Digestive physiology in the small intestine

2021-06-13 10:38 PM

When intestinal obstruction, in order to push the chyme to pass through the obstruction, peristalsis increases greatly, causing symptoms of intermittent abdominal pain, and signs of a snake crawling.

The small intestine has the function of completing the digestion of food, so it plays the most important role in indigestion.

The structure of the small intestine is very favourable for the digestive process:

It is the longest part of the digestive tract.

There are many types of digestive juices poured in, a very rich enzyme system capable of breaking down all food into an absorbable form.

To complete digestion, the small intestine has the following functional activities:

Mechanical activity of the small intestine

The small intestine has four forms of mechanical activity:

Contractions

Has the effect of dividing the chyme into short pieces for easy absorption of digestive juices.

Pendulum movement

Has the effect of mixing chyme with digestive juices to speed up digestion.

Peristaltic

These are waves of contractions that spread from the beginning to the end of the small intestine, which have the effect of pushing food through the intestines.

When intestinal obstruction (tumour, worms, volvulus...), in order to push the chyme to pass through the obstruction, peristalsis is greatly increased, causing intermittent abdominal pain and the appearance of snake signs (signs of snakes). Koenig), a marker for the diagnosis of intestinal obstruction.

Anti-peristaltic

Are contraction waves opposite to peristalsis but appear sparse and weaker than peristalsis.

Anti-peristaltic works in coordination with peristalsis to slow down the movement of chyme for more thorough digestion and absorption.

Excretion activity in the small intestine

Digestive juice in the small intestine is very rich because it is secreted from 3 places: pancreas, bile and small intestine.

Pancreatic secretions

Pancreatic juice is a product of the exocrine pancreas. After secretion, pancreatic juice follows the pancreatic ducts (Wirsung and Santorini) into the duodenum.

The quantity is about 1 - 1.5 litres/24 hours.

Composition and effects of pancreatic juice:

Pancreatic juice is a clear, colourless liquid with the most alkaline pH in digestive juices (about 7.8 - 8.5). Consists of the following components:

Group of enzymes that digest proteins: 

Chymotrypsin:

Excreted as inactive form chymotrypsinogen (pro-enzyme). Under the action of trypsin, it will convert to active chymotrypsin, which has the effect of breaking down peptide bonds whose (-CO -) part belongs to aromatic amino acids.

Carboxypeptidase:

Excreted as an inactive procarboxypeptidase. Under the action of trypsin, it will convert to an active carboxypeptidase, which cuts the amino acids at the C-terminus of the polypeptide chain into individual amino acids.

Trypsin:

There are 2 effects:

Break down peptide bonds where the (- CO -) part belongs to alkaline amino acids (lysine, arginine)

Activates chymotrypsinogen and procarboxypeptidase to the active form. In addition, trypsin activates its own pro-enzyme

Initially, trypsin is secreted in an inactive form, trypsinogen, and is converted to active trypsin by three mechanisms:

Due to the activation of entering peptidase of intestinal juice, this is the primary mechanism that initiates the activation of pancreatic juice protein-digesting enzymes in the intestine.

Due to newly formed trypsin activation.

Due to the auto-activation mechanism: trypsinogen can be automatically converted to active trypsin when there is an accumulation of pancreatic juice in the pancreas. This is one of the causes of acute pancreatitis.

Acute pancreatitis usually occurs in people with a history of pancreatic head tumour or common bile duct stones and occurs after a delicious meal. In such meals, because there are many proteins and lipids, the digestive products stimulate the secretion of pancreatic juice very strongly. Pancreatic secretions are abundant, but the outlet is blocked (due to tumours, stones), so it accumulates in the pancreas, causing trypsinogen to automatically convert to trypsin.

The newly formed trypsin activates all three proenzymes: chymotrypsin, PR carboxypeptidase and trypsinogen. These three enzymes convert to their active form in the pancreas, which destroys the pancreas itself, causing acute pancreatitis and often death.

The group of lipid-digesting enzymes:

Pancreatic Lipase:

Has the effect of breaking down emulsified triglycerides into fatty acids and monoglycerides? This effect is greatly supported by bile salts.

Phospholipase:

Remove fatty acids from phospholipid molecules.

Group of enzymes that digest glucose:

Pancreatic juice amylase:

Has the effect of breaking down ripe and living starch into double sugar maltose?

Small amounts of pancreatic amylase are absorbed into the bloodstream. In acute pancreatitis, blood amylase is increased. Therefore, blood amylase quantification is valuable for the diagnosis of acute pancreatitis.

Maltase:

Breakdown of the disaccharide maltose into glucose.

HCO3-:

Not a digestive enzyme but plays a very important role:

Create a favourable environment for enzymes to work.

Neutralize HCl acid of gastric juice to protect intestinal mucosa.

Contributes to the mechanism of opening and closing the pylorus.

Regulate pancreatic secretion:

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

Nervous mechanism:

Because the X cord is stimulated by two types of reflexes similar to the secretion of saliva and gastric juice.

Translation mechanism:

Secretin is secreted by two hormones secreted by cells lining the small intestine: secretin and pancreozymins.

Secretin:

Excreted under the stimulant action of hydrochloric acid in the chyme. Secretin stimulates the secretion of pancreatic juice containing a lot of water and HCO3-.

Pancreozymin:

Excreted under the stimulant effect of digestive products protide, lipid, glucid in the intestine. Pancreozymin secretes pancreatic juice containing many enzymes.

Thus, under the effect of the humoral mechanism, the composition of the secreted pancreatic juice depends entirely on the properties of the chyme:

When the chyme is too acidic, the pancreatic juice is dilute, with more HCO3- and few enzymes.

When the chyme is rich in digestive products, the pancreatic juice is rich in enzymes.

Secretion of bile

Bile is a product excreted by the liver. Once produced, the bile is stored in the gallbladder and concentrated. When necessary, the gallbladder will contract to expel bile into the intestine. The amount of bile is about 0.5 litres/24 hours.

Composition and effects of bile:

Bile is a clear liquid, blue or yellow, slightly alkaline pH (about 7 -7.7), consisting of the following main components:

Bile salt:

Potassium or sodium salts of glycocholic and taurocholic bile acids derived from cholesterol.

Bile salts are the only components in bile that have a digestive effect:

Emulsify triglycerides so that lipase in the small intestine can break down all the triglycerides in the food.

Helps absorb lipid digestive products: fatty acids, monoglycerides, cholesterol. Thereby, it also helps to absorb lipid-soluble vitamins: A, D, E and K. In the absence of bile salts, the absorption of these substances is reduced.

In addition, bile salts also help cholesterol dissolve easily in bile to prevent gallstones.

Once it reaches the ileum, 95% of the bile salts are reabsorbed back into the blood and taken to the liver for re-elimination (entero-hepatic cycle).

Cholesterol:

Cholesterol in bile is the raw material for the production of bile salts. It is also possible that this is the body's way to eliminate cholesterol to regulate blood cholesterol.

Normally, the amount of cholesterol excreted is correlated with bile salts, so bile salts help cholesterol dissolve in bile. When there is hypercholesterolemia or cholangitis, the gallbladder causes the biliary mucosa to increase the absorption of bile salts, this correlation is lost, cholesterol becomes dominant and will precipitate to form cholesterol stones, which are common in many patients. European countries or in people with a high-lipid diet.

Bile pigment:

Also known as direct bilirubin (bilirubin diglucuronide) is produced during the metabolism of haemoglobin in the liver.

When excreted normally into the intestines, bile pigments give stool a yellow colour.

When bile is blocked (hepatitis, stones...), bile pigments cannot go to the intestines, but are absorbed back into the blood and excreted in the urine, causing the following symptoms:

White faeces (stork droppings).

Skin and mucous membranes are yellow.

Dark yellow urine.

These symptoms contribute to the diagnosis of biliary obstruction syndrome.

Regulates bile secretion:

Bile secretion is regulated by two mechanisms:

Nervous mechanism:

Due to the X wire under the effect of two types of reflections as above.

Translation mechanism:

Also due to 2 hormones secretin and pancreozymins.

Secretin:

Stimulates hepatocytes to increase bile production, so also known as hepatocrinin.

Pancreozymin:

Stimulates gallbladder contractions to expel bile into the intestine, also known as cholecystokinin (CCK).

Excretion of intestinal juices

Secreted by cells of the intestinal lining and glands located just above the intestinal wall:

Brunner Gland: secretes mucus and HCO3-.

Lieberkühn gland: excretes water.

Mucosal cells: secrete enzymes.

Thus, the cells lining the small intestine play an important role in the secretion of intestinal juices while the intestinal glands only secrete the by-products.

The amount of intestinal fluid is about 2 - 3 litres/24 hours.

Composition and effects of intestinal juice:

Group of enzymes that digest proteins:

Aminopeptidase:

Has the effect of cutting off each amino acid standing at the N-terminus of the polypeptide chain?

Dipeptidase, tripeptides:

Break down dipeptides and tripeptides into individual amino acids.

Group of enzymes that digest glucose:

Intestinal amylase:

Breaks down both raw and cooked starches into double sugar maltose.

Maltase:

Break down maltose into glucose.

Sucrase:

Break down sucrose (cane sugar) into glucose and fructose.

Lactase:

Break down lactose (milk sugar) into glucose and galactose.

Intestinal fluid lipase:

Break down emulsified triglycerides into glycerol and fatty acids.

Regulate intestinal secretions:

Intestinal fluid secretion is regulated mainly by mechanical mechanisms. When food passes through the intestine, it stimulates the glands to secrete alkaline fluid and mucus and causes the cells lining the small intestine to shed and burst, releasing enzymes into the intestinal lumen. Therefore, the cells lining the small intestine are renewed every 3-5 days.

Absorption in the small intestine

Absorption in the small intestine plays a very important role. Most substances needed by the body (digestive products, water, electrolytes, drugs) are taken from the lumen of the digestive tract into the bloodstream through the small intestine. This is because the small intestine has very favourable structural features for absorption:

The small intestine is very long, about 3 m. The mucosa has many folds, many villi and microvilli that make up the brush border with a very large contact area, about 
300 m2. Inside the villi, there is a vascular, lymphatic and nervous system that is very convenient for the absorption

The small intestinal mucosal cells contain many elements necessary for the absorption of substances across the membrane such as enzymes, carriers, and energy.

All food that reaches the small intestine is broken down into absorbable products

Protein absorption

Proteins are absorbed in the small intestine from food (50%), digestive juices (25%) and intestinal mucosal cells (25%). The duodenum is the site of greatest absorption, followed by the jejunum and lowest in the ileum.

Amino acids are absorbed by active transport.

Di-tripeptides are also absorbed by active transport.

In addition, in breast-fed infants, the small intestine is capable of absorbing some unresolved proteins in the form of moist cells. Thanks to this ability, babies can absorb antibodies (globulins) contained in breast milk to help them fight infections.

Glucose absorption

Most absorbed in the jejunum mainly as monosaccharides in 3 forms:

Simple diffusion: ribose, mannose.

Diffusion mediated: fructose.

Active transport: glucose, galactose.

Among them, glucose is the most important monosaccharide. The absorption of glucose (as well as galactose) is greatly enhanced in the presence of Na+ by the secondary active transporter as follows:

Na+ and glucose have the same carrier, which transports Na+ and glucose into the intestinal mucosal cells. Here, Na+ will be actively transported into the interstitial fluid, so Na+ in the cell is always at a lower concentration than the intestinal lumen, motivating the carrier to continue transporting Na+ and glucose into the cell.

After entering the cell, glucose enters the interstitial fluid by simple diffusion or mediated diffusion.

In addition to monosaccharides, small amounts of disaccharides are also absorbed.

Absorption of lipids

Lipids are absorbed mainly in the form of fatty acids, monoglycerides, cholesterol and glycerol. Glycerol is absorbed as a simple sugar by simple diffusion. In contrast, fatty acids, monoglycerides and cholesterol are required to have bile salts in order to be absorbed by the following mechanism:

Bile salts interact with fatty acids, monoglycerides and cholesterol to form spherical micelles, the outer surface of which is hydrophilic so the micelles are water-soluble and readily come into contact with the brush fringing. Here, fatty acids, monoglycerides and cholesterol simply diffuse into the cells, while bile salts return to the intestinal lumen to continue forming new micelles.

In mucosal cells, short-chain fatty acids (10 carbons) go directly into the interstitial fluid and then into blood vessels, while long-chain fatty acids (>10 carbons) will be synthesized into triglycerides and enter cholesterol along with cholesterol. lymph.

When there is a lack of bile salts, lipid absorption is markedly reduced, there are many fatty acids and monoglycerides in the stool.

Absorption of vitamins

Vitamins are absorbed in their intact form by simple diffusion.

Water-soluble vitamins (C, PP, group B) are absorbed very quickly, except vitamin B12, which requires an intrinsic factor.

In contrast, lipid-soluble vitamins (A, D, E, K) to be absorbed need to be accompanied by lipid absorption. When lipid absorption is reduced (bile salt deficiency, lipase deficiency) these vitamins decrease absorption.

Most vitamins are absorbed in the first part of the small intestine except vitamin B12, which is absorbed in the ileum.

Absorb ions

Absorption of Na+:

Absorbed throughout the length of the small intestine by active transport as follows:

At the basal border, under the action of the Na+ pump (Na+- K+ ATPase), Na+ is actively transported into the interstitial fluid, causing the Na+ concentration in intestinal mucosal cells to drop lower than in the intestinal lumen, creating a ladder difference. electrochemistry. Therefore, from the lumen of the intestine, Na+ diffuses across the brush border into the intestinal mucosal cells by means of a carrier protein (mediated diffusion).

When the carrier protein transports Na+, it also simultaneously transports glucose from the intestinal lumen into the intestinal mucosal cells (secondary active transport). Carrier proteins will be transported faster if both Na+ and glucose are transported at the same time. Thus, Na + and glucose support each other's absorption, which is very important in the treatment of dehydration diarrhoea with ORS electrolyte solution.

Cl- absorption:

Most of it is absorbed passively by Na+ in the early small intestine.

Besides the absorption process, the small intestinal mucosal cells also secrete small amounts of Cl- into the intestinal lumen under the action of cyclic AMP. Some bacteria such as cholera, Escherichia coli can produce a toxin that increases the amount of cyclic AMP in the intestinal mucosal cells, causing increased secretion of Cl- into the intestinal lumen, leading to Na + and water, causing diarrhoea.

Ca2+ absorption:

About 30-80% of Ca2+ in food is absorbed depending on the body's needs. Most Ca2+ is absorbed by active transport in the early small intestine with the help of 2 factors:

1,25-dihydroxycholecalciferol: is a metabolite of vitamin D produced in the kidney that increases the load of Ca2+.

Parahormone: Parathyroid hormone that converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol in the kidney.

When vitamin D deficiency or hypoparathyroidism, absorption of Ca2+ decreases, children will have rickets.

Fe2+ ​​absorption:

Iron is absorbed mainly in the duodenum in the form of active transport, readily absorbed in the ferrous form (Fe2+), but iron in food is usually in the ferric form (Fe3+). Factors such as hydrochloric acid and vitamin C convert Fe3+ to Fe2+, thus increasing iron absorption. Therefore, gastrectomy patients often develop iron deficiency anaemia. In treatment, when using iron, it is necessary to add vitamin C.

Absorb water

The process of water absorption in the small intestine plays a very important role. Every day, the small intestine receives about 10 litres of water, of which 2 litres come from eating and 8 litres from digestive juices, most of which is intestinal juice. This amount of water must be absorbed almost completely.

The process of absorption and excretion of water in the small intestine forms a two-way flow in which absorption is always stronger than excretion. For some medical reason, absorption is weaker than excretion, causing diarrhoea.

Water is absorbed dynamically according to the solute to balance the osmotic pressure, in which Na+ and glucose play an important role in water absorption. These two substances support each other's absorption, and their absorption involves water. Therefore, in the presence of Na+ and glucose, water absorption is greatly increased, which is an important basis for ORS rehydration and electrolytes to treat dehydration diarrhoea.