. Tonic contractions narrow the lumen of the intestine over a large extent.
Regulation of motility of the small intestine. Motility of the small intestine is regulated by nervous and humoral mechanisms; the role of myogenic mechanisms, which are based on the properties of smooth muscle automation, is quite large.
Parasympathetic nerve fibers predominantly excite, while sympathetic fibers inhibit contractions of the small intestine. These fibers are conductors of the reflex regulation of the motility of the small intestine. The act of eating conditioned and unconditioned reflex first briefly slows down, and then enhances intestinal motility. In the future, it is determined by the physical and chemical properties of the chyme: coarse food, rich in dietary fibers and fats that are indigestible in the small intestine, enhances it.
Local irritants that enhance intestinal motility are the products of digestion of nutrients, especially fats, acids, alkalis, salts (in concentrated solutions).
The cerebral cortex influences gut motility mainly through the hypothalamus and limbic system. The important role of the cerebral cortex and the second signaling system in the regulation of intestinal motility is proved by the fact that when talking or even thinking about tasty food, intestinal motility increases, and with a negative attitude towards food, motility is inhibited. With anger, fear and pain, it is also inhibited. Sometimes, with some strong emotions, such as fear, there is a violent intestinal motility ("nervous diarrhea").
Adequate irritation of any part of the gastrointestinal tract (GIT) causes excitation in the irritated and underlying areas and enhances the movement of the contents in the caudal direction from the site of irritation. At the same time, it inhibits motility and delays the progress of chyme in the overlying sections of the gastrointestinal tract.
Humoral substances change intestinal motility, acting directly on muscle fibers and through receptors on neurons of the intramural nervous system. Strengthen the motility of the small intestine serotonin, histamine, gastrin, cholecystokinin-pancreozymin.
EXCURSION TO THE PHYSIOLOGY OF DIGESTION. Part two.
Today we will talk about what happens to food in the small and large intestines.
Everything that happened to food in the mouth and stomach was a preparation for further transformations. There was practically no assimilation and absorption of nutrients. The real alchemy of digestion takes place in the small intestine, more precisely, in its initial part - the duodenum, so named because its length is measured by 12 fingers folded together - fingers.
The food processed by gastric secrets, already completely different from what we have eaten, is moving towards the exit from the stomach, to its pyloric part. Here is a sphincter (valve) that separates the stomach from the intestines, which in portions releases chyme into the duodenum (another name for the duodenum), where the environment is no longer acidic, as in the stomach, but alkaline. Valve regulation is a very complex mechanism that depends, among other things, on signals from receptors that respond to acidity, composition, consistency and degree of food processing, and pressure in the stomach. Normally, at the exit from the stomach, food should already have a slightly acid reaction of the environment in which other proteolytic (protein-splitting) enzymes continue to work. In addition, there should always be free space in the stomach for gases that are formed as a result of fermentation and fermentation. Gas pressure especially promotes the opening of the sphincter. That is why it is recommended to eat such an amount of food that 1/3 of the stomach is filled with solid food, 1/3 of the liquid and 1/3 of the space remains free, which will help to avoid many unpleasant consequences (belching, reflux formation, premature passage of unprocessed food into the intestines). and the formation of persistent, which have become chronic disorders). In other words, it is better not to overeat, and for this it is necessary to eat slowly, since signals about satiety begin to enter the brain only after 20 minutes.
Digestion in the small intestine
A well-processed food slurry (chyme) in the stomach enters the small intestine through a valve, which consists of three parts, the most important of which is the duodenum. It is here that the complete digestion of all food nutrients occurs under the action of intestinal secretions, including pancreatic juices, bile and the secrets of the intestine itself. People can live without a stomach (as they do after surgery) on a strict diet, but they cannot live at all without this important part of the small intestine. The absorption of the products we eat, split (hydrolyzed) to the final components (amino acids, fatty acids, glucose and other macro and micro molecules), occurs in two other parts of the small intestine. The inner layer lining them, the villous epithelium, has a total surface area many times greater than the size of the intestine itself (the lumen of which is as thick as a finger). Such a structure of this amazing layer of the intestine is intended for the passage of final monomers (absorption) into the intestinal space - into the blood and lymph (inside each "papilla" there are blood and lymphatic vessels), from where they rush to the liver, spread throughout the body and are embedded in its cells .
Let's return to the processes taking place in the duodenum, which is rightfully called the "brain" of digestion and not only digestion ... This section of the intestine is also actively involved in the hormonal regulation of many processes in the body, in providing immune protection, and in many others, which we will discuss talk about future topics.
There should be an alkaline environment in the small intestine, but acidic chyme comes from the stomach, what happens? Abundant release into the lumen of the duodenum of intestinal juices, secretions of the pancreas and bile containing bicarbonates, can quickly neutralize the incoming acid in just 16 seconds (during the day, each of the secrets is released from 1.5 to 2.5 liters). Thus, the necessary slightly alkaline environment is created in the intestine, in which pancreatic enzymes are activated.
The pancreas is a vital organ. It not only performs a secretory digestive function, but also produces the hormones insulin and glucagon, which are not released into the intestinal lumen, but immediately enter the bloodstream and play the most important role in the regulation of sugar in the body.
Pancreatic juice is rich in enzymes that hydrolyze (break down) proteins, fats and carbohydrates. Proteolytic enzymes (trypsin, chymotrypsin, elastase, etc.) break down the internal bonds of a protein molecule to form amino acids and low molecular weight peptides that can pass through the villous layer of the small intestine into the blood. Enzymatic hydrolysis of fats is carried out by pancreatic lipase, phospholipase, cholesterolesterase. But these enzymes can only work with emulsified fats (emulsification is the splitting of large fat molecules into smaller ones by bile, preparation for processing with lipases). The end product of lipid hydrolysis is fatty acids, which then enter the lymphatic vessels in the intestinal space.
The breakdown of dietary carbohydrates (starch, sucrose, lactose), which began in the oral cavity, continues in the small intestine under the action of pancreatic enzymes in a slightly alkaline environment to the final monosaccharides (glucose, fructose, galactose).
Absorption is the process of transferring the products of hydrolysis of nutrients from the cavity of the gastrointestinal tract into the blood, lymph and intercellular space. As I mentioned, enzymes enter the intestinal lumen in an inactive form. Why? Because, if they were initially active, they would have digested the gland itself, which is what happens with acute pancreatitis (from the word "pancreas" - pancreas), which is accompanied by unbearable pain and requires immediate medical attention. Fortunately, chronic inflammation of the pancreas is more common due to digestive disorders, resulting in insufficient production of enzymes, which can be adjusted by diet and atraumatic (non-drug) treatment.
Let's pay a little more attention to the role of bile. Bile is produced by the liver, this process goes on continuously both day and night (1-2 liters are produced per day), but it increases during meals and is stimulated by certain chemical compounds (mediators) and hormones. I will mention only one substance - cholecystokinin-pancreozymin - an important stimulant of bile secretion, produced by the cells of the small intestine and entering the liver with the bloodstream. With inflammatory changes in the intestines, this hormone may not be produced. Of the products, the main stimulants of bile secretion are: oils (fats), egg yolks (contain bile acids), milk, meat, bread, magnesium sulfate. Through the bile ducts of the liver, bile enters the common bile duct, where on the way it can accumulate in the gallbladder (up to 50 ml), in which water is reabsorbed, leading to thickening of bile (another reason to drink enough water). If the bile is thick, and there are anatomical features of the location of the gallbladder (kinks, twists), then its movement becomes difficult, which can lead to stagnation and the formation of stones.
What is in bile? Bile acids; bile pigments (bilirubin); cholesterol and lecithin; slime; drug metabolites (if taken, the liver cleanses the body and removes them with bile). Bile must be sterile and have a pH of 7.8-8.2 (an alkaline environment allows for a bactericidal effect).
Functions of bile: emulsification of fats (preparation for further hydrolysis by pancreatic enzymes); dissolution of hydrolysis products (which ensures their absorption in the small intestine); increased activity of intestinal and pancreatic enzymes; ensuring the absorption of fat-soluble vitamins (A, D, E), cholesterol, calcium salts; bactericidal action on putrefactive flora; stimulation of the processes of bile formation and bile secretion, motor and secretory activity; participation in the programmed death and renewal of erythrocytes (apoptosis and proliferation of erythrocytes); removal of toxins.
How many functions it performs! And if, due to inflammation, thickening and other reasons, the secretion of bile is disturbed? But what if the liver (whose versatility should be singled out as a separate topic), with its toxic loads and disorders, does not produce enough bile? How many digestive mechanisms fail! And for the most part, we don’t want to pay attention to the signals with which the body notifies us of digestive disorders: increased gas formation, bloating after eating, belching, heartburn, bad breath, smell of secretions, pain and spasms, nausea and vomiting, and many other manifestations of non-digestion of food, the cause of which must be found and corrected, and not “suppress” the symptoms with medication.
Digestion in the large intestine
Further, everything that is not absorbed in the small intestine passes into the large intestine, where water is absorbed and fecal masses are formed for a long time. In the large intestine, friendly and unfriendly microorganisms live, which share the rest of the meal with us, fighting among themselves for the environment, and sometimes with our body. Do you think that no one lives in us? This is a whole world and a war of worlds ... Their diversity cannot be accurately calculated. Only in the intestines there are several hundred species of microorganisms. Some of them are friendly and beneficial to us, others give us trouble. Scientists have proven that bacteria can transmit information to each other, and that this is how resistance (resistance) to antibiotics and other medications is rapidly growing. They can hide from the immune cells of our body, releasing certain substances and becoming invisible to them. They mutate and adapt.
There is a real problem all over the world: how to prevent epidemics from developing again in conditions of insensitivity of microorganisms to existing drugs. One of its causes is the uncontrolled use of antibacterial drugs and immunomodulators, which are often used to quickly get rid of the symptoms of the disease, and are not always prescribed justifiably, just in case for prevention.
An important role in the development of pathogenic microflora is played by the internal environment. Friendly (symbiotic) microorganisms feel good in a slightly alkaline environment and love fiber. By eating it, they produce vitamins for us and normalize metabolism. Unfriendly (conditionally pathogenic), feeding on protein decay products, cause decay with the formation of substances toxic to humans - the so-called ptomains or "cadaveric poisons" (indoles, skatoles). The former help us maintain health, the latter take it away. Do we have the ability to choose who we will be friends with? Fortunately, yes! To do this, it is enough, at least, to be picky in food.
Pathogenic microorganisms grow and multiply using protein breakdown products as food. And this means that the more protein, indigestible foods (meat, eggs, dairy) and refined sugars in the diet, the more actively the processes of decay in the intestines will develop. As a result, acidification will occur, which will make the environment even more favorable for the development of conditionally pathogenic microflora. Our symbiote friends prefer food rich in plant fiber. Therefore, a diet with a low protein content and an abundance of vegetables, fruits and whole grain carbohydrates favorably affects the state of a healthy human microflora, which, in the course of its life, produces vitamins and breaks down fiber and other complex carbohydrates into simple substances that can be used as an energy resource for the intestinal epithelium. . In addition, food rich in fiber promotes peristaltic movements in the gastrointestinal tract, thereby preventing unwanted stagnation of food masses.
How does rotting food affect human health? Protein decay products are toxins that easily pass through the intestinal mucosa and enter the bloodstream, and then to the liver, where they are neutralized. But in addition to toxins, pathogenic microorganisms that produce them can also enter the bloodstream, which becomes a burden not only for the liver, but also for the immune system. If the flow of toxins is very rapid, the liver does not have time to neutralize them, as a result, poisons are spread throughout the body, poisoning every cell. All this does not pass without a trace for a person, and as a result of chronic poisoning, a person feels chronic fatigue. On a high-protein diet, due to the increased activity of immune cells, the permeability of capillaries and small blood vessels may increase, through which harmful bacteria and decay products can pass, which gradually leads to the development of foci of inflammation in the internal organs. And then the inflamed tissues swell, the blood supply and metabolic processes in them are disturbed, which ultimately contributes to the development of a wide variety of pathological conditions and diseases.
Stagnation of feces in violation of peristalsis and irregular emptying of the intestine also contributes to the maintenance of putrefactive processes, the release of toxins and the formation of inflammatory processes, both in the intestine itself and in the organs located nearby. So, for example, a sagging large intestine overstretched from feces can put pressure on the reproductive organs of women and men, causing inflammatory changes in them. The state of our physical and psycho-emotional health directly depends on the state of the processes in the large intestine and its regular emptying.
What I want you to remember
Our digestive organs work strictly according to the laws. Each section of the gastrointestinal tract has its own processes. It is very important to help your body be healthy. It is very important to pay attention to how and what you eat, since we need to eat to live. It is really important and physiological to maintain the correct acid-base balance, which is normally weakly alkaline, with the exception of the stomach. Food processing is a very complex, energy-intensive process, which is helped not by counting calories and useful components in the original product, but by simple actions.
These include:
- regular, preferably at the same time, intake of balanced meals;
- mindfulness while eating (understand what you are doing, enjoy the taste, do not “swallow” food in pieces, take your time, do not do other things while eating, do not mix incompatible, for example, protein and carbohydrate foods);
- following the biorhythms of the organs (the digestive organs are most active in the morning and not at all active in the evening, when other organs are already engaged in cleansing and restoring the body).
It is important to ensure that bowel movements are regular. And it is very important to drink enough water, which is needed not only to start enzyme systems, produce mucus, but also to cleanse the body as a whole.
Take care of yourself and stay healthy!
From the stomach, food enters the duodenum (the name refers to its length - 12 fingers folded horizontally). Here the food is not for long - only in order to be even more crushed.
In the structure of the duodenum, the so-called bulb is distinguished - a thickening in its upper part. As we have already said, in the chemical processing of food, in addition to salivary enzymes and enzymes of gastric juice, enzymes of the juice secreted by the pancreas and bile (which the liver secretes) are involved. It is in the bulb of the duodenum that the ducts from the liver and pancreas open.
Thus, in this bulb, the final breakdown of food occurs before it enters the small intestine, from where, in turn, the nutrients obtained during digestion enter the blood and lymph by absorption and are carried throughout the body. And it is the bulb that is fraught with danger - just in this place, under certain unfavorable conditions, a duodenal ulcer occurs.
As the processed food moves through the small intestine, its processing with intestinal juice ends and the substances necessary for the body are absorbed: proteins, fats and carbohydrates. (In the process of digestion, these substances have acquired the form of soluble compounds: amino acids, fatty acids and glucose, respectively.) All the blood flowing from the intestines passes through the liver - an extremely important digestive organ. The liver cleanses the blood, neutralizing the toxic substances formed during digestion.
Undigested food remnants enter the large intestine and move through it for about 12 hours. Thus, they gradually turn into feces, which exit the body through the rectum and anus.
Digestion in the small intestine
The small intestine performs excretory, secretory, motor and absorption functions.
The excretory function is that some of their ingredients enter the intestinal cavity from the blood and lymph, especially if their concentration in these biological fluids increases (water, salts, urea, etc.).
The secretory function is associated with the release of intestinal juice into the intestinal cavity, which plays an important role in digestion, which is the result of the active activity of enterocytes. Let us dwell on the secretory function of the small intestine.
Intestinal juice is 98% water and 2% dry residue: (pH - 8-8.6), which contains organic and inorganic substances. The latter include bicarbonates and salts of Na +, K +, Ca2 +, etc. The organic ones include urea, uric acid, amino acids, mucus and numerous enzymes that act on intermediate decomposition products, actually completing hydrolysis. 22 enzymes were found in intestinal juice: various proteases - leucine aminopeptidase, aminopeptidase, carboxypeptidase, tripeptidase, dipeptidase, acid cathepsins, enteropeptidase, etc. In addition, intestinal juice contains phosphatase, phosphorylase, nuclease, etc. Carbohydrases - sucrase, maltase, lactase, hydrolyzing the corresponding disaccharides.
The secretory function of the intestine is regulated by nervous and humoral mechanisms. Nervous regulation of secretion is carried out at the systemic (food center) and organ levels (ganglionic nerve cells of the intestinal tube wall, within which short reflex arcs are closed). At their expense, secretion can be enhanced (cholinergic and serotonergic systems) or inhibited (adrenergic systems). However, the latter type of regulation is under the control of the system.
Systemic regulation includes conditioned (visa, smell of food) and unconditioned food reflexes (irritation of numerous chemo- and mechanoreceptors of the intestinal mucosa with food gruel). It has been shown that the vagus nerve stimulates the secretion of juice only in 1/3 of the small intestine, in the remaining 2/3 it inhibits, however, the secretory function is also inhibited by sympathetic nerves. Known importance in the regulation of the secretory function of the small intestine have humoral mechanisms - enterocrinin, duocrinin (intestinal hormones), as well as acetylcholine, which stimulate its secretion, catecholamines (adrenaline and norepinephrine), which inhibit it.
The secretory function of the small intestine is studied experimentally by removing one or two ends of the intestine under the skin (Tiri or Tiri-Vella), in humans - by duodenal sounding (only for the secretory function of the duodenum).
Involvement of the pancreas in digestion
The main role in the digestion of food is played by the duodenum. Digestive enzymes enter it from the pancreas, and bile from the liver. Pancreatic juice is extremely important for digestion. During the day, the pancreas produces about 1.5-2 liters of thick juice. In addition to digestive juice, water and bicarbonate, it contains many digestive enzymes that break down nutrients. Only in this form can they be absorbed through the walls of the intestine and enter the cells with blood or lymph. The main nutrients needed by the human body are proteins, fats and carbohydrates. Proteins are made up of amino acids and are water soluble. Carbohydrates are the so-called. polysaccharides. Under the action of enzymes, these polysaccharides are broken down into water-soluble monosaccharides. Fats are made up of glycerol and fatty acids and must also be broken down into their constituent parts. Digestion begins in the mouth and continues in the stomach. Saliva contains enzymes that break down carbohydrates into monosaccharides; in gastric juice - hydrochloric acid and pepsin, which break down proteins.
The food mass coming from the stomach into the duodenum is mixed with pancreatic juice containing digestive enzymes: some of them break down proteins, others - carbohydrates, and others - fats. Proteases break down proteins, carbohydrases break down carbohydrates, and esterases break down fats. Pancreatic juice contains enzymes that break down nucleic acids. The secretion of pancreatic juice depends on food, for example, when eating bread, it is released more, when eating dairy products - less.
In violation of the secretion of pancreatic juice from the pancreas, the breakdown of proteins, carbohydrates, fats and nucleic acids is inevitably disturbed. As a result, the cells of the human body receive insufficient nutrients, and the glands are not able to function normally. As a rule, the secretion of pancreatic juice is impaired when the pancreas is diseased or in the presence of a tumor.
The role of bile in digestion
Bile in the duodenum creates favorable conditions for the activity of pancreatic enzymes, especially lipases. Bile acids emulsify fats, reducing the surface tension of fat droplets, which creates conditions for the formation of fine particles that can be absorbed without prior hydrolysis, and increase the contact of fats with lipolytic enzymes. Bile provides absorption in the small intestine of water-insoluble higher fatty acids, cholesterol, fat-soluble vitamins (D, E, K, A) and calcium salts, enhances the hydrolysis and absorption of proteins and carbohydrates, promotes the resynthesis of triglycerides in enterocytes.
Bile has a stimulating effect on the activity of intestinal villi, as a result of which the rate of absorption of substances in the intestine increases, participates in parietal digestion, creating favorable conditions for the fixation of enzymes on the intestinal surface. Bile is one of the stimulators of the secretion of the pancreas, small intestine juice, gastric mucus, along with enzymes involved in the processes of intestinal digestion, prevents the development of putrefactive processes, has a bacteriostatic effect on the intestinal flora. The daily secretion of bile in humans is 0.7-1.0 liters. Its constituent parts are bile acids, bilirubin, cholesterol, inorganic salts, fatty acids and neutral fats, lecithin.
Digestion in the large intestine
Gradually left with almost no substances useful to the body, the remnants of the food bolus are sent straight to the exit.
To do this, they pass through several sections of the large intestine, which has massive walls and a large lumen of the internal cavity.
Here, by the way, is the so-called caecum, which is very developed in herbivores and almost does not perform useful functions in humans. The fact is that it is in it that a large number of bacteria that digest fiber accumulate. And since fiber makes up a very small part of the human diet, the need for a massive caecum has disappeared.
Digestion in the large intestine is a simple process - only a mass of undigested residues remains from the food bolus. This, as a rule, is still the same fiber moistened with water, mixed with useful substances that the intestines simply did not have time to digest. All sections of the digestive system are separated from each other by one-way valves that do not allow food to pass in the opposite direction.
However, with severe vomiting, some of these valves may work in the opposite direction, and food can enter, for example, from the duodenum into the stomach. Medicine also describes extremely severe, sometimes incompatible with life, cases of the so-called "nightmare vomiting", when the urges reach such strength that masses from the farthest parts of the intestine are ejected into the stomach and esophagus.
Well, then the remnants of the food bolus linger for a short time in the lowest sections of the large intestine in order to be excreted from the body at the right time. On this, digestion in the intestines, as in the whole human body, comes to an end.
Digestion is the first step in metabolism. For the renewal and growth of body tissues, the intake of appropriate substances with food is necessary. Food products contain proteins, fats and carbohydrates, as well as vitamins, mineral salts and water necessary for the body. However, proteins, fats and carbohydrates contained in food cannot be absorbed by its cells in their original form. In the digestive tract, not only the mechanical processing of food takes place, but also chemical breakdown under the influence of the enzymes of the digestive glands, which are located along the gastrointestinal tract.
Digestion in the mouth. IN oral cavity hydrolysis of polysaccharides (starch, glycogen). os-Amylase of saliva cleaves the glycosidic bonds of glycogen and amylase and amylopectin molecules, which are part of the starch structure, with the formation of dextrins. The action of os-amylase in the oral cavity is short-term, but the hydrolysis of carbohydrates under its influence continues in the stomach due to the saliva entering here. If the contents of the stomach are processed under the influence of hydrochloric acid, then osamylase is inactivated and stops its action.
Digestion in the stomach. IN Digestion of food occurs in the stomach under the influence of gastric juice. The latter is produced by morphologically heterogeneous cells that are part of the digestive glands.
The secretory cells of the bottom and body of the stomach secrete acidic and alkaline secretions, and the cells of the antrum secrete only alkaline secretions. In humans, the volume of daily secretion of gastric juice is 2-3 liters. On an empty stomach, the reaction of gastric juice is neutral or slightly acidic, after eating it is strongly acidic (pH 0.8-1.5). The composition of gastric juice includes enzymes such as pepsin, gastrixin and lipase, as well as a significant amount of mucus - mucin.
In the stomach, the initial hydrolysis of proteins occurs under the influence of proteolytic enzymes of gastric juice with the formation of polypeptides. Here, about 10% of peptide bonds are hydrolyzed. The above enzymes are active only at the appropriate level of HC1. The optimal pH value for pepsin is 1.2-2.0; for gastrixin - 3.2-3.5. Hydrochloric acid causes swelling and denaturation of proteins, which facilitates their further cleavage by proteolytic enzymes. The action of the latter is realized mainly in the upper layers of the food mass adjacent to the wall of the stomach. As these layers are digested, the food mass shifts to the pyloric section, from where, after partial neutralization, it moves to the duodenum. In the regulation of gastric secretion, acetylcholine, gastrin, and histamine occupy the main place. Each of them excites secretory cells.
There are three phases of secretion: cerebral, gastric and intestinal. The stimulus for the appearance of secretion of the gastric glands in cerebral phase are all the factors that accompany the meal. At the same time, conditioned reflexes arising from the sight and smell of food are combined with unconditioned reflexes that are formed during chewing and swallowing.
IN gastric phase secretion stimuli arise in the stomach itself, when it is stretched, when exposed to the mucous membrane of the products of protein hydrolysis, some amino acids, as well as extractive substances of meat and vegetables.
Influence on the glands of the stomach occurs in third, intestinal, phase of secretion, when insufficiently processed gastric contents enter the intestines.
Duodenal secretin inhibits HCl secretion but increases pepsinogen secretion. A sharp inhibition of gastric secretion occurs when fat enters the duodenum. .
Digestion in the small intestine. In humans, the glands of the mucous membrane of the small intestine form intestinal juice, the total amount of which reaches 2.5 liters per day. Its pH is 7.2-7.5, but with increased secretion it can increase to 8.6. Intestinal juice contains over 20 different digestive enzymes. A significant release of the liquid part of the juice is observed with mechanical irritation of the intestinal mucosa. The products of digestion of nutrients also stimulate the secretion of juice rich in enzymes. Vasoactive intestinal peptide also stimulates intestinal secretion.
There are two types of food digestion in the small intestine: abdominal And membranous (parietal). The first is carried out directly by intestinal juice, the second - by enzymes adsorbed from the cavity of the small intestine, as well as by intestinal enzymes synthesized in intestinal cells and built into the membrane. The initial stages of digestion occur exclusively in the cavity of the gastrointestinal tract. Small molecules (oligomers) formed as a result of cavity hydrolysis enter the brush border zone, where they are further split. Due to membrane hydrolysis, predominantly monomers are formed, which are transported into the blood.
Thus, according to modern concepts, the assimilation of nutrients is carried out in three stages: cavity digestion - membrane digestion - absorption. The last stage includes processes that ensure the transfer of substances from the lumen of the small intestine into the blood and lymph. Absorption occurs mostly in the small intestine. The total absorptive surface area of the small intestine is approximately 200 m 2 . Due to the numerous villi, the surface of the cell increases by more than 30 times. Through the epithelial surface of the intestine, substances enter in two directions: from the lumen of the intestine into the blood and simultaneously from the blood capillaries into the intestinal cavity.
Physiology of bile formation and bile secretion. The process of bile formation occurs continuously both by filtering a number of substances (water, glucose, electrolytes, etc.) from the blood into the bile capillaries, and by active secretion of bile salts and sodium ions by hepatocytes. .
The final formation of bile occurs as a result of the reabsorption of water and mineral salts in the bile capillaries, ducts and gallbladder.
A person produces 0.5-1.5 liters of bile during the day. The main components are bile acids, pigments and cholesterol. In addition, it contains fatty acids, mucin, ions (Na +, K + , Ca 2+ , Cl - , NCO - 3), etc.; The pH of hepatic bile is 7.3-8.0, cystic - 6.0 - 7.0.
Primary bile acids (cholic, chenodeoxycholic) are formed in hepatocytes from cholesterol, combine with glycine or taurine and are excreted in the form of sodium salt of glycocholic and potassium salts of taurocholic acids. In the intestine, under the influence of microflora, they are converted into secondary bile acids - deoxycholic and lithocholic. Up to 90% of bile acids are actively reabsorbed from the intestine into the blood and returned to the liver through the portal vessels. Bile pigments (bilirubin, biliverdin) are products of the breakdown of hemoglobin, they give bile a characteristic color.
The process of bile formation and its secretion is associated with food, secretin, cholecystokinin. Among the products strong causative agents of bile secretion are egg yolks, milk, meat and fats. Eating and associated conditioned and unconditional reflex stimuli activate bile secretion. Initially, the primary reaction occurs: the gallbladder relaxes and then contracts. 7-10 minutes after a meal, a period of evacuation activity of the gallbladder begins, which is characterized by alternating contractions and relaxation and lasts 3-6 hours. After this period, the contractile function of the gallbladder is inhibited and hepatic bile begins to accumulate in it again.
Physiology of the pancreas. Pancreatic juice is a colorless liquid. During the day, the human pancreas produces 1.5-2.0 liters of juice; its pH is 7.5-8.8. Under the influence of pancreatic juice enzymes, intestinal contents are broken down to final products suitable for absorption by the body. -Amylase, lipase, nuclease are secreted in the active state, and trypsinogen, chymotrypsinogen, prophospholipase A, proelastase and procarboxypeptidases A and B are secreted as proenzymes. Trypsinogen is converted to trypsin in the duodenum. The latter activates prophospholipase A, proelastase, and procarboxypeptidases A and B, which are converted into phospholipase A, elastase, and carboxypeptidases A and B, respectively.
The enzymatic composition of pancreatic juice depends on the type of food taken: when carbohydrates are taken, mainly the secretion of amylase increases; proteins - trypsin and chymotrypsin; fatty foods - lipases. The composition of pancreatic juice includes bicarbonates, chlorides Na + , K + , Ca 2+ , Mg 2+ , Zn 2+ .
Pancreatic secretion is regulated by neuro-reflex and humoral pathways. Distinguish spontaneous (basal) and stimulating secretion. The first is due to the ability of pancreatic cells to automatism, the second - the influence on cells of neurohumoral factors that are included in the process of eating.
The main stimulators of pancreatic exocrine cells are acetylcholine and gastrointestinal hormones - cholecystokinin and secretin. They enhance the secretion of enzymes and bicarbonates by pancreatic juice. Pancreatic juice begins to be secreted 2-3 minutes after the start of eating as a result of reflex excitation of the gland from the receptors of the oral cavity. And then the impact of gastric contents on the duodenum releases the hormones cholecystokinin and secretin, which determine the mechanisms of pancreatic secretion.
Digestion in the large intestine. Digestion in the large intestine is practically absent. The low level of enzymatic activity is due to the fact that the chyme entering this section of the digestive tract is poor in undigested nutrients. However, the colon, unlike other sections of the intestine, is rich in microorganisms. Under the influence of bacterial flora, the remains of undigested food and components of digestive secretions are destroyed, resulting in the formation of organic acids, gases (CO 2, CH 4, H 2 S) and substances toxic to the body (phenol, skatole, indole, cresol). Some of these substances are neutralized in the liver, the other is excreted with feces. Of great importance are bacterial enzymes that break down cellulose, hemicellulose and pectins, which are not affected by digestive enzymes. These hydrolysis products are absorbed by the large intestine and used by the body. In the colon, microorganisms synthesize vitamin K and B vitamins. The presence of normal microflora in the intestine protects the human body and improves immunity. The remains of undigested food and bacteria, glued together by the mucus of the juice of the large intestine, form fecal masses. With a certain degree of stretching of the rectum, there is an urge to defecate and there is an arbitrary emptying of the intestine; reflex involuntary center of defecation is located in the sacral spinal cord.
Suction. The products of digestion pass through the mucous membrane of the gastrointestinal tract and are absorbed into the blood and lymph through transport and diffusion. Absorption occurs mainly in the small intestine. The mucous membrane of the oral cavity also has the ability to absorb, this property is used in the use of certain drugs (validol, nitroglycerin, etc.). Absorption practically does not occur in the stomach. It absorbs water, mineral salts, glucose, medicinal substances, etc. The duodenum also absorbs water, minerals, hormones, protein breakdown products. In the upper small intestine, carbohydrates are mainly absorbed in the form of glucose, galactose, fructose, and other monosaccharides. Protein amino acids are absorbed into the blood by active transport. The hydrolysis products of the main dietary fats (triglycerides) are able to penetrate the intestinal cell (enterocyte) only after appropriate physicochemical transformations. Monoglycerides and fatty acids are absorbed in enterocytes only after interaction with bile acids by passive diffusion. Having formed complex compounds with bile acids, they are transported mainly to the lymph. Some of the fats can enter directly into the bloodstream, bypassing the lymphatic vessels. The absorption of fats is closely related to the absorption of fat-soluble vitamins (A, D, E, K). Water-soluble vitamins can be absorbed by diffusion (eg, ascorbic acid, riboflavin). Folic acid is absorbed in a conjugated form; vitamin B 12 (cyanocobalamin) - in the ileum with the help of an internal factor, which is formed on the body and bottom of the stomach.
In the small and large intestines, water and mineral salts are absorbed, which come with food and are secreted by the digestive glands. The total amount of water that is absorbed in the human intestine during the day is about 8-10 liters, sodium chloride - 1 mol. The transport of water is closely related to the transport of Na + ions and is determined by it.
The concept of physiology can be interpreted as the science of the laws of operation and regulation of a biological system in conditions of health and the presence of diseases. Physiology studies, among other things, the vital activity of individual systems and processes, in a particular case, this is, i.e. the vital activity of the digestive process, the patterns of its work and regulation.
The very concept of digestion means a complex of physical, chemical and physiological processes, as a result of which, in the process, they are split into simple chemical compounds - monomers. Passing through the wall of the gastrointestinal tract, they enter the bloodstream and are absorbed by the body.
The digestive system and the process of digestion in the oral cavity
A group of organs is involved in the process of digestion, which is divided into two large sections: the digestive glands (salivary glands, glands of the liver and pancreas) and the gastrointestinal tract. Digestive enzymes are divided into three main groups: proteases, lipases, and amylases.
Among the functions of the digestive tract, one can note: the promotion of food, the absorption and excretion of undigested food residues from the body.
The process is born. During chewing, the food supplied in the process is crushed and moistened with saliva, which is produced by three pairs of large glands (sublingual, submandibular and parotid) and microscopic glands located in the mouth. Saliva contains the enzymes amylase and maltase, which break down nutrients.
Thus, the process of digestion in the mouth consists in the physical crushing of food, exerting a chemical effect on it and moisturizing it with saliva for ease of swallowing and continuing the digestion process.
Digestion in the stomach
The process begins with the fact that food, crushed and moistened with saliva, passes through the esophagus and enters the organ. Within a few hours, the food bolus experiences mechanical (muscle contraction when moving to the intestines) and chemical effects (gastric juice) inside the organ.
Gastric juice consists of enzymes, hydrochloric acid and mucus. The main role belongs to hydrochloric acid, which activates enzymes, promotes fragmentary cleavage, has a bactericidal effect, destroying a lot of bacteria. The enzyme pepsin in the composition of gastric juice is the main one, splitting proteins. The action of mucus is aimed at preventing mechanical and chemical damage to the shell of the organ.
What composition and amount of gastric juice will depend on the chemical composition and nature of food. The sight and smell of food contributes to the release of the necessary digestive juice.
As the digestion process progresses, food gradually and portionwise moves into the duodenum.
Digestion in the small intestine
The process begins in the cavity of the duodenum, where the food bolus is affected by pancreatic juice, bile and intestinal juice, since it contains the common bile duct and the main pancreatic duct. Inside this organ, proteins are digested into monomers (simple compounds) that are absorbed by the body. Learn more about the three components of chemical exposure in the small intestine.
The composition of pancreatic juice includes the enzyme trypsin, which breaks down proteins, which converts fats into fatty acids and glycerol, the enzyme lipase, as well as amylase and maltase, which break down starch into monosaccharides.
Bile is synthesized by the liver and stored in the gallbladder, from where it enters the duodenum. It activates the lipase enzyme, participates in the absorption of fatty acids, increases the synthesis of pancreatic juice, and activates intestinal motility.
Intestinal juice is produced by special glands in the inner lining of the small intestine. It contains over 20 enzymes.
There are two types of digestion in the intestine and this is its feature:
- cavitary - carried out by enzymes in the cavity of the organ;
- contact or membrane - performed by enzymes that are located on the mucous membrane of the inner surface of the small intestine.
Thus, food substances in the small intestine are actually completely digested, and the end products - monomers are absorbed into the blood. Upon completion of the digestion process, the digested food remains from the small intestine into the large intestine.
Digestion in the large intestine
The process of enzymatic processing of food in the large intestine is rather insignificant. However, in addition to enzymes, obligate microorganisms (bifidobacteria, Escherichia coli, streptococci, lactic acid bacteria) are involved in the process.
Bifidobacteria and lactobacilli are extremely important for the body: they have a beneficial effect on the functioning of the intestines, participate in the breakdown, ensure the quality of protein and mineral metabolism, enhance the body's resistance, and have an antimutagenic and anticarcinogenic effect.
Intermediate products of carbohydrates, fats and proteins are broken down here to monomers. Colon microorganisms produce (groups B, PP, K, E, D, biotin, pantothenic and folic acids), a number of enzymes, amino acids and other substances.
The final stage of the digestion process is the formation of fecal masses, which are 1/3 composed of bacteria, and also contain epithelium, insoluble salts, pigments, mucus, fiber, etc.
Absorption of nutrients .jpg)
Let's dwell on the process separately. It represents the ultimate goal of the digestion process, when food components are transported from the digestive tract to the internal environment of the body - blood and lymph. Absorption occurs in all parts of the gastrointestinal tract.
Absorption in the mouth is practically not carried out due to the short period (15 - 20 s) of food in the cavity of the organ, but not without exceptions. In the stomach, the absorption process partially covers glucose, a number of amino acids, dissolved alcohol. Absorption in the small intestine is the most extensive, largely due to the structure of the small intestine, which is well adapted to the suction function. Absorption in the large intestine concerns water, salts, vitamins and monomers (fatty acids, monosaccharides, glycerol, amino acids, etc.).
The central nervous system coordinates all nutrient absorption processes. Humoral regulation is also involved.
The process of protein absorption occurs in the form of amino acids and water solutions - 90% in the small intestine, 10% in the large intestine. Absorption of carbohydrates is carried out in the form of various monosaccharides (galactose, fructose, glucose) at different rates. Sodium salts play a role in this. Fats are absorbed in the form of glycerol and fatty acids in the small intestine into the lymph. Water and mineral salts begin to be absorbed in the stomach, but this process proceeds more intensively in the intestines.
Thus, it covers the process of digestion of nutrients in the mouth, in the stomach, in the small and large intestines, as well as the process of absorption.