Regulation of gastric secretion I.P. Pavlov conditionally subdivided into three phases. I phase - complex reflex(cerebral, cephalic) consists of conditioned and unconditional reflex mechanisms. The type of food, the smell of food, talking about it cause a conditioned reflex secretion of juice. Standing out juice I.P. Pavlov called appetizing, "fuse".
This juice prepares the stomach for food intake, has high acidity and enzymatic activity, so this juice in an empty stomach can have a damaging effect (for example, the type of food and the inability to eat it, chewing gum on an empty stomach). The unconditioned reflex is activated when food stimulates the receptors in the oral cavity.
Fig. 6 Scheme of the unconditioned reflex of the regulation of gastric secretion
1 - facial nerve, 2 - glossopharyngeal nerve, 3 - upper laryngeal nerve, 4 - sensory fibers of the vagus nerve, 5 - efferent fibers of the vagus nerve, 6 - postganglionic sympathetic fiber, G - gastrin-secreting cell.
The presence of a complex reflex phase of gastric secretion proves the experience of "imaginary feeding". The experiment is carried out on a dog that had previously undergone gastric fistula and esophagotomy (the esophagus was cut, and its ends were sutured into an incision in the skin of the neck). Experiments are carried out after the recovery of the animal. When feeding such a dog, food fell out of the esophagus without getting into the stomach, but gastric juice was released through the open fistula of the stomach. When feeding raw meat for 5 minutes, gastric juice is secreted for 45-50 minutes. The juice separated at the same time has high acidity and proteolytic activity. In this phase, the vagus nerve activates not only the cells of the gastric glands, but also G-cells that secrete gastrin (Fig. 6).
II phase of gastric secretion - gastric- associated with the flow of food into the stomach. Filling the stomach with food excites mechanoreceptors, information from which is sent along the sensitive fibers of the vagus nerve to its secretory nucleus. Efferent parasympathetic fibers of this nerve stimulate gastric secretion. Thus, the first component of the gastric phase is purely reflex (Fig. 6).
The contact of food and products of its hydrolysis with the gastric mucosa excites chemoreceptors and activates local reflex and humoral mechanisms. As a result Gpyloric cells secrete the hormone gastrin activating the main cells of the glands and, especially, parietal cells. Mast cells (ECL) secrete histamine, which stimulates parietal cells. Central reflex regulation is supplemented by long-term humoral regulation. The secretion of gastrin increases when the products of protein digestion appear - oligopeptides, peptides, amino acids and depends on the pH value in the pyloric section of the stomach. If the secretion of hydrochloric acid is increased, then less gastrin is released. At pH-1.0, its secretion stops, while the volume of gastric juice decreases sharply. Thus, self-regulation of the secretion of gastrin and hydrochloric acid is carried out.
Gastrin: stimulates the secretion of HCl and pepsinogens, enhances the motility of the stomach and intestines, stimulates pancreatic secretion, activates the growth and restoration of the gastric and intestinal mucosa.
In addition, food contains biologically active substances (for example, meat extractives, vegetable juices), which also excite mucosal receptors and stimulate sap secretion in this phase.
The synthesis of HCl is associated with the aerobic oxidation of glucose and the formation of ATP, the energy that is used by the system of active transport of H + ions. Built into the apical membrane H + / TO + ATPase, which pumps out of the cellH + ions in exchange for potassium. One theory suggests that the main supplier of hydrogen ions is carbonic acid, which is formed as a result of the hydration of carbon dioxide, this reaction is catalyzed by carbonic anhydrase. The carbonic acid anion leaves the cell through the basement membrane in exchange for chlorine, which is then excreted through the chloride channels of the apical membrane. Another theory considers water as a source of hydrogen (Fig. 7).
Fig.7. SecretionHClparietal cell and regulation of secretion. Ions H + are transferred into the lumen with the participation of H-K-ATPase, built into the apical membrane. ionsCl - enter the cell in exchange for HCO ions 3 - and excreted through the chloride channels of the apical membrane; H ions + formed from H 2 SO 3 and to a lesser extent from water.
It is believed that the parietal cells of the gastric glands are excited in three ways:
the vagus nerve has a direct effect on them through muscarinic cholinergic receptors (M-cholinergic receptors) and indirectly, by activating G-cells of the pyloric stomach.
gastrin has a direct effect on them through specific G-receptors.
gastrin activates ECL (mast) cells that secrete histamine. Histamine through H 2 receptors activates parietal cells.
Blockade of cholinergic receptors by atropine reduces the secretion of hydrochloric acid. Blockers of H 2 receptors and M-cholinergic receptors are used in the treatment of hyperacid conditions of the stomach. Inhibition of the secretion of hydrochloric acid causes the hormone secretin. Its secretion depends on the pH of the contents of the stomach: the higher the acidity of the chyme entering the duodenum, the more secretin is released. Fatty foods stimulate the secretion of cholecystokinin (HC). HC reduces the secretion of juice in the stomach and inhibits the activity of parietal cells. Reduce the secretion of hydrochloric acid and other hormones and peptides: glucagon, GIP, VIP, somatostatin, neurotensin.
III phase - intestinal- begins with the evacuation of chyme from the stomach into the small intestine. Irritation of the mechano-, chemoreceptors of the small intestine by the products of food digestion regulates secretion mainly due to local nervous and humoral mechanisms. Enterogastrin, bombesin, motilin are secreted by the endocrine cells of the mucous layer, these hormones increase sap secretion. VIP (vasoactive intestinal peptide), somatostatin, bulbogastron, secretin, GIP (gastroinhibiting peptide) - inhibit gastric secretion when fats, hydrochloric acid, and hypertonic solutions act on the small intestinal mucosa.
Thus, the secretion of gastric juice is under the control of central and local reflexes, as well as many hormones and biologically active substances.
The amount of juice, the rate of secretion and its composition depend on the quality of the food, as evidenced by the curves of juice secretion obtained in the laboratory of I.P. Pavlov when equal volumes of bread, meat, and milk are introduced into the stomach of dogs. The most powerful stimulants of gastric secretion are meat and bread. When consumed, a lot of juice with high proteolytic activity is released.
The formation and secretion of gastric juice is controlled by nervous and humoral mechanisms.
The separation of gastric juice occurs in 2 phases:
1) The first phase of secretion reflex secretion:
Definitely a reflex gastric juice is secreted by irritation of the olfactory receptors of the oral cavity, pharynx, esophagus;
conditioned reflex sap secretion occurs when visual, olfactory, auditory receptors are stimulated, i.e. the sight, the smell of food, etc.
The juice separated at the same time, Pavlov called fiery or appetizing - it prepares the stomach to receive food. This was studied in experiments with "imaginary feeding ”, when food is only in the oral cavity, but does not enter the stomach, but falls out through an opening in the esophagus.
2) Second phase of secretion gastric or neurohumoral, is associated with irritation of food receptors of the gastric mucosa: mechanical and chemical irritation → sensory neuron → medulla oblongata → motor neuron → working organ (juice secretion). Starts immediately after eating and lasts 2 hours.
Centers of nervous regulation:
Digestion, salivation,
juice secretion - medulla oblongata;
Hunger and satiety - diencephalon;
Taste area - forebrain
Defecation - spinal cord.
Strong irritants are the products of digestion of proteins (meat, fish, vegetable broths), mineral salts, water. The secretion of gastric juice occurs as long as there is food in the stomach: fatty foods are digested for 7-8 hours, carbohydrate foods are digested much faster.
Humoral phase of regulation : Gastric mucosa secretes hormone into blood gastrin, it enters the glands and occurs activation of secretion of gastric juice and regulation of peristalsis of the stomach and intestines (begins 2 hours after a meal, is carried out by the gastrointestinal tract's own hormones ( histamine, gastrin, secretin)). In addition, the hormones of the anterior pituitary and adrenal cortex contribute to the synthesis of digestive enzymes. sympathetic autonomic nervous system slows down, but parasympathetic – stimulates secretion of digestive juices.
A great merit in the study of the physiology of digestion belongs to Pavlov, who proposed and used the following methods: fistula method; The method of gastric fistula with transection of the esophagus (imaginary feeding); Formation of an "isolated ventricle".
With the help of the first two methods, the existence of the first phase of gastric secretion was proved, the third - the existence of the second phase of secretion.
The fistula of the stomach is displayed on the outer side of the abdominal wall. In experiments on the formation "isolated ventricle" when a small ventricle was surgically separated from the stomach, and a fistula was placed on it with preservation of innervation and blood supply, it was possible to obtain pure gastric juice. This made it possible to find out that the amount and composition of the secreted juice depends on the chemical composition of the food - more juice with the highest content of enzymes is released for protein foods, less for carbohydrates, and even less for fats.
Functions of the stomach:
Mechanical
The digestive tract (or gastrointestinal tract - GIT) is a muscular tube lined with a mucous membrane, the lumen of the tube is the external environment. The mucosa contains lymphatic follicles and may include simple exocrine glands (eg, in the stomach). The submucosa of some parts of the digestive tract (esophagus, duodenum) has complex glands. The excretory ducts of all exocrine glands of the digestive tract (including salivary, liver and pancreas) open on the surface of the mucous membrane. The gastrointestinal tract has its own nervous apparatus (enteric nervous system) and own system of endocrine cells (enteroendocrine system). The gastrointestinal tract, together with its large glands, forms a digestive system focused on the processing of incoming food. (digestion) and the flow of nutrients, electrolytes and water into the internal environment of the body (suction).
Each part of the gastrointestinal tract performs specific functions: the oral cavity - chewing and wetting with saliva, the pharynx - swallowing, the esophagus - the passage of food boluses, the stomach - deposition and initial digestion, the small intestine - digestion and absorption (2-4 hours after food enters the gastrointestinal tract) , colon and rectum - preparation and removal of feces (defecation occurs from 10 hours to several days after eating). Thus, the digestive system provides: - the movement of food, the contents of the small intestine (chyme) and feces from the mouth to the anus; - secretion of digestive juices and digestion of food; -absorption of digested foods, water and electrolytes; - the movement of blood through the digestive organs and the transfer of absorbed substances; -o excretion of feces; -o humoral and nervous control of all these functions.
Nervous regulation of gastrointestinal functions
Enteral nervous system- a set of own nerve cells (intramural neurons with a total number of about 100 million) of the gastrointestinal tract, as well as processes of autonomic neurons located outside the gastrointestinal tract (extramural neurons). Regulation of the motor and secretory activity of the gastrointestinal tract is the main function of the enteric nervous system. The wall of the gastrointestinal tract contains powerful networks of nerve plexuses.
Plexus(Fig. 22-1). The proper nervous apparatus of the digestive tract is represented by submucosal and intermuscular plexuses.
Intermuscular nerve plexus(Auerbach) is located in the muscular membrane of the digestive tract, consists of a ganglion-containing network of nerve fibers. The number of neurons in the ganglion varies from units to hundreds. The intermuscular nerve plexus is necessary primarily to control the motility of the digestive tube.
Rice. 22-1. enteric nervous system. 1 - longitudinal layer of the muscular membrane; 2 - intermuscular (Auerbach) nerve plexus; 3 - circular layer of the muscular membrane; 4 - submucosal (Meissner) nerve plexus; 5 - muscular layer of the mucous membrane; 6 - blood vessels; 7 - endocrine cells; 8 - mechanoreceptors; 9 - chemoreceptors; 10 - secretory cells
0 Submucosal nerve plexus(Meissner) is located in the submucosa. This plexus governs the contractions of the SMC of the muscular layer of the mucosa, as well as the secretion of the glands of the mucosa and submucosa.
Innervation of the gastrointestinal tract
0 parasympathetic innervation. Excitation of the parasympathetic nerves stimulates the intestinal nervous system, increasing the activity of the digestive tract. The parasympathetic motor pathway consists of two neurons.
0 sympathetic innervation. Excitation of the sympathetic nervous system inhibits the activity of the digestive tract. A neural circuit contains two or three neurons.
0 Afferents. Sensitive chemo- and mechanoreceptors in the membranes of the gastrointestinal tract form terminal branches of the own neurons of the enteric nervous system (Dogel cells of the 2nd type), as well as afferent fibers of the primary sensory neurons of the spinal nodes.
Humoral regulatory factors. In addition to classical neurotransmitters (for example, acetylcholine and norepinephrine), nerve cells of the enteric system, as well as nerve fibers of extramural neurons, secrete many biologically active substances. Some of them act as neurotransmitters, but most act as paracrine regulators of gastrointestinal functions.
Local reflex arcs. In the wall of the digestive tube there is a simple reflex arc, consisting of two neurons: sensitive (Dogel cells of the 2nd type), the terminal branches of the processes of which register the situation in different membranes of the digestive tract; and motor (Dogel cells of the 1st type), the terminal branches of the axons of which form synapses with muscle and glandular cells and regulate the activity of these cells.
Gastrointestinal reflexes. The enteric nervous system is involved in all reflexes that control the gastrointestinal tract. According to the level of closure, these reflexes are divided into local (1), closed at the level of the sympathetic trunk (2) or at the level of the spinal cord and the CNS stem (3).
0 1. Local reflexes control the secretion of the stomach and intestines, peristalsis and other activities of the gastrointestinal tract.
0 2. Reflexes involving the sympathetic trunk include gastrointestinal reflex, causing, when the stomach is activated, the evacuation of the contents of the large intestine; gastrointestinal a reflex that inhibits the secretion and motility of the stomach; ki-
gastrointestinal reflex(reflex from the colon to the ileum), inhibiting the emptying of the contents of the ileum into the colon. 0 3. Reflexes that close at the level of the spinal cord and brain stem include reflexes from the stomach and duodenum with pathways to the brainstem and back to the stomach via the vagus nerve(control the motor and secretory activity of the stomach); pain reflexes, causing general inhibition of the digestive tract, and defecation reflexes with tracts, going from the colon and rectum to the spinal cord and back (cause strong contractions of the colon and rectum and abdominal muscles necessary for defecation).
Humoral regulation of gastrointestinal functions
Humoral regulation of various functions of the gastrointestinal tract is carried out by various biologically active substances of an informational nature (neurotransmitters, hormones, cytokines, growth factors, etc.), i.e. paracrine regulators. Molecules of these substances (substance P, gastrin, gastrin-releasing hormone, histamine, glucagon, gastric inhibitory peptide, insulin, methionine-enkephalin, motilin, neuropeptide Y, neurotensin, calcitonin gene-related peptide, secretin, serotonin, somatostatin, cholecystokinin, epidermal growth factor, VIP, urogastron) come from enteroendocrine, nerve and some other cells located both in the wall of the gastrointestinal tract and outside it.
Enteroendocrine cells are found in the mucous membrane and are especially numerous in the duodenum. When food enters the lumen of the gastrointestinal tract, various endocrine cells, under the influence of wall stretching, under the influence of the food itself or changes in pH in the lumen of the gastrointestinal tract, begin to release hormones into the tissues and into the blood. The activity of enteroendocrine cells is under the control of the autonomic nervous system: vagus nerve stimulation (parasympathetic innervation) promotes the release of hormones that enhance digestion, and increased activity of the splanchnic nerves (sympathetic innervation) has the opposite effect.
Neurons. secreted from nerve endings gastrinreleasing hormone; peptide hormones come from the endings of nerve fibers, from the blood and from the own (intramural) neurons of the gastrointestinal tract: neuropeptide Y(secreted together with norepinephrine), related to the calcitonin gene peptide.
Other sources.Histamine secreted by mast cells, come from various sources serotonin, bradykinin, prostaglandin E.
Functions of biologically active substances in the digestive tract
Adrenaline and norepinephrinesuppress intestinal peristalsis and gastric motility, constrict lumen of blood vessels.
Acetylcholinestimulates all types of secretion in the stomach, duodenum, pancreas, as well as gastric motility and intestinal motility.
Bradykininstimulates motility of the stomach. Vasodilator.
VIPstimulates motility and secretion in the stomach, peristalsis and secretion in the intestines. Powerful vasodilator.
Substance P causes a slight depolarization of neurons in the ganglia of the intermuscular plexus, reduction MMC.
Gastrinstimulates secretion of mucus, bicarbonate, enzymes, hydrochloric acid in the stomach, suppresses evacuation from the stomach stimulates intestinal peristalsis and insulin secretion, stimulates cell growth in the mucosa.
Gastrin-releasing hormonestimulates secretion of gastrin and pancreatic hormones.
Histaminestimulates secretion in the glands of the stomach and peristalsis.
Glucagonstimulates secretion of mucus and bicarbonate, suppresses intestinal peristalsis.
Gastric inhibitory peptidesuppresses gastric secretion and gastric motility.
Motilinstimulates motility of the stomach.
Neuropeptide Ysuppresses gastric motility and intestinal peristalsis, reinforces vasoconstrictor effect of norepinephrine in many vessels, including celiac.
Peptide related to the calcitonin genesuppresses secretion in the stomach, vasodilator.
Prostaglandin Estimulates secretion of mucus and bicarbonate in the stomach.
Secretinsuppresses intestinal peristalsis, activates evacuation from the stomach stimulates secretion of pancreatic juice.
Serotoninstimulates peristalsis.
Somatostatinsuppresses all processes in the digestive tract.
Cholecystokininstimulates intestinal peristalsis, but suppresses motility of the stomach; stimulates Bile enters the intestines and secreted by the pancreas reinforces release-
insulin. Cholecystokinin is important for the process of slow evacuation of the contents of the stomach, relaxation of the sphincter Oddy.
epidermal growth factorstimulates regeneration of epithelial cells in the mucous membrane of the stomach and intestines.
The influence of hormones on the main processes in the digestive tract
Secretion of mucus and bicarbonate in the stomach.Stimulate: gastrin, gastrin-releasing hormone, glucagon, prostaglandin E, epidermal growth factor. Suppresses somatostatin.
Secretion of pepsin and hydrochloric acid in the stomach.Stimulate acetylcholine, histamine, gastrin. suppress somatostatin and gastric inhibitory peptide.
Motility of the stomach.Stimulate acetylcholine, motilin, VIP. suppress somatostatin, cholecystokinin, epinephrine, norepinephrine, gastric inhibitory peptide.
Intestinal peristalsis.Stimulate acetylcholine, histamine, gastrin (suppresses evacuation from the stomach), cholecystokinin, serotonin, bradykinin, VIP. suppress somatostatin, secretin, epinephrine, norepinephrine.
Secretion of pancreatic juice.Stimulate acetylcholine, cholecystokinin, secretin. Suppresses somatostatin.
bile secretion.Stimulate gastrin, cholecystokinin.
MOTOR FUNCTION OF THE DIGESTIVE TRACT
Electrical properties of myocytes. The rhythm of contractions of the stomach and intestines is determined by the frequency of slow waves of smooth muscles (Fig. 22-2A). These waves are slow, undulating changes in MP, on the crest of which action potentials (APs) are generated, which cause muscle contraction. The contraction occurs when the MP decreases to -40 mV (smooth muscle MP at rest ranges from -60 to -50 mV).
0 Depolarization. Factors that depolarize the SMC membrane: ♦ muscle stretch, ♦ acetylcholine, ♦ parasympathetic stimulation, ♦ gastrointestinal hormones.
0 Hyperpolarization myocyte membranes. It is caused by adrenaline, noradrenaline and stimulation of postganglionic sympathetic fibers.
Types of motor skills. Distinguish between peristalsis and mixing movements.
Rice. 22-2. Peristalsis. BUT.Above - slow waves of depolarization with numerous APs, at the bottom- recording abbreviations. B. Propagation of the wave of peristalsis. IN. Segmentation of the small intestine
^ Peristaltic movements- Propulsive (propulsive) movements. Peristalsis is the main type of motor activity that promotes food (Fig. 22-2B, C). Peristaltic contraction - the result of a local reflex - peristaltic reflex, or myoenteric reflex. Normally, the wave of peristalsis moves in the anal direction. The peristaltic reflex, together with the anal direction of movement of peristalsis, is called gut law.^ Mixing movements. In some departments, peristaltic contractions perform the function of mixing, especially where the movement of food is delayed by sphincters. Local alternating contractions may occur, clamping the intestine from 5 to 30 seconds, then new clamping in another place, etc. Peristaltic and pinching contractions are adapted to move and mix food in various parts of the digestive tract. CHEWING- the combined action of the chewing muscles, the muscles of the lips, cheeks and tongue. The movements of these muscles are coordinated by cranial nerves (V, VII, IX-XII pairs). Chewing control involves not only the nuclei of the brain stem, but also the hypothalamus, amygdala and cerebral cortex.
chewing reflex participates in a voluntarily controlled act of chewing (regulation of stretching of the masticatory muscles).
Teeth. The front teeth (incisors) provide cutting action, the back teeth (molars) - grinding. The chewing muscles develop, when compressing the teeth, a force of 15 kg for the incisors and 50 kg for the molars.
SWALLOWING subdivided into arbitrary, pharyngeal and esophageal phases.
Arbitrary phase begins with the completion of chewing and determining the moment the food is ready to be swallowed. The food bolus moves into the pharynx, pressing on the root of the tongue from above and having a soft palate behind. From this point on, swallowing becomes involuntary, almost completely automatic.
pharyngeal phase. The food bolus stimulates the receptor zones of the pharynx, nerve signals enter the brain stem (swallowing center) causing a series of contractions of the muscles of the pharynx.
Esophageal phase of swallowing reflects the main function of the esophagus - the rapid passage of food from the pharynx to the stomach. Normally, the esophagus has two types of peristalsis - primary and secondary.
F- Primary peristalsis- continuation of the wave of peristalsis, which begins in the pharynx. The wave passes from the pharynx to the stomach within 5-10 s. Fluid flows faster.
F- secondary peristalsis. If the primary peristaltic wave cannot move all the food from the esophagus into the stomach, then a secondary peristaltic wave occurs, caused by the stretching of the esophageal wall by the remaining food. Secondary peristalsis continues until all the food passes into the stomach.
F- Lower esophageal sphincter(gastroesophageal smooth muscle sphincter) is located near the junction of the esophagus with the stomach. Normally, there is a tonic contraction that prevents the contents of the stomach (reflux) from entering the esophagus. As the peristaltic wave moves down the esophagus, the sphincter relaxes. (receptive relaxation).
Motility of the stomach
In the wall of all parts of the stomach, the muscular membrane is strongly developed, especially in the pyloric (pyloric) part. The circular layer of the muscular membrane at the junction of the stomach into the duodenum forms the pyloric sphincter, which is constantly in a state of tonic contraction. The muscular membrane provides the motor functions of the stomach - the accumulation of food, mixing food with gastric secretions and turning it into a semi-dissolved form (chyme) and emptying the chyme from the stomach into the duodenum.
Hungry stomach contractions occurs when the stomach remains without food for several hours. Hungry contractions - rit-
mimic peristaltic contractions of the body of the stomach - can merge into a continuous tetanic contraction, which lasts 2-3 minutes. The severity of hungry contractions increases with a low level of sugar in the blood plasma.
Deposition of food. Food enters the cardiac region in separate portions. New portions push back the previous ones, which puts pressure on the wall of the stomach and causes vago-vagal reflex reducing muscle tone. As a result, conditions are created for the receipt of new and new portions, up to complete relaxation of the stomach wall, which occurs when the volume of the stomach cavity is from 1.0 to 1.5 liters.
Mixing food. In a stomach filled with food and relaxed, against the background of slow spontaneous fluctuations in the MP of smooth muscles, weak peristaltic waves arise - mixing waves. They spread along the wall of the stomach in the direction of the pyloric part every 15-20 s. These slow and weak peristaltic waves against the background of the appearance of PD are replaced by more powerful contractions of the muscular membrane. (peristaltic contractions), which, passing to the pyloric sphincter, also mixes the chyme.
Emptying of the stomach. Depending on the degree of digestion of food and the formation of liquid chyme, peristaltic contractions become more and more powerful, capable of not only mixing, but also moving chyme into the duodenum (Fig. 22-3). As gastric emptying progresses, peristaltic push contractions start from the upper parts of the body and the bottom of the stomach, adding their contents to the pyloric chyme. The intensity of these contractions is 5-6 times greater than the force of contractions of mixing peristalsis. Each strong wave of peristalsis squeezes out several
Rice. 22-3. Sequential phases of gastric emptying. A, B- pyloric sphincter closed.IN- pyloric sphincter open
milliliters of chyme into the duodenum, exerting a propulsive pumping action (pyloric pump).
Regulation of gastric emptying
❖ Rate of gastric emptying regulated by signals from the stomach and duodenum.
❖ Increasing the volume of chyme in the stomach promotes intensive emptying. This is not due to an increase in pressure in the stomach, but due to the implementation of local reflexes and increased activity of the pyloric pump.
❖ gastrin, released during stretching of the stomach wall, enhances the work of the pyloric pump and potentiates the peristaltic activity of the stomach.
❖ Evacuation stomach contents inhibited by gastrointestinal reflexes from the duodenum.
❖ Factors causing inhibitory gastrointestinal reflexes: acidity of the chyme in the duodenum, stretching of the wall and irritation of the mucous membrane of the duodenum, an increase in the osmolality of the chyme, an increase in the concentration of cleavage products of proteins and fats.
❖ Cholecystokinin, gastric inhibitory peptideinhibit gastric emptying.
Motility of the small intestine
Contractions of the smooth muscles of the small intestine mix and move the chyme in the intestinal lumen towards the large intestine.
Stirring abbreviations(Fig. 22-2B). Stretching of the small intestine causes agitating contractions (segmentations). Periodically squeezing the chyme with a frequency of 2 to 3 times per minute (the frequency is set slow electrical waves) segmentation ensures the mixing of food particles with digestive secretions.
Peristalsis. Peristaltic waves move through the intestine at a speed of 0.5 to 2.0 cm/sec. Each wave fades after 3-5 cm, so the movement of chyme is slow (about 1 cm / min): it takes 3 to 5 hours to pass from the pyloric sphincter to the ileocecal valve.
peristalsis control. Entry of chyme into the duodenum reinforces peristalsis. The gastrointestinal reflex, which occurs when the stomach is stretched and spreads along the intermuscular plexus from the stomach, as well as gastrin, cholecystokinin, insulin and serotonin, have the same effect. secretin and glucagon slow down motility of the small intestine.
Ileocecal sphincter(circular thickening of the muscular membrane) and the ileocecal valve (semilunar folds of the mucous membrane) prevent reflux - the contents of the large intestine enter the small intestine. The flap folds close tightly with increasing pressure in the caecum, withstanding a pressure of 50-60 cm of water. A few centimeters from the valve, the muscular membrane is thickened, this is the ileocecal sphincter. The sphincter normally does not completely cover the intestinal lumen, which provides slow emptying jejunum into the cecum. Caused by the gastrointestinal reflex fast emptying relaxes the sphincter, which significantly increases the movement of chyme. Normally, about 1500 ml of chyme enters the caecum daily.
Control of ileocecal sphincter function. Reflexes from the caecum control the degree of contraction of the ileocecal sphincter and the intensity of jejunal peristalsis. Stretching the caecum increases the contraction of the ileocecal sphincter and inhibits jejunal motility, delaying its emptying. These reflexes are realized at the level of the enteric plexus and extramural sympathetic ganglia.
Motility of the large intestine
In the proximal colon, absorption occurs predominantly (mainly the absorption of water and electrolytes), in the distal - the accumulation of feces. Any irritation of the colon can cause intense peristalsis.
Mixing abbreviations. The smooth muscle of the longitudinal layer of the muscular membrane from the caecum to the rectum is grouped in the form of three bands called ribbons. (taenia coli) which gives the large intestine the appearance of segmental sac-like extensions. The alternation of sac-like extensions along the colon ensures slow progression, mixing and tight contact of the contents with the mucous membrane. Pendulum contractions occur predominantly in segments, develop within 30 s and slowly relax.
Moving contractions- propulsive peristalsis in the form of slow and constant pendulum contractions. It takes at least 8-15 hours for the chyme to move from the ileocecal valve through the colon for the chyme to turn into a fecal mass.
Massive movement. From the beginning of the transverse colon to the sigmoid colon 1 to 3 times a day passes enhanced peristaltic wave- massive movement, promote-
the contents towards the rectum. During increased peristalsis, pendulum and segmental contractions of the colon temporarily disappear. A full series of enhanced peristaltic contractions lasts from 10 to 30 minutes. If the fecal masses are advanced into the rectum, then there is an urge to defecate. The occurrence of a massive movement of fecal masses after a meal is accelerated gastrointestinal and duodeno-intestinal reflexes. These reflexes arise as a result of stretching of the stomach and duodenum and are carried out by the autonomic nervous system.
Other reflexes also affect colonic motility. Abdomino-intestinal reflex occurs when the peritoneum is irritated, it strongly inhibits intestinal reflexes. Renal-intestinal and vesico-intestinal reflexes, arising from irritation of the kidneys and bladder, inhibit intestinal motility. Somato-intestinal reflexes inhibit intestinal motility when the skin of the abdominal surface is irritated.
defecation
functional sphincter. Usually the rectum is free of feces. This is the result of the tension of the functional sphincter located at the junction of the sigmoid colon with the rectum and the presence of an acute angle at this junction, which creates additional resistance to filling the rectum.
anal sphincters. The constant flow of feces through the anus is prevented by tonic contraction of the internal and external anal sphincters (Fig. 22-4A). internal anal sphincter- a thickening of the circular smooth muscle located inside the anus. External anal sphincter consists of striated muscles surrounding the internal sphincter. The external sphincter is innervated by the somatic nerve fibers of the pudendal nerve and is under conscious control. The unconditioned reflex mechanism constantly keeps the sphincter contracted until signals from the cerebral cortex slow down the contraction.
Defecation reflexes. The act of defecation is regulated by defecation reflexes.
❖ Own recto-sphincter reflex occurs when the wall of the rectum is stretched by fecal masses. Afferent signals through the intermuscular plexus activate peristaltic waves in the descending, sigmoid, and rectum, forcing the movement of feces toward the anus.
At the same time, the internal anal sphincter relaxes. If at the same time conscious signals are received to relax the external anal sphincter, then the act of defecation begins
❖ Parasympathetic defecation reflex involving segments of the spinal cord (Fig. 22-4A), enhances its own recto-sphincter reflex. Signals from the nerve endings in the wall of the rectum enter the spinal cord, the reverse impulse goes to the descending colon, sigmoid and rectum and anus along the parasympathetic fibers of the pelvic nerves. These impulses greatly increase the peristaltic waves and the relaxation of the internal and external anal sphincters.
❖ afferent impulses, entering the spinal cord during defecation, activate a number of other effects (deep breath, closing of the glottis and contraction of the muscles of the anterior abdominal wall).
GAS GASTROINTESTINAL TRACT. Sources of gases in the lumen of the gastrointestinal tract: swallowing air (aerophagy), bacterial activity, diffusion of gases from the blood.
Rice. 22-4. REGULATION OF MOTORITY (A) AND SECRETION(B). BUT- Parasympathetic mechanism of the defecation reflex. B- Phases of gastric secretion. II. Gastric phase (local and vagal reflexes, stimulation of gastrin release). III. Intestinal phase (nervous and humoral mechanisms). 1 - the center of the vagus nerve (medulla oblongata); 2 - afferents; 3 - trunk of the vagus nerve; 4 - secretory fibers; 5 - nerve plexuses; 6 - gastrin; 7 - blood vessels
Stomach. Gases in the stomach are a mixture of nitrogen and oxygen from the swallowed air, which is removed by belching.
Small intestine contains little gas coming from the stomach. In the duodenum, CO 2 accumulates due to the reaction between gastric hydrochloric acid and pancreatic bicarbonates.
Colon. The main amount of gases (CO 2 , methane, hydrogen, etc.) is created by the activity of bacteria. Some types of food cause significant gas from the anus: peas, beans, cabbage, cucumbers, cauliflower, vinegar. On average, 7 to 10 liters of gases are formed in the large intestine every day and about 0.6 liters are pushed out through the anus. The remaining gases are absorbed by the intestinal mucosa and excreted through the lungs.
SECRETORY FUNCTION OF THE DIGESTIVE TRACT
The exocrine glands of the digestive system secrete digestive enzymes from the oral cavity to the distal jejunum and secrete slime throughout the GI tract. Secretion is regulated by autonomic innervation and numerous humoral factors. Parasympathetic stimulation, as a rule, stimulates secretion, and sympathetic - suppresses.
SALIVA SECRETION. Three pairs of salivary glands (parotid, mandibular, sublingual), as well as many buccal glands, secrete from 800 to 1500 ml of saliva daily. Hypotonic saliva contains a serous component (including α-amylase for starch digestion) and a mucous component (mainly mucin for enveloping the food bolus and protecting the mucous membrane from mechanical damage). Parotid glands secrete serous secretions mandibular and sublingual- mucous and serous, buccal glands are only mucous. The pH of saliva ranges from 6.0 to 7.0. Saliva contains a large number of factors that inhibit the growth of bacteria (lysozyme, lactoferrin, thiocyanate ions) and bind Ag (secretory IgA). Saliva moistens food, envelops the food bolus for easier passage through the esophagus, carries out the initial hydrolysis of starch (a-amylase) and fats (lingual lipase). Stimulation of saliva secretion carries out impulses coming along parasympathetic nerve fibers from the upper and lower salivary nuclei of the brain stem. These nuclei are excited by gustatory and tactile stimuli from the tongue and other areas of the oral cavity and pharynx, as well as reflexes that occur in the stomach and upper intestine. Parasympathetic
This stimulation also increases blood flow in the salivary glands. Sympathetic stimulation affects the blood flow in the salivary glands in two phases: first it reduces, causing vasoconstriction, and then increases it.
SECRETARY FUNCTION OF THE ESOPHAGUS. The wall of the esophagus contains simple mucous glands throughout; and closer to the stomach and in the initial part of the esophagus - complex mucous glands of the cardiac type. The secret of the glands protects the esophagus from the damaging effect of incoming food and from the digestive action of the gastric juice thrown into the esophagus.
secretory function of the stomach
The exocrine function of the stomach is aimed at protecting the stomach wall from damage (including self-digestion) and digesting food. Surface epithelium The gastric mucosa produces mucins (mucus) and bicarbonate, thereby protecting the mucosa by forming a mucus-bicarbonate barrier. The mucous membrane in various parts of the stomach contains cardiac, fundic and pyloric glands. Cardiac glands produce mainly mucus, fundic glands (80% of all gastric glands) - pepsinogen, hydrochloric acid, internal factor of Castle and a certain amount of mucus; pyloric glands secrete mucus and gastrin.
Mucus bicarbonate barrier
The muco-bicarbonate barrier protects the mucosa from acid, pepsin, and other potential damaging agents.
Slime constantly secreted on the inner surface of the stomach wall.
Bicarbonate(ions HCO 3 -), secreted by superficial mucous cells (Fig. 22-5.1), has a neutralizing effect.
pH. The slime layer has a pH gradient. On the surface of the mucus layer, the pH is 2, and in the near-membrane part it is more than 7.
H+. The permeability of the plasmolemma of the gastric mucosal cells for H+ is different. It is insignificant in the cell membrane facing the lumen of the organ (apical), and quite high in the basal part. With mechanical damage to the mucous membrane and when it is exposed to oxidation products, alcohol, weak acids or bile, the concentration of H + in cells increases, which leads to cell death and destruction of the barrier.
Rice. 22-five. GASTROINTESTINAL SECRETION. I-. The mechanism of secretion of HC0 3 ~ by epithelial cells of the mucous membrane of the stomach and duodenum: A - the release of HC0 3 ~ in exchange for C1 ~ stimulates some hormones (for example, glucagon), and suppresses the transport blocker C1 ~ furosemide. B- active transport of HC0 3 ~, independent of C - transport. IN And G- transport of HC0 3 ~ through the membrane of the basal part of the cell into the cell and through the intercellular spaces (depends on hydrostatic pressure in the subepithelial connective tissue of the mucous membrane). II - Parietal cell. The system of intracellular tubules greatly increases the surface area of the plasma membrane. IN ATP is produced by numerous mitochondria to ensure the operation of the ion pumps of the plasma membrane
Rice. 22-5.
Continuation.III - Parietal cell: ion transport and secretion of HC1. Na+ ,K + -ATPase is involved in the transport of K + into the cell. C1 ~ enters the cell in exchange for HC0 3 ~ through the membrane of the lateral surface (1), and exits through the apical membrane; 2 - exchange of Na + for H +. One of the most important links is the release of H + through the apical membrane over the entire surface of the intracellular tubules in exchange for K + with the help of H +, K + -ATPase. IV - Regulation of the activity of parietal cells. The stimulatory effect of histamine is mediated through cAMP, while the effects of acetylcholine and gastrin are mediated through an increase in Ca 2+ influx into the cell. Prostaglandins reduce the secretion of HC1 by inhibiting adenylate cyclase, which leads to a decrease in the level of intracellular cAMP. H + , K + -ATPase blocker (for example, omeprazole) reduces the production of HC1. PC - protein kinase activated by cAMP; phosphorylates membrane proteins, enhancing the work of ion pumps.
Regulation. Secretion of bicarbonate and mucus amplify glucagon, prostaglandin E, gastrin, epidermal growth factor. To prevent damage and to restore the damaged barrier, antisecretory agents (eg, histamine receptor blockers), prostaglandins, gastrin, and sugar analogs (eg, sucralfate) are used.
Destruction of the barrier. Under unfavorable conditions, the barrier is destroyed within a few minutes, epithelial cells die, edema and hemorrhages occur in the own layer of the mucous membrane. Factors known to be unfavorable for maintaining the barrier: -Fnesteroid anti-inflammatory drugs (eg, aspirin, indomethacin); -Fethanol; -Psalts of bile acids; -f- Helicobacter pylori is a Gram-negative bacterium that survives in the acidic environment of the stomach. H. pylori affects the superficial epithelium of the stomach and destroys the barrier, contributing to the development of gastritis and ulcerative defect of the stomach wall. This microorganism is isolated from 70% of patients with gastric ulcer and 90% of patients with duodenal ulcer.
Regeneration epithelium, which forms a layer of bicarbonate mucus, occurs due to stem cells located at the bottom of the gastric pits; cell renewal time - about 3 days. Stimulants of regeneration: o gastrin from the endocrine cells of the stomach; o gastrin-releasing hormone from endocrine cells and vagus nerve fiber endings; o epidermal growth factor from salivary, pyloric, duodenal and other sources.
Slime. In addition to the superficial cells of the gastric mucosa, cells of almost all gastric glands secrete mucus.
Pepsinogen. The chief cells of the fundic glands synthesize and secrete pepsin precursors (pepsinogen) as well as small amounts of lipase and amylase. Pepsinogen has no digestive activity. Under the influence of hydrochloric acid and especially previously formed pepsin, pepsinogen is converted into active pepsin. Pepsin is a proteolytic enzyme active in an acidic environment (optimum pH from 1.8 to 3.5). At a pH of about 5, it has practically no proteolytic activity and is completely inactivated in a short time.
internal factor. For the absorption of vitamin B 12 in the intestine, the (intrinsic) factor of Castle, synthesized by the parietal cells of the stomach, is necessary. The factor binds vitamin B 12 and protects it from degradation by enzymes. The complex of intrinsic factor with vitamin B 12 in the presence of Ca 2 + ions interacts with epithelial receptors.
lial cell of the distal ileum. In this case, vitamin B 12 enters the cell, and the intrinsic factor is released. The absence of an intrinsic factor leads to the development of anemia.
Hydrochloric acid
Hydrochloric acid (HCl) is produced by parietal cells, which have a powerful system of intracellular tubules (Fig. 22-5.11), which significantly increase the secretory surface. The cell membrane facing the lumen of the tubules contains proton pump(H + ,K + -LTPase), pumping out H + from the cell in exchange for K +. Chlorine Bicarbonate Anion Exchanger built into the membrane of the lateral and basal surface of cells: Cl - enters the cell in exchange for HCO 3 - through this anion exchanger and exits into the lumen of the tubules. Thus, both components of hydrochloric acid are in the lumen of the tubules: both Cl - and H +. All other molecular components (enzymes, ion pumps, transmembrane carriers) are aimed at maintaining the ionic balance inside the cell, primarily at maintaining intracellular pH.
Regulation of hydrochloric acid secretion shown in fig. 22-5, IV. The parietal cell is activated through muscarinic cholinergic receptors (blocker - atropine), H 2 -histamine receptors (blocker - cimetidine) and gastrin receptors (blocker - proglumid). These blockers or their analogues, as well as vagotomy, are used to suppress the secretion of hydrochloric acid. There is another way to reduce the production of hydrochloric acid - the blockade of H +, K + -ATPase.
gastric secretion
The clinical terms "gastric secretion", "gastric juice" mean the secretion of pepsin and the secretion of hydrochloric acid, i.e. combined secretion of pepsin and hydrochloric acid.
Stimulants secretion of gastric juice: o pepsin(optimal enzyme activity at acidic pH values); about Cl- and H+(hydrochloric acid); about gastrin; about histamine; about acetylcholine.
Inhibitors and blockers secretion of gastric juice: o gastric inhibitory peptide; about secretin; about somatostatin; about receptor blockers gastrin, secretin, histamine and acetylcholine.
Phases of gastric secretion
Gastric secretion is carried out in three phases - cerebral, gastric and intestinal (Fig. 22-4B).
brain phase begins before food enters the stomach, at the time of eating. The sight, smell, taste of food increase secretion
gastric juice. The nerve impulses that trigger the brain phase come from the cerebral cortex and hunger centers in the hypothalamus and amygdala. They are transmitted through the motor nuclei of the vagus nerve and then through its fibers to the stomach. The secretion of gastric juice in this phase is up to 20% of the secretion associated with food intake.
Gastric phase begins when food enters the stomach. The incoming food causes vago-vagal reflexes, local reflexes of the enteric nervous system, and the release of gastrin. Gastrin stimulates the secretion of gastric juice within a few hours of food in the stomach. The amount of juice released into the gastric phase is 70% of the total secretion of gastric juice (1500 ml).
Intestinal phase is associated with the entry of food into the duodenum, which causes a slight increase in the secretion of gastric juice (10%) due to the release of gastrin from the intestinal mucosa under the influence of stretching and the action of chemical stimuli.
Regulation of gastric secretion by intestinal factors
Food that has entered the small intestine from the stomach inhibits the secretion of gastric juice. The presence of food in the small intestine causes inhibitory gastrointestinal reflex, carried out through the enteric nervous system, sympathetic and parasympathetic fibers. The reflex is initiated by stretching of the small intestine wall, the presence of acid in the cranial small intestine, the presence of protein breakdown products, and irritation of the intestinal mucosa. This reflex is part of a complex reflex mechanism that slows down the passage of food from the stomach to the duodenum.
The presence of acid, fat and protein breakdown products, hyper or hypoosmotic fluids, or any other irritant in the cranial small intestine causes the release of several intestinal peptide hormones - secretin, gastric inhibitory peptide and VIP. Secretin- the most important factor in stimulating the secretion of the pancreas - inhibits the secretion of the stomach. Gastric inhibitory peptide, VIP and somatostatin have a moderate inhibitory effect on gastric secretion. As a result, inhibition of gastric secretion by intestinal factors leads to a slowdown in the flow of chyme from the stomach to the intestine when it is already full. Secretion of the stomach after eating. The secretion of the stomach some time after eating (2-4 hours) is several
milliliters of gastric juice for every hour of the "interdigestive period". Mostly mucus and traces of pepsin are secreted, with little or no hydrochloric acid. However, emotional stimuli often increase secretion to 50 ml or more per hour with high levels of pepsin and hydrochloric acid.
secretory function of the pancreas
Every day the pancreas secretes about 1 liter of juice. Pancreatic juice (enzymes and bicarbonates) in response to gastric emptying flows through the long excretory duct. This duct, having connected with the common bile duct, forms the hepato-pancreatic ampulla, which opens on the large duodenal (Vater) papilla into the duodenum, being surrounded by a pulp from the MMC (sphincter of Oddi). The pancreatic juice entering the intestinal lumen contains digestive enzymes necessary for the digestion of carbohydrates, proteins and fats, and a large amount of bicarbonate ions that neutralize acidic chyme.
Proteolytic Enzymes- trypsin, chymotrypsin, carboxypeptidase, elastase, as well as nucleases that degrade DNA and RNA macromolecules. Trypsin and chymotrypsin break down proteins into peptides, while carboxypeptidase breaks down peptides into individual amino acids. Proteolytic enzymes are inactive (trypsinogen, chymotrypsinogen and procarboxypeptidase) and become active only after entering the intestinal lumen. Trypsinogen activates enterokinase from the cells of the intestinal mucosa, as well as trypsin. Chymotrypsinogen is activated by trypsin, and procarboxypeptidase is activated by carboxypeptidase.
Lipases. Fats are broken down by pancreatic lipase (hydrolyzes triglycerides, lipase inhibitor - bile salts), cholesterol esterase (hydrolyzes cholesterol esters) and phospholipase (cleaves fatty acids from phospholipids).
α-amylase(pancreatic) breaks down starch, glycogen and most carbohydrates into di- and monosaccharides.
Bicarbonate ions secreted by epithelial cells of small and medium ducts. The mechanism of secretion of HCO 3 - is considered in fig.
Phases of secretion pancreas are the same as gastric secretion - cerebral (20% of all secretion), gastric (5-10%) and intestinal (75%).
secretion regulation. The secretion of pancreatic juice is stimulated acetylcholine and parasympathetic stimulation cholecystokinin, secretin(especially with very acidic chyme) and progesterone. The action of secretion stimulants has a multiplier effect, that is, the effect of the simultaneous action of all stimuli is much greater than the sum of the effects of each stimulus separately.
bile secretion
One of the diverse functions of the liver is bile-forming (from 600 to 1000 ml per day). Bile is a complex aqueous solution consisting of organic compounds and inorganic substances. The main components of bile are cholesterol, phospholipids (mainly lecithin), bile salts (cholates), bile pigments (bilirubin), inorganic ions, and water. Bile (the first portion of bile) is constantly secreted by hepatocytes and through the duct system (here the second portion stimulated by secretin, containing many bicarbonate and sodium ions, is added to the bile) enters the common hepatic and then into the common bile duct. From here, hepatic bile is emptied directly into the duodenum or enters the cystic duct leading to the gallbladder. The gallbladder stores and concentrates bile. From the gallbladder, concentrated bile (cystic bile) is ejected in portions through the cystic and further through the common bile duct into the lumen of the duodenum. In the small intestine, bile is involved in the hydrolysis and absorption of fats.
Bile concentration. The volume of the gallbladder - from 30 to 60 ml,
but in 12 hours, up to 450 ml of hepatic bile can be deposited in the gallbladder, since water, sodium, chlorides and other electrolytes are constantly absorbed through the mucous membrane of the bladder. The main absorption mechanism is the active transport of sodium, followed by the secondary transport of chloride ions, water, and other components. Bile is concentrated 5 times, maximum - 20 times.
Emptying the gallbladder due to rhythmic contractions of its wall occurs when food (especially fatty) enters the duodenum. Efficient emptying of the gallbladder occurs with simultaneous relaxation of the sphincter of Oddi. The intake of a significant amount of fatty foods stimulates the complete emptying of the gallbladder within 1 hour. The stimulator of gallbladder emptying is cholecystokinin, additional stimuli come from the cholinergic fibers of the vagus nerve.
Functions of bile acids. Daily hepatocytes synthesize about 0.6 g of glycocholic and taurocholic bile acids. Bile acids - detergents, they reduce the surface tension of the fat particles, which leads to the emulsification of the fat. Moreover, bile acids promote the absorption of fatty acids, monoglycerides, cholesterol and other lipids. Without bile acids, more than 40% of dietary lipids are lost in the feces.
Enterohepatic circulation of bile acids. Bile acids are absorbed from the small intestine into the blood and through the portal vein enter the liver. Here they are almost completely absorbed by hepatocytes and secreted back into the bile. In this way, bile acids are circulated up to 18 times before they are gradually eliminated in the feces. This process is called enterohepatic circulation.
Secretory function of the small intestine
Up to 2 liters of secretions are produced daily in the small intestine (intestinal juice) with a pH of 7.5 to 8.0. The sources of the secret are the glands of the submucosa of the duodenum (Brunner's glands) and part of the epithelial cells of the villi and crypts.
Brunner's glands secrete mucus and bicarbonates. The mucus secreted by the Brunner glands protects the duodenal wall from the action of gastric juice and neutralizes the hydrochloric acid coming from the stomach.
Epithelial cells of villi and crypts. Goblet cells secrete mucus, and enterocytes secrete water, electrolytes, and enzymes into the intestinal lumen.
Enzymes. On the surface of enterocytes in the villi of the small intestine are peptidases(break down peptides into amino acids) disaccharidases sucrase, maltase, isomaltase and lactase (break down disaccharides into monosaccharides) and intestinal lipase(breaks down neutral fats to glycerol and fatty acids).
secretion regulation. secretion stimulate mechanical and chemical irritation of the mucous membrane (local reflexes), excitation of the vagus nerve, gastrointestinal hormones (especially cholecystokinin and secretin). Secretion is inhibited by influences from the sympathetic nervous system.
secretory function of the colon. Colon crypts secrete mucus and bicarbonates. The amount of secretion is regulated by mechanical and chemical irritation of the mucous membrane and local reflexes of the enteric nervous system. Excitation of the parasympathetic fibers of the pelvic nerves causes an increase in the separation of the
zi with simultaneous activation of peristalsis of the colon. Strong emotional factors can stimulate bowel movements with periodic mucus discharge without fecal content ("bear disease").
FOOD DIGESTION
Proteins, fats and carbohydrates in the digestive tract are converted into products that can be absorbed (digestion, digestion). Digestion products, vitamins, minerals and water pass through the epithelium of the mucous membrane and enter the lymph and blood (absorption). The basis of digestion is the chemical process of hydrolysis carried out by digestive enzymes.
Carbohydrates. The food contains disaccharides(sucrose and maltose) and polysaccharides(starches, glycogen), as well as other organic carbohydrate compounds. Cellulose in the digestive tract is not digested, since a person does not have enzymes capable of hydrolyzing it.
about Oral cavity and stomach.α-Amylase breaks down starch into the disaccharide maltose. During the short stay of food in the oral cavity, no more than 5% of all carbohydrates are digested. In the stomach, carbohydrates continue to be digested for an hour before the food is completely mixed with gastric juice. During this period, up to 30% of starches are hydrolyzed to maltose.
about Small intestine.α-Amylase of pancreatic juice completes the breakdown of starches to maltose and other disaccharides. Lactase, sucrase, maltase and α-dextrinase contained in the brush border of enterocytes hydrolyze disaccharides. Maltose is broken down to glucose; lactose - to galactose and glucose; sucrose - to fructose and glucose. The resulting monosaccharides are absorbed into the blood.
Squirrels
about Stomach. Pepsin, active at pH 2.0 to 3.0, converts 10-20% of proteins to peptones and some polypeptides. about Small intestine
♦ Pancreatic enzymes trypsin and chymotrypsin in the intestinal lumen cleave polypeptides into di- and tripeptides, carboxypeptidase cleaves amino acids from the carboxyl end of the polypeptides. Elastase digests elastin. In general, few free amino acids are formed.
♦ On the surface of the microvilli of the bordered enterocytes in the duodenum and jejunum, there is a three-dimensional dense network - the glycocalyx, in which numerous
peptidases. It is here that these enzymes carry out the so-called parietal digestion. Aminopolypeptidases and dipeptidases cleave polypeptides into di- and tripeptides, and di- and tripeptides are converted into amino acids. Then amino acids, dipeptides and tripeptides are easily transported into enterocytes through the microvilli membrane.
♦ In the border enterocytes there are many peptidases specific for the bonds between specific amino acids; within a few minutes, all remaining di- and tripeptides are converted into individual amino acids. Normally, more than 99% of the products of protein digestion are absorbed in the form of individual amino acids. Peptides are very rarely absorbed.
Fats are found in food mainly in the form of neutral fats (triglycerides), as well as phospholipids, cholesterol and cholesterol esters. Neutral fats are part of the food of animal origin, they are much less in plant foods. about Stomach. Lipases break down less than 10% of triglycerides. about Small intestine
♦ Digestion of fats in the small intestine begins with the transformation of large fat particles (globules) into tiny globules - fat emulsification(Fig. 22-7A). This process begins in the stomach under the influence of the mixing of fats with gastric contents. In the duodenum, bile acids and the phospholipid lecithin emulsify fats to a particle size of 1 µm, increasing the total surface of fats by 1000 times.
♦ Pancreatic lipase breaks down triglycerides into free fatty acids and 2-monoglycerides and is able to digest all chyme triglycerides within 1 minute if they are in an emulsified state. The role of intestinal lipase in the digestion of fats is small. The accumulation of monoglycerides and fatty acids at the sites of fat digestion stops the hydrolysis process, but this does not happen because micelles, consisting of several tens of bile acid molecules, remove monoglycerides and fatty acids at the moment of their formation (Fig. 22-7A). Cholate micelles transport monoglycerides and fatty acids to enterocyte microvilli, where they are absorbed.
♦ Phospholipids contain fatty acids. Cholesterol esters and phospholipids are cleaved by special pancreatic juice lipases: cholesterol esterase hydrolyzes cholesterol esters, and phospholipase L 2 breaks down phospholipids.
ABSORPTION IN THE DIGESTIVE TRACT
Absorption - the movement of water and substances dissolved in it - digestion products, as well as vitamins and inorganic salts from the intestinal lumen through a single-layered epithelium into the blood and lymph. In reality, absorption occurs in the small and partly in the large intestine; only liquids, including alcohol and water, are absorbed in the stomach.
Absorption in the small intestine
In the mucous membrane of the small intestine there are circular folds, villi and crypts. Due to the folds, the suction area increases 3 times, due to the villi and crypts - 10 times, and due to the microvilli of the border cells - 20 times. In total, folds, villi, crypts and microvilli provide a 600-fold increase in the area of absorption, and the total suction surface of the small intestine reaches 200 m 2 . The single-layered cylindrical squamous epithelium contains squamous, goblet, enteroendocrine, Panetian, and cambial cells. Absorption occurs through the border cells. Border cells(enterocytes) have more than 1000 microvilli on the apical surface. This is where the glycocalyx is present. These cells absorb digested proteins, fats and carbohydrates. about microvilli form a suction or brush border on the apical surface of enterocytes. Through the absorptive surface, active and selective transport occurs from the lumen of the small intestine through the border cells, through the basement membrane of the epithelium, through the intercellular substance of its own layer of the mucous membrane, through the wall of the blood capillaries into the blood, and through the wall of the lymphatic capillaries (tissue gaps) into the lymph. about Intercellular contacts. Since the absorption of amino acids, sugars, glycerides, etc. occurs through cells, and the internal environment of the body is far from being indifferent to the contents of the intestine (recall that the intestinal lumen is the external environment), the question arises of how the penetration of the intestinal contents into the internal environment through the spaces between epithelial cells is prevented. The “closing” of actually existing intercellular spaces is carried out due to specialized intercellular contacts that bridge the gaps between epithelial cells. Each cell in the epithelium along the entire circumference in the apical region has a continuous belt of tight junctions that prevent the entry of intestinal contents into the intercellular gaps.
about Water. The hypertonicity of the chyme causes the movement of water from the plasma into the chyme, while the transmembrane movement of water itself occurs through diffusion, obeying the laws of osmosis. Kamchatye crypt cells secrete Cl - into the intestinal lumen, which initiates the flow of Na +, other ions and water in the same direction. In the same time villus cells"pump" Na + into the intercellular space and thus compensate for the movement of Na + and water from the internal environment into the intestinal lumen. Microorganisms leading to the development of diarrhea cause water loss by inhibiting the absorption of Na + by the cells of the villi and by increasing the hypersecretion of Cl - by the cells of the crypts. The daily turnover of water in the digestive canal - income is equal to consumption - is 9 liters.
about Sodium. Daily intake of 5 to 8 g of sodium. From 20 to 30 g of sodium is secreted with digestive juices. To prevent loss of sodium excreted in the feces, the intestines need to absorb 25 to 35 g of sodium, which is approximately equal to 1/7 of the total sodium content in the body. Most Na + is absorbed through active transport (Fig. 22-6). Active transport of Na + is associated with the absorption of glucose, some amino acids and a number of other substances. The presence of glucose in the intestine facilitates the reabsorption of Na+. This is the physiological basis for restoring the loss of water and Na + in diarrhea by drinking salted water with glucose. Dehydration increases aldosterone secretion. Aldosterone within 2-3 hours activates all the mechanisms for enhancing the absorption of Na +. An increase in the absorption of Na + entails an increase in the absorption of water, Cl - and other ions.
about Chlorine. Ions Cl - are secreted into the lumen of the small intestine through ion channels activated by cAMP. Enterocytes absorb Cl - together with Na + and K +, and sodium serves as a carrier (Fig. 22-6, III). The movement of Na+ through the epithelium creates electronegativity of the chyme and electropositivity in the intercellular spaces. The Cl - ions move along this electrical gradient, "following" the Na + ions.
about Bicarbonate. The absorption of bicarbonate ions is associated with the absorption of Na+ ions. In exchange for Na+ absorption, H+ ions are secreted into the intestinal lumen, combine with bicarbonate ions and form H 2 CO 3 which dissociates into H 2 O and CO 2 . Water remains in the chyme, while carbon dioxide is absorbed into the blood and excreted by the lungs.
about Potassium. Some K+ ions are secreted along with mucus into the intestinal cavity; most of the K+ ions are absorbed
Rice. 22-6. ABSORPTION IN THE SMALL INTESTINE. I- Emulsification, breakdown and entry of fats into the enterocyte. II- Entry and exit of fats from the enterocyte. 1 - lipase; 2 - microvilli; 3 - emulsion; 4 - micelles; 5 - salts of bile acids; 6 - monoglycerides; 7 - free fatty acids; 8 - triglycerides; 9 - protein; 10 - phospholipids; 11 - chylomicron. III- The mechanism of secretion of HCO 3 - epithelial cells of the mucous membrane of the stomach and duodenum. BUT- release of HCO 3 - in exchange for Cl - stimulates some hormones (for example, glucagon), and suppresses the blocker of Cl transport - furosemide. B- active HCO 3 - transport, independent of Cl - transport. IN And G- transport of HCO 3 - through the membrane of the basal part of the cell into the cell and through the intercellular spaces (depends on hydrostatic pressure in the subepithelial connective tissue of the mucous membrane).
is transported through the mucosa by diffusion and active transport.
about Calcium. From 30 to 80% of the absorbed calcium is absorbed in the small intestine by active transport and diffusion. Active transport of Ca 2 + enhances 1,25-dihydroxycalciferol. Proteins activate Ca 2+ absorption, phosphates and oxalates inhibit it.
about other ions. Ions of iron, magnesium, phosphates are actively absorbed from the small intestine. With food, iron enters in the form of Fe 3 +, in the stomach iron passes into a soluble form of Fe 2 + and is absorbed in the cranial sections of the intestine.
about Vitamins. Water-soluble vitamins are absorbed very quickly; Absorption of fat-soluble vitamins A, D, E, and K is dependent on fat absorption. If there are no pancreatic enzymes or bile does not enter the intestine, then the absorption of these vitamins is impaired. Most vitamins are absorbed in the cranial small intestine, with the exception of vitamin B 12 This vitamin combines with intrinsic factor (a protein secreted in the stomach) and the resulting complex is absorbed in the ileum.
about Monosaccharides. The absorption of glucose and fructose in the brush border of enterocytes of the small intestine is provided by the GLUT5 carrier protein. GLUT2 of the basolateral part of enterocytes implements the release of sugars from the cells. 80% of carbohydrates are absorbed mainly in the form of glucose - 80%; 20% are fructose and galactose. The transport of glucose and galactose depends on the amount of Na + in the intestinal cavity. A high concentration of Na + on the surface of the intestinal mucosa facilitates, and a low concentration inhibits the movement of monosaccharides into epithelial cells. This is because glucose and Na+ share a common carrier. Na + moves into the intestinal cells along the concentration gradient (glucose moves with it) and is released in the cell. Further, Na + actively moves into the intercellular spaces, and glucose, due to secondary active transport (the energy of this transport is provided indirectly due to the active transport of Na +), enters the blood.
about Amino acids. Absorption of amino acids in the intestine is realized with the help of carriers encoded by genes SLC. Neutral amino acids - phenylalanine and methionine - are absorbed through secondary active transport due to the energy of active sodium transport. Na +-independent carriers carry out the transfer of a part of neutral and alkaline amino acids. Special carriers transport dipeptides and tripepep
Tids into enterocytes, where they are broken down into amino acids and then, by simple and facilitated diffusion, enter the intercellular fluid. Approximately 50% of digested proteins come from food, 25% from digestive juices, and 25% from discarded mucosal cells. Fats(Fig. 22-6, II). Monoglycerides, cholesterol and fatty acids delivered by micelles to enterocytes are absorbed depending on their size. Fatty acids containing less than 10-12 carbon atoms pass through the enterocytes directly into the portal vein and from there enter the liver in the form of free fatty acids. Fatty acids containing more than 10-12 carbon atoms are converted into triglycerides in enterocytes. Some of the absorbed cholesterol is converted into cholesterol esters. Triglycerides and cholesterol esters are sheathed with proteins, cholesterol, and phospholipid to form chylomicrons that leave the enterocyte and enter the lymphatic vessels. absorption in the large intestine. About 1500 ml of chyme passes through the ileocecal valve every day, but the colon absorbs 5 to 8 liters of fluid and electrolytes daily. Most of the water and electrolytes are absorbed in the large intestine, leaving no more than 100 ml of liquid and some Na + and Cl - in the stool. Absorption occurs predominantly in the proximal colon, with the distal colon serving to store waste and form feces. The mucous membrane of the large intestine actively absorbs Na + and with it Cl - . The absorption of Na + and Cl - creates an osmotic gradient that causes the movement of water through the intestinal mucosa. The colonic mucosa secretes bicarbonates in exchange for an equivalent amount of absorbed Cl - . Bicarbonates neutralize the acidic end products of colon bacteria.
The formation of feces. The composition of feces includes 3/4 water and 1/4 solid matter. The dense substance contains 30% bacteria, 10 to 20% fat, 10-20% inorganic substances, 2-3% protein and 30% undigested food residues, digestive enzymes, and desquamated epithelium. Colon bacteria are involved in the digestion of a small amount of cellulose, form vitamins K, B 12, thiamine, riboflavin and various gases (carbon dioxide, hydrogen and methane). The brown color of feces is determined by bilirubin derivatives - stercobilin and urobilin. The smell is created by the activity of bacteria and depends on the bacterial flora of each individual and the composition of the food taken. Substances that give feces a characteristic odor are indole, skatole, mercaptans and hydrogen sulfide.
Questions at the beginning of the paragraph.
Question 1. What methods were used to study digestion by IP Pavlov?
To study digestion, Pavlov used the fistula method. Fistula - an artificially created opening for the removal of products that are in the cavity organs or glands. So, in order to investigate the secretions of the salivary gland, IP Pavlov brought one of its ducts out and collected saliva. This made it possible to obtain it in its pure form and study the composition. It was found that saliva is released both when food enters the oral cavity and when it is seen, but on condition that the animal is familiar with the taste of this food.
Question 2. What is the difference between unconditioned and conditioned reflexes?
At the suggestion of IP Pavlov, reflexes were divided into unconditional and conditional.
Unconditioned reflexes are innate reflexes inherent in all individuals of a given species. With age, they can change, but according to a strictly defined program, the same for all individuals of this species. Unconditioned reflexes are a reaction to vital events: food, danger, pain, etc.
Conditioned reflexes are reflexes acquired during life. They enable the body to adapt to changing conditions, to accumulate life experience.
Question 3. How does hunger and satiety occur?
Question 4. How is the humoral regulation of digestion carried out?
After the nutrients are absorbed into the blood, the humoral separation of gastric juice begins. Among the nutrients there are biologically active substances, which, for example, are found in vegetable and meat broths. The products of their breakdown through the gastric mucosa are absorbed into the blood. With the blood flow, they enter the glands of the stomach, and they begin to intensely secrete gastric juice. This allows for long-term juice secretion: proteins are digested slowly, sometimes for 6 hours or more. Thus, gastric juice secretion is regulated by both nervous and humoral pathways.
Questions at the end of the paragraph.
Question 1. Is salivation in a dog that looks like a feeder with food - a conditioned or unconditioned reflex?
This reflex is conditional.
Question 2. How do sensations of hunger and satiety arise?
The feeling of hunger occurs when the stomach is empty and disappears when it is filled, and there is a feeling of satiety. There is an inhibitory reflex to the filling of the stomach, which warns against overeating.
Question 3. How is the humoral regulation of gastric juice secretion carried out?
The cleavage products of biologically active substances are absorbed into the blood through the gastric mucosa. With the blood flow, they enter the gastric glands and cause juice secretion, which continues throughout the entire time that the food is in the stomach.
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