Gastrointestinal physiology in the stomach

2021-06-13 10:45 PM

When you are hungry, your stomach muscles contract, when you swallow a food pill, the muscle relaxes just enough to hold that food pill, so the pressure in the stomach does not increase.

The stomach is the middle part of the alimentary canal, the upper part connects to the oesophagus through the cardia, the lower part connects to the small intestine through the pylorus, divided into 3 parts: the fundus, the body and the cavernous (figure).

 

Figure: Structure of the stomach.

The stomach has two digestive functions:

Contains food.

Continue preliminary digestion of food.

Food storage function

Because the stomach is the largest part of the digestive tract and its muscles are very elastic, the stomach has a very large capacity, up to several litres.

When you are hungry, your stomach muscles contract. When we swallow a food pill, the muscle relaxes just enough to hold that food pill, so the pressure in the stomach does not increase, making it easier for food to continue to enter the stomach.

The more food comes in, the more the stomach muscles relax, and when the muscles are fully relaxed, the pressure in the stomach suddenly increases, causing a feeling of fullness.

In gastritis, gastric muscle tone increases, stomach capacity decreases, patients eat quickly and lose appetite.

Towards the end of the meal, food is stored in the trunk in an orderly fashion:

The food that enters first lies around in contact with the gastric mucosa.

Food that comes in is in the middle.

Due to such arrangement, in the first stage after eating, there are 2 processes of food digestion in the stomach:

The food lying around has absorbed gastric juice and is digested by gastric juice.

The food in the middle has not yet absorbed the gastric juice, the pH is still neutral, so the salivary amylase will continue to break down the cooked starch for a while longer until the food in the middle is also absorbed by the gastric juice, then the salivary amylase will stop working. .

Mechanical activity of the stomach

Open and close the heart

Normally, the stomach is closed, when the swallowing action brings a food tablet down right above the cardia, the food will stimulate the intestinal reflex, causing the cardia to open and the food to enter the stomach. The food that has just entered will stimulate the stomach, causing the intestinal reflex to close the cardia. The caldarium will continue to open as the swallowing action continues to bring another pellet down just above the cardia.

When the food in the stomach is too acidic, the stomach can easily open even though there is no food in the oesophagus, causing heartburn symptoms in some patients with peptic ulcers.

The cardia is also easy to open when the pressure in the stomach increases too high: either by eating too much or by some stimulants acting on the vomiting centre, causing the stomach muscles, diaphragm, abdominal wall muscles to contract strongly. and suddenly, the contents of the stomach will be vomited out.

In children, the heart is often not closed tightly, so the child is prone to vomiting after eating.

Peristalsis of the stomach

When food enters the stomach, peristalsis begins. Those are contraction waves that spread from the body area to the stomach cave area, about every 15-20 seconds, the closer to the cave area, the stronger the peristalsis.

Peristalsis of the stomach has 2 effects:

Grind the food further and mix the food with the gastric juices to form chyme.

Push the chyme lying around down into the antrum and press on this chyme mass with great pressure to open the pylorus, pushing the chyme down into the duodenum. When the patient has pyloric stenosis, in order to push the chyme through the pylorus, peristalsis must increase very strongly, causing abdominal pain and Bouveret's sign, one of the signs to diagnose pyloric stenosis.

In addition, when the environment in the stomach is too acidic, peristalsis also increases sharply, causing abdominal pain in some patients with peptic ulcers.

Open and close the subject

Whenever peristalsis spreads to the cavernous region, the chyme is strongly pressed, causing the pylorus to open and a small amount of the chyme to be pushed into the duodenum. The chyme that has just entered will stimulate the duodenum causing intestinal reflex to close the pylorus.

The pylorus will continue to open under the effect of 2 conditions:

New peristalsis spread to the cave area.

The chyme that has just entered the duodenum has been alkalized.

The opening and closing of the pylorus have the following effects:

Introduce the chyme into the duodenum slowly little by little for more thorough digestion and absorption.

Although we eat several meals a day, digestion and absorption take place almost throughout the day. Therefore, the process of supplying materials to the body also takes place continuously and regularly, keeping homeostasis.

Prevent the duodenum from being irritated by a large amount of overly acidic chyme. When the mechanism of opening and closing the pylorus is lost, for example, a patient with pyloric stenosis has to undergo gastrojejunostomy surgery, the chyme from the stomach through the opening goes down to the duodenum massively, stimulating the duodenum very strongly, causing a syndrome. Dumping syndrome has the following manifestations: a short time after eating, the patient has symptoms of sweating, pale skin, weakness in the limbs, epigastric pain, diarrhoea, low blood pressure and low blood pressure. may faint. To prevent this situation, we should give the patient several meals a day, eating only a little at a time.

Excretion of gastric juice

Figure: Structure of the gastric gland.

Gastric juice is the digestive juice of the stomach secreted by the gastric mucosal glands. Depending on the composition of secretions, these glands can be divided into 2 groups:

Glands in the cardia and pylorus: secrete mucus

Gland in the trunk region: is the main digestive gland of the stomach, consisting of 3 types of cells:

Main cells: secrete enzymes.

Parietal cells: secrete hydrochloric acid and endogenous factor.

Glandular cells: secrete mucus.

In addition, the entire gastric mucosa secretes HCO3- and some mucus.

Gastric juice is a mixture of secretions from the upper regions about 2 - 2.5 liters/24 hours (figure).

Digestive enzyme group

Pepsin:

Is a protid-digesting enzyme that is excreted in an inactive form as pepsinogen, in a pH < 5.1 environment, pepsinogen is activated into active pepsin, which has the effect of cutting peptide bonds (- CO - NH -) part (- NH -) belongs to aromatic amino acids (tyrosin, phenylalanine). So it just hydrolyzes the protein into different long and short polypeptide chains:

Long chain: called protein.

Short chain: called peptone.

gastric lipase.

Is a lipid-digesting enzyme operating in an acidic environment, which hydrolyzes the already emulsified triglycerides in food (triglycerides in milk, egg yolks) into glycerol and fatty acids.

Chymosin (rennin, presur, lab- ferment):

As an enzyme that digests milk, it plays an important role in breastfed babies. It has the effect of breaking down a special protein in milk, caseinogen, into casein, which causes the milk to curdle, the casein will be retained in the stomach for digestion by pepsin, and the other parts in milk called the emulsion are quickly taken to the intestines. Thanks to that, the baby's stomach is small, but in one feeding it can absorb a much larger amount of milk than the stomach volume.

Acid HCl

It is not a digestive enzyme but plays a very important role in the digestive process because it has the following effects:

Increases pepsin activity through the following mechanisms:

Activation of pepsinogen to pepsin.

Create a suitable pH environment for pepsin to work.

Break down the connective tissue that surrounds the muscle mass so that pepsin breaks down the protein portion of the muscle mass. The combination of hydrochloric acid and pepsin has a very strong protid digestion effect.

Antiseptic: destroy bacteria from the outside entering the stomach with food to avoid infection through the digestive tract.

Hydrolysis of cellulose of young vegetables.

Contributes to the opening and closing mechanism of the cardia and pylorus.

However, HCl acid is a double-edged sword, when its secretion increases or in case the resistance of the gastric mucosa decreases, HCl acid will coordinate with pepsin to destroy the gastric mucosa causing gastric ulcers. thick.

HCl acid is secreted by parietal cells by the following mechanism:

The parietal cells secrete HCl in the form of H+ and Cl-. H+ is actively transported from the parietal cells into the gastric juice to exchange with K+ from the gastric juice under the action of the enzyme H+-K+ATPase (this enzyme is also known as the proton pump).

Therefore, one of the principles of gastric ulcer treatment is to use drugs that inhibit the enzyme H +-K + ATPase to reduce the secretion of HCl acid by the parietal cells. These drugs are called proton pump inhibitors (omeprazole, lansoprazole, etc.).

Intrinsic factors

Secreted by parietal cells, it is essential for the absorption of vitamin B12 in the small intestine. When B12 enters the stomach, it is taken up by endothelial factor to form a B12-internal factor complex. When it reaches the ileum, this complex is received by a specific type of receptor and vitamin B12 is absorbed into the bloodstream.

Because B12 is a vitamin involved in the production of red blood cells, this factor is also known as the internal anti-anemia factor.

When there is a lack of intrinsic factor (gastrectomy, gastric mucosal atrophy...) the patient will have megaloblastic anemia (Biermer).

HCO3-

Due to secretion by gastric mucosal cells, there are protective effects of gastric mucosa through 2 mechanisms:

Neutralize a part of hydrochloric acid in gastric juice when there is an increase in acid secretion.

Binds to mucus forming a protective barrier of the gastric mucosa.

Gum

It is a glycoprotein secreted from the pyloric glands, the cardia, the gonadal cells of the body glands and from the entire gastric mucosal cells.

Mucus combines with HCO3- to form a durable membrane about 1 - 1.5 mm thick that covers the entire gastric mucosa forming a mucous-bicarbonate barrier that protects the gastric mucosa against reverse diffusion of H+ from gastric juice into the gastric mucosa.

However, when there is an abnormal increase in the secretion of HCl and pepsin acid or there is a decrease in the secretion of mucus and HCO3-, the H+ and pepsin will penetrate into the gastric mucosa, damage and cause a gastric ulcer.

Therefore, agents that damage the mucous-bicarbonate barrier such as alcohol, spicy substances, sour substances, bile salts, pain relievers and anti-inflammatory drugs... can cause stomach ulcers. Conversely, factors that increase the strength of this barrier will be used to treat gastric ulcers (eg, Cytotec, sucralfate, colloidal bismuth subcitrate...).      

Regulate gastric secretion

Gastric juice is secreted by two regulatory mechanisms: nervous and humoral.

Nervous mechanism:

There are two nervous systems involved in the regulation of gastric secretion:

Intrinsic nerves:

As the Meissner plexus located just below the gastric mucosa, this plexus secretes gastric juice under the action of food stimulation in the stomach or from the stimulation of the central nervous system.

Central neutral system:

Is the number X nerve. The X cord secretes gastric juice under the stimulation effect of two types of reflexes: unconditioned reflex and conditioned reflex, the agents causing these two reflexes are similar to those in the salivary secretion mechanism.

Therefore, in the treatment of peptic ulcer disease, we can use methods to inhibit the effect of the X cord to reduce the secretion of HCl acid and pepsin.

Mechanism of translation:

There are many factors that regulate gastric secretion through the humoral mechanism:

Gastrin:

Is a hormone secreted by G cells in the cavernous region under the stimulation effect of the X cord or of the digestive products of protid in the stomach (peptone, protease). In addition, when the tension of the stomach wall increases, gastrin secretion is also stimulated.

After secretion, gastrin follows the blood to the body of the stomach, stimulating the glands to secrete HCl acid and pepsinogen. When the food in the cave is too acidic, it will inhibit gastrin secretion [feed back (-)]

In the surgical treatment of peptic ulcer disease, it is common to cut the cavernous area (where gastrin is secreted), to reduce the secretion of HCl acid.

Gastrin-like:

As a hormone secreted by the duodenal and pancreatic mucosa, the effect is similar to gastrin. When the patient has a pancreatic tumour, the tumour cells increase gastrin-like secretion leading to increased secretion of hydrochloric acid and pepsin causing peptic ulcer in many places (Zollinger-Ellison syndrome). For treatment, the tumour must be removed.

Histamine:

It is a product of metabolism from histidine of gastric mucosal cells. Histamine stimulates the H2 receptors of the parietal cells (H2-receptors) to increase the secretion of HCl acid.

Therefore, in the treatment of peptic ulcer, people use H2-receptor blocking drugs to reduce the HCl secretion effect of histamine (eg, cimetidine, ranitidine, famotidine...).

Glucocorticoid:

It is a hormone of the adrenal cortex that stimulates the secretion of HCl and pepsin acid and inhibits the secretion of mucus.

Therefore, in people with prolonged nervous tension (psychological stress) due to increased glucocorticoid secretion, gastric ulcers are common.

In treatment, the use of glucocorticoid drugs (Dexamethasone, Prednisolone...) is contraindicated for patients with gastric ulcer or a history of gastric ulcer.

Prostaglandin E2:

As a hormone of gastric mucosal cells that inhibits the secretion of hydrochloric acid and pepsin and stimulates the secretion of mucus, it is considered a protective factor for the gastric mucosa. Therefore, in the treatment Of peptic ulcers, drugs derived from prostaglandins (eg, cytotec) or drugs that increase gastric secretion of prostaglandin E2 (eg, colloidal bismuth subcitrate) are used.

In contrast, agents that inhibit prostaglandin secretion will cause stomach ulcers, which are analgesics, anti-inflammatory drugs such as aspirin, Voltaren, piroxicam, ibuprofen... These drugs have strong anti-inflammatory effects through a reduction mechanism. Prostaglandin synthesis is an inflammatory agent at the site of inflammation but also reduces the secretion of prostaglandin E2 in the stomach causing gastric ulcer. These drugs are contraindicated in patients with gastric ulcer.

Absorption in the stomach

The stomach can absorb sugar, iron, water and alcohol.

Iron

Iron when entering the stomach is dissolved by gastric juice and becomes Fe2+, a small part is absorbed by the stomach in the form of active transport, the rest is absorbed by the duodenum.

Street

The stomach can absorb some of the glucose.

Country

Water is partially absorbed in the stomach by passive transport to balance osmotic pressure. Therefore, when gastric juice is hypotonic, water absorption increases.

Alcohol

Absorbed mainly in the stomach by passive transport.

Particularly in breastfed babies, the stomach can absorb 25% of the nutrients in breast milk.