Digestion in intestine and colon.
Absorption in gastrointestinal tract
Motility of gastrointestinal tract
1. Digestion in the small intestine
It has three parts: the Duodenum, Jejunum, and Ileum.
After being processed in the stomach, food is passed to the small intestine via the pyloric sphincter. The majority of digestion and absorption occurs here after the milky chyme enters the duodenum. Here it is further mixed with three different liquids:
Bile, which emulsifies fats to allow absorption, neutralizes the chyme and is used to excrete waste products such as bilin and bile acids. Bile is produced by the liver and then stored in the gallbladder where it will be released to the small intestine via the bile duct. The bile in the gallbladder is much more concentrated.[clarification needed]
Pancreatic juice made by the pancreas, which secretes enzymes such as pancreatic amylase, pancreatic lipase, and trypsinogen (inactive form of protease).
Intestinal juice secreted by the intestinal glands in the small intestine. It contains enzymes such as enteropeptidase, erepsin, trypsin, chymotrypsin, maltase, lactase and sucrase (all three of which process only sugars).
The pH level increases in the small intestine as all three fluids are alkaline. A more basic environment causes more helpful enzymes to activate and begin to help in the breakdown of molecules such as fat globules. Small, finger-like structures called villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine and enhancing speed at which nutrients are absorbed. Blood containing the absorbed nutrients is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.
The small intestine and remainder of the digestive tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes. When the circular muscles contract, the lumen becomes narrower and longer and the food is squeezed and pushed forward. When the longitudinal muscles contract, the circular muscles relax and the gut dilates to become wider and shorter to allow food to enter.
a) Role of duodenum in the digestive system (There are two secretor functions of pancreas – external and internal. The external secretor function of pancreas means that exsogenic cells of pancreas and ducts cells produce pancreatic juice. It helps to hydrolyzed protein to peptides and amino acids, carbohydrates to monosaccharides, lipids to the fat acids and glycerine. It neutralizes acidic chymus, which come from stomach.)
b) External secretor function of pancreas (The external secretor function of pancreas means that exsogenic cells of pancreas and ducts cells produce pancreatic juice).
c) Composition and property of pancreas juice
(Quantity of pancreatic juice per day – 1,5-
d) Regulation of pancreas secretion (Regulation act by complex of neuro-humoral mechanisms. There are three phases of pancreatic secretion: cephalic, stomach and intestine. The first stage caused by act of nervous influences. Nervus vagus realizes this effect by means of conditioned and unconditioned reflexes. Secretion begins after 1-2 minutes of food. This juice consists of enzymes, small quantity of water and ions. Sympathetic influences have a trophic role. During the second phase there are two kinds of influences: nervous and humoral, for example, gastrin from stomach. The third phase caused by chymus contents. The main is humoral factors. In that time secrete 2 hormons – secretin and cholecystokinin-pancreasemin. Secretin stimulates production of a big quantity of juice with a high concentration of hydro carbonates and a small quantity of enzymes in ducts cells. Cholecystokinin-pancreasemin stimulates production of a less quantity of juice with a big concentration of enzymes in acinars cells.)
e) Bile production and bile secrete (Secretion of bile occur all time and increase by influences of bile acids, cholecystokinin-pancreasemin, secretin. Bile secretion in the duodenum depends from take food. It depends of nervus vagus and humoral influences – concentration of cholecystokinin-pancreasemin, secretin, fats.)
f) Composition of bile, their role in digestive processes (Composition: bilirubin, bile acids, cholesterol, leukocytes, some epitheliocytes, cristalls of bilirubin, calcium, cholesterol. The role of bile: 1. Neutrolyze the stomach acid; 2. Inhibit he act of stomach proteases; 3. Increase the activity of pancreatic lipase; 4. Emulgate the lipids; 5. Increase the absorption of fat acids, vitamins K, D, E; 6. Increase tone and motor function of intestines; 7. Decrease the activity of intestine microflora.)
g) Composition and properties of intestine juice
(Composition of intestine juice: mucus, enzymes – peptidase, saccharase,
maltase, lactase, lipase, immunoglobulins, leukocytes; epitheliocytes (
h) Cavity and membrane hydrolyses of substances (On the glicocalix of micro fibers present enzymes, which are adsorbed and digest small molecules of nutritive substances – membrane hydrolyses of substances. Cavity hydrolyses of substances provide by enzymes, which are in intestine space.)
Digestion in the large intestine
After the food has been passed through the small intestine, the food enters the large intestine. Within it, digestion is retained long enough to allow fermentation due to the action of gut bacteria, which breaks down some of the substances that remain after processing in the small intestine; some of the breakdown products are absorbed. In humans, these include most complex saccharides (at most three disaccharides are digestible in humans). In addition, in many vertebrates, the large intestine reabsorbs fluid; in a few, with desert lifestyles, this reabsorbtion makes continued existence possible.
In general, the large
intestine is less vigorous in absorptive activity. It produces sacculation,
renews epithelial cells, and provides protective mucus and mucosal immunity. In
humans, the large intestine is roughly
a) Composition of intestine juice and their properties (Composition of intestine juice: mucus, epithelial cells. Functions: protective from mechanical, chemical irritations; formed of base reaction of intestine contents.)
b) Role of the micro flora of big intestine (1. Ending decompose of all nutritive substances, which are do not digestive; synthesis of some vitamins – of B group, vitamin K; take place in metabolic processes.)
The small intestine is the longest organ of the digestive tract. It is divided up indiscriminately into three sections: the duodenum, the jejunum, and the ilium.
This is the place where the ultimate destruction of food digestion reaches its completion and where the acidity of chyme is nullified. The nutrients in the food eaten many hours ago have almost been diminished to molecules small enough to be absorbed through the intestinal walls into the bloodstream. Carbohydrates are diminished into simpler sugars; proteins to amino acids; and fats to fatty acids and glycerol. Enzymes are secreted by the walls of the duodenum and unite with the bile (essential for the digestion and absorption of tenacious fatty materials) and pancreatic enzymes in the duodenum.
Peristalsis pushes the nutrient liquid out of the duodenum into the first reaches of the jejunum. A greater number of villi , microscopic, hair like structures, begin to absorb amino acids , sugars, fatty acids and glycerol from the digested contents of the small intestine, and starts them on their way to other parts of the body. This part of the small intestine executes a digestive operation so that what is passed on to the large intestine is a thin watery substance almost completely devoid of nutrients.
Legitimately, these three organs lie outside of the gastrointestinal tract. Nevertheless, digestive fluids from all three meet like intersections of a railway track at the common bile duct, and their movement from there into the duodenum is controlled by a sphincter muscle.
The pancreas is a producer of digestive enzymes. The gallbladder is a small reservoir for bile. The liver reproduces nutrients so that they can be used for cell-rebuilding and energy.
There is a
merger between the illium and the cecum,
the first section of the large intestine. Any solid substances that flow into
the large intestine through the ileocecal valve
(which prevents back flow into the small intestine) are as a rule indigestible,
or are bile constituents. What the cecum primarily
inherits is water.
What the large intestine essentially does, other than act as a passageway for removal of body wastes, is to act as a provisional reservoir for water. There are no villi in the large intestine and peristalsis is much less forceful than in the small intestine. As water is absorbed, the contents of the large intestine change from a watery liquid and are compressed into semisolid feces. Nerve endings in the large intestine signal the brain that it is time for a bowel movement. The fecal material moves through the colon down to several remaining inches known as the rectum and out through the anus an opening controlled by the outlet valves of the large intestine.
Site of Enzyme Origin
Nutrient It Breacks Down
Product Of Enzyme Action
Place of Enzyme Action
Maltase, Lactase, Sucrase
Trypsin, Lipase, Amylase
Proteins, Fats/Lipids, Carbohydrates
Amino acids, Glycerol/Fatty Acids, Simple Sugars
The gastrointestinal tract is supplied with a number of muscular valves. These control and direct the quantity of food that goes through the digestive tract and inhibits the back movement of partially digested food.
Action of the major digestive hormones
There are at least five hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds. Arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years.
Gastrin - is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH .
Secretin - is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver. This hormone responds to the acidity of the chyme.
Cholecystokinin (CCK) - is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.
Gastric inhibitory peptide (GIP) - is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach. Another function is to induce insulin secretion.
Motilin - is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.
Significance of pH in digestion
Digestion is a complex process controlled by several factors. pH plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx, and esophagus, pH is typically about 6.8, very weakly acidic. Saliva controls pH in this region of the digestive tract. Salivary amylase is contained in saliva and starts the breakdown of carbohydrates into monosaccharides. Most digestive enzymes are sensitive to pH and will denature in a high or low pH environment.
The stomach's high acidity inhibits the breakdown of carbohydrates within it. This acidity confers two benefits: it denatures proteins for further digestion in the small intestines, and provides non-specific immunity, damaging or eliminating various pathogens.
In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralize the acidic conditions from the stomach, and the pancreatic duct empties into the duodenum, adding bicarbonate to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline with a pH of about 8.5.
1. Common characteristic of absorption process
a)Determine of notion “absorption”
Absorption is a complex of processes, which are provide transport of substances from digestive tract into internal surroundings of organism (blood, lymph, intercellular substances).
Main types of transport of nutritive substances in internal surroundings of organism are: passive and active.
1.Passive transport include diffusion and osmosis. This transport do not need presents of energy. In this case substances transport through the mucus shell by help of concentrative gradient. This way of transport have water, water disolved vitamins (C, B6, B2).
2.Active transport include pinocytosis and active transport by help of protein and energy. Active transport need energy of ATP. This way characteristic of amino acids, monosaccharids, vitamin B12, ions of calcium, enzymes. Pinocytosis – by help of pynocytic bulb, where secreted enzymes for proteins hydrolysis. Products of hydrolysis adsorbed by cell.
1. ANATOMICAL BASIS OF ABSORPTION
The total quantity of fluid
that must be absorbed each day is equal to the ingested fluid (about
The stomach is a poor absorptive area of the gastrointestinal tract because it lacks the typical villus type of absorptive membrane and also because the junctions between the epithelial cells are tight junctions. Only a few highly lipid-soluble substances, such as alcohol and some drugs like aspirin, can be absorbed in small quantities.
The Absorptive Surface of the Intestinal Mucosa – The Villi.
The absorptive surface of the
intestinal mucosa, showing many folds called valvulae conniventes (or folds of Kerckring), which increase the
surface area of the absorptive mucosa about threefold. These folds extend
circularly most of the way around the intestine and are especially well
developed in the duodenum and jejunum, where they often protrude as much as
Located over the entire
surface of the small intestine, from approximately the point at which the
common bile duct empties into the duodenum down to the ileocecal
valve, are literally millions of small villi, which
The intestinal epithelial
cells are characterized by a brush border, consisting of about 600 microvilli
1 μm in length and 0,1 μm
in diameter protruding from each cell. This increases the surface area exposed
to the intestinal materials another 20-fold. Thus, the combination of the folds
of Kerckring, the villi,
and the microvilli increases the absorptive area of
the mucosa about 600-fold, making a tremendous total area of about
The general organization of a villus, emphasizing especially the advantageous arrangement of the vascular system for absorption of fluid and dissolved material into the portal blood, and the arrangement of the central lacteal for absorption into the lymph. Many small pinocytic vesicles, which are pinched-off portions of infolded epithelium surrounding extracellular materials that have been entrapped inside the cells. Small amounts of substances are absorbed by this physical process of pinocytosis, though, as noted later in the chapter, most absorption occurs by means of single molecular transfer. Located near the brush border of the epithelial cell are many mitochondria, which supply the cell with oxidative energy needed for active transport of materials through the intestinal epithelium. Also, extending linearly into each microvillus of the brush border are multiple actin filaments that are believed to contract and cause continual movement of the microvilli, keeping them constantly exposed to new quantities of intestinal fluid.
BASIC MECHANISMS OF ABSORPTION
Absorption through the gastrointestinal mucosa occurs by active transport and by diffusion, as is also true for other membranes.
Briefly, active transport imparts energy to the substance as it is being transported for the purpose of concentrating it on the other side of the membrane or for moving it against an electrical potential On the other hand, the term diffusion means simply transport of substances through the membrane as a result of molecular movement along, rather than against, an electrochemical gradient.
c)Absorption in the mouth cavity and stomach
In the mouth cavity absorbed water, water soluble medicines (for example, validol, nitroglycerin, adelphan, furosemid, corinfar and others). In our oral cavity, under the tongue present a big quantity of vessels. That is why all water soluble substances absorbed in this place. They go to the bloodstream, and have immediately action on our receptors. They do not go through the liver, and do not desintoxicated, that is why may be toxic effect of some substances, for example products of food, drugs.
In esophagus do not absorbed nutritive substances as a rule.
In stomach absorbed alcohol, water and small quantity of other substances.
ABSORPTION IN THE SMALL INTESTINE
d) Absorption in intestines
Virtually all nutrients from the diet are absorbed into blood across the mucosa of the small intestine. In addition, the intestine absorbs water and electrolytes, thus playing a critical role in maintenance of body water and acid-base balance.
It's probably fair to say that the single most important process that takes place in the small gut to make such absorption possible is establishment of an electrochemical gradient of sodium across the epithelial cell boundary of the lumen. This is a critical concept and actually quite interesting. Also, as we will see, understanding this process has undeniably resulted in the saving of millions of lives.
To remain viable, all cells are required to maintain a low intracellular concentration of sodium. In polarized epithelial cells like enterocytes, low intracellular sodium is maintained by a large number of Na+/K+ ATPases - so-called sodium pumps - embedded in the basolateral membrane. These pumps export 3 sodium ions from the cell in exchange for 2 potassium ions, thus establishing a gradient of both charge and sodium concentration across the basolateral membrane.
In rats, as a model of all mammals, there are about 150,000 sodium pumps per small intestinal enterocyte which collectively allow each cell to transport about 4.5 billion sodium ions out of each cell per minute (J Membr Biol 53:119-128, 1980). Pretty impressive! This flow and accumulation of sodium is ultimately responsible for absorption of water, amino acids and carbohydrates.
Aside from the electrochemical gradient of sodium just discussed, several other concepts are required to understand absorption in the small intestine. Also, dietary sources of protein, carbohydrate and fat must all undergo the final stages of chemical digestion just prior to absorption of, for example, amino acids, glucose and fatty acids.
At this point, its easiest to talk separately about absorption of each of the major food groups, recognizing that all of these processes take place simultaneously.
Water and electrolytes
Carbohydrates, after digestion to monosaccharides
Proteins, after digestion to small peptides and amino acids
Neutral fat, after digestion to monoglyceride and free fatty acids
Absorption in the Small Intestine:
Normally, absorption from the
small intestine each day consists of several hundred grams of carbohydrates,
100 or more grams of fat, 50 to
ABSORPTION IN THE LARGE INTESTINE
Approximately 1500 ml of chyme pass through the ileocecal valve into the large intestine each day. Most of the water and electrolytes in this are absorbed in the colon, usually leaving less than 100 ml of fluid to be excreted in the feces. Also, essentially all the ions are also absorbed, leaving only about 1 mEq each of sodium and chloride ions to be lost in the feces.
Most of the absorption in the large intestine occurs in the proximal half of the colon, giving this portion the name absorbing colon, whereas the distal colon functions principally for storage and is therefore called the storage colon.
Absorption and Secretion of Electrolytes and Water.
The mucosa of the large intestine, like that of the small intestine, has a high capability for active absorption of sodium, and the electrical potential created by the absorption of the sodium causes chloride absorption as well. The „tight junctions“ between the epithelial cells of the large intestinal epithelium are much tighter than those of the small intestine. This prevents significant amounts of back-diffusion of ions through these junctions, thus allowing the large intestinal mucosa to absorb sodium ions far more completely – that is, against a much higher concentration gradient – than can occur in the small intestine.
In addition, as in the distal portion of the small intestine, the mucosa of the large intestine actively secretes bicarbonate ions while it simultaneously actively absorbs an equal amount of chloride ions in an exchange transport process. The bicarbonate helps neutralize the acidic end-products of bacterial action in the colon.
The absorption of sodium and chloride ions creates an osmotic gradient across the large intestinal mucosa, which in turn causes absorption of water.
Action in the
Numerous bacteria, especially colon bacilli, are present in the absorbing colon. These are capable of digesting small amounts of cellulose, in this way providing a few calories of nutrition to the body each day. In herbivorous animals this source of energy is very significant, though it is of negligible importance in the human being. Other substances formed as a result of bacterial activity are vitamin K, vitamin B12, thiamin, riboflavin, and various gases that contribute to flatus in the colon – especially carbon dioxide, hydrogen gas, and methane. Vitamin K is especially important, for the amount of this vitamin in the ingested foods is normally insufficient to maintain adequate blood coagulation.
Composition of the Feces. The feces normally are about three-fourths water and one-fourth solid matter composed of about 30 per cent dead bacteria, 10 to 20 per cent fat, 10 to 20 per cent inorganic matter, 2 to 3 per cent protein, and 30 per cent undigested roughage of the food and dried constituents of digestive juices, such as bile pigment and sloughed epithelial cells. The large amount of fat derives mainly from fat formed by bacteria and fat in the sloughed epithelial cells.
The brown color of feces is caused by stercobilin and urobilin, which are derivatives of bilirubin. The odor is caused principally by the products of bacterial action; these vary from one person to another, depending on each person's colonic bacterial flora and on the type of food eaten. The actual odoriferous products include indole, skatole, mercaptans, and hydrogen sulfide.
ABSORPTION OF WATER
Isosmotic Absorption. Water is transported through the intestinal membrane entirely by the process of diffusion. Furthermore, this diffusion obeys the usual laws of osmosis. Therefore, when the chyme is dilute, water is absorbed through the intestinal mucosa into the blood of the villi by osmosis.
On the other hand, water can also be transported in the opposite direction, from the plasma into the chyme. This occurs especially when hyperosmotic solutions are discharged from the stomach into the duodenum Usually within minutes, sufficient water is transferred by osmosis to make the chyme isosmotic with the plasma Thereafter, the chyme remains almost exactly isosmotic throughout its total passage through the small and large intestines.
dissolved substances are absorbed from the lumen of the gut into the blood the
absorption tends to decrease the osmotic pressure of the chyme,
but water diffuses so readily through the intestinal membrane (because of large
e) Methods of absorptions’ investigation
1. Angiostoma (experimental method). Surgeon put stoma, aperture, on one of the gastrointestinal vessels in which absorbed nutritive substances. He add it by help of catheter of body surface. In this case he investigate absorption processes in anybody part of intestines.
In the case of angiostoma he may investigate each stage of digestion in different organs – oral cavity, esophagus, stomach, small and large intestines. He may determining the speed of absorption; quantity of different substances, which are absorbed in different part of digestive tract; components of food, which can absorbed in different part of gastro-intestinal tract; speed of bloodstream in the different part of gastro-intestinal tract, which help to absorbed some substances; mechanism of absorption in different part of gastrointestinal tract.
2. X-ray investigation (experimental or clinical method). In this case by help of different substances, for example, suspension of barium for determining motor function of gastrointestinal tract and other water-soluble substances to determining absorption. Doctor do X-ray investigation and determining place of absorption, place of increase or decrease speed of absorption, part of digestive tract, where present decrease of absorption. This method may be act on animal too, for example, if we need to determining absorption of new substances.
3. Biochemical method of investigation (experimental or clinical method). In this case laboratory assistant investigate blood, urine, saliva to content of different substances – glucose, amino acids, fat acids, lactose, mannose, sugar and others. For example, to determining pathology of carbohydrates absorption in intestines doctor laboratory assistant investigate quantity of glucose, or galactose, or lactose, or mannose in blood and urine and if he know the quantity of glucose which are coming into organism, he may value absorption of glucose, or galactose, or lactose, or mannose in digestive tract. For example, to determining pathology of sodium or potassium absorption in digestive tract doctor laboratory assistant investigate concentration of sodium or potassium in saliva, blood, urine and after that doctor may value their absorption.
4. Radioisotopic investigation (clinical method). Nurses inject intravenously radioisotop, which absorbed in digestive tract, into the organism of patient. After some time, which is necessary for investigation, doctor scan the places, where this isotop must absorbed. Then he determining the absorptive function of intestines, as he see the speed of absorption, quantity of radioisotop, which are absorbed and place of absorption of radioisotop.
Water and mineral salts
Active Transport of Sodium.
The principles of sodium absorption from the intestine are also essentially the same as those for absorption of sodium from the renal tubules. The motive power for the sodium absorption is provided by active transport of sodium from inside the epithelial cells through the side walls of these cells into the intercellular spaces. This active transport obeys the usual laws of active transport it requires energy, and it is catalyzed by appropriate ATPase carrier enzymes in the cell membrane. Part of the sodium is transported along with chloride ions that are passively „dragged“ along by the positive electrical charges of the sodium ion. However, other sodium ions are absorbed while either potassium or hydrogen ions are transported into the lumen of the gut in exchange for the sodium ions. In the membrane of the brush border are special transport proteins that facilitate these exchanges between sodium and potassium or sodium and hydrogen.
The active transport of sodium reduces its concentration in the cell to a low value (about 50 mEq/liter). Since the sodium concentration in the chyme is normally about 142 mEq/liter (that is, approximately equal to that in the plasma), sodium moves by passive absorption from the chyme through the brush border of the epithelial cell into the epithelial cell cytoplasm. This replaces the sodium that is actively transported out of the epithelial cells into the intercellular spaces.
The next step in the transport process is osmosis of water into the intercellular spaces. This movement is caused by the osmotic gradient created by the elevated concentration of ions in the intercellular space. Most of this osmosis occurs through the „tight junctions“ between the apical borders of the epithelial cells, as discussed earlier, but a smaller proportion occurs through the cells themselves. The osmotic movement of water creates a flow of fluid into the intercellular space, then through the basement membrane of the epithelium, and finally into the circulating blood of the villi.
Absorption of Chloride Ions in the Duodenum and Jejunum. In the upper part of the small intestine chloride absorption is mainly by passive diffusion. The absorption of sodium ions through the epithelium creates electronegativity in the chyme and electropositivity on the basal side of the epithelial cells. Then chloride ions move along this electrical gradient to „follow“ the sodium ions.
„Active“ Absorption of Bicarbonate Ions in the Duodenum and Jejunum. Often, large quantities of bicarbonate ions must be reabsorbed from the upper small intestine because of the large amounts of bicarbonate ions in both the pancreatic secretion and bile. However, the bicarbonate ion is absorbed in an indirect way as follows: When sodium ions are absorbed, moderate amounts of hydrogen ions are secreted into the lumen of the gut in exchange for some of the sodium, as explained earlier. These hydrogen ions in turn combine with the bicarbonate ion to form carbonic acid (H2CO3), and this then dissociates to form H2O and CO3. The water remains part of the chyme in the intestines, but the carbon dioxide is readily absorbed into the blood and subsequently expired through the lungs. Thus, this is the so-called „active“ absorption of bicarbonate ions. It is the same mechanism that occurs in the tubules of the kidneys.
Active Absorption of Chloride Ions and Active Secretion of Bicarbonate Ions in the Ileum and Large Intestine. The epithelial cells of the ileum and of the large intestine have the special capability of actively absorbing chloride ions by means of a tightly coupled transport mechanism in which an equivalent number of bicarbonate ions are secreted. The functional role of this mechanism is to provide bicarbonate ions for neutralization of acidic products formed by bacteria – especially in the large intestine.
Various bacterial toxins, particularly those of cholera, colon bacilli, and staphylococci, can strongly stimulate this chloride-bicarbonate exchange mechanism.
Absorption of Other Ions. Calcium ions are actively absorbed, especially from the duodenum, and calcium ion absorption is exactly controlled in relation to the need of the body for calcium. One important factor controlling calcium absorption is parathyroid hormone secreted by the parathyroid glands, and another is vitamin D. The parathyroid hormone activates vitamin D in the kidneys, and the activated vitamin D in turn greatly enhances calcium absorption.
Iron ions are also actively absorbed from the small intestine. The principles of iron absorption and the regulation of its absorption in proportion to the body’s need for iron.
Potassium, magnesium, phosphate, and probably still other ions can also be actively absorbed through the mucosa. In general, the monovalent ions are absorbed with ease and in great quantities. On the other hand, the bivalent ions are normally absorbed in only small amounts; for instance, the maximum absorption of calcium ions is only 1/50 as great as the normal absorption of sodium ions. Fortunately, only small quantities of the divalent ions are normally needed by the body.
b) Products of proteins hydrolyses
Absorption of Proteins
Most proteins are absorbed in the form of amino acids. However, small quantities of dipeptides and even tripeptides are also absorbed, and extremely minute quantities of whole proteins can at times be absorbed by the process of pinocytosis, though not by the usual absorptive mechanisms.
The absorption of amino acids also obeys the principles listed above for active absorption of glucose; that is, the different types of amino acids are absorbed selectively and certain ones interfere with the absorption of others, illustrating that common carrier systems exist. Finally, metabolic poisons block the absorption of amino acids in the same way that they block the absorption of glucose.
Absorption of amino acids through the intestinal mucosa can occur far more rapidly than can protein digestion in the lumen of the intestine. As a result, the normal rate of absorption is determined not by the rate at which they can be absorbed but by the rate at which they can be released from the proteins during digestion. For these reasons, essentially no free ammo acids can be found in the intestine during digestion – that is, they are absorbed as rapidly as they are formed. Since most protein digestion occurs in the upper small intestine, most protein absorption occurs in the duodenum and jejunum.
Basic Mechanisms of Amino Acid Transport. As is true for monosaccharide absorption, very little is known about the basic mechanisms of amino acid transport. However, at least four different carrier systems transport different amino acids – one transports neutral amino acids, a second transports basic amino acids, a third transports acidic amino acids, and a fourth has specificity for the two imino acids proline and hydroxyproline. Also, the transport mechanisms have far greater affinity for transporting L-stereoisomers of amino acids than D-stereoisomers.
Amino acid transport (at least for most of the amino acids), like glucose transport, occurs only in the presence of simultaneous sodium transport. Furthermore, the carrier systems for amino acid transport, like those for glucose transport, are in the brush border of the epithelial cell. It is believed that amino acids are transported by the same sodium cotransport mechanism as that explained above for glucose transport. That is, the theory postulates that the carrier has receptor sites for both an amino acid molecule and a sodium ion. Only when both of the sites are filled will the carrier move both the sodium and the amino acid to the interior of the cell at the same time. Because of the sodium gradient across the brush border, the sodium diffusion to the cell interior pulls the amino acid to the interior where the amino acid becomes trapped. Therefore, amino acid concentration increases within the cell, and it then diffuses through the sides or base of the cell into the portal blood, probably by a facilitated diffusion process.
c)Products of carbohydrates hydrolyses
Essentially all the carbohydrates are absorbed in the form of monosaccharides, only a small fraction of a per cent being absorbed as disaccharides and almost none as larger carbohydrate compounds. Furthermore, little carbohydrate absorption results from simple diffusion, for the pores of the mucosa through which diffusion occurs are essentially impermeable to water-soluble solutes with molecular weights greater than 100.
That the transport of most monosaccharides through the intestinal membrane is an active process is demonstrated by several important experimental observations:
1. Transport of most of them, especially glucose and galactose, can be blocked by metabolic inhibitors, such as iodoacetic acid, cyanides, and phlorhizin.
2. The transport is selective, specifically transporting certain monosaccharides without transporting others. The order of preference for transporting different monosaccharides and their relative rates of transport in comparison with glucose are:
3. There is a maximum rate of transport for each type of monosaccharide. The most rapidly transported monosaccharide is galactose, with glucose running a close second. Fructose, which is also one of the three important monosaccharides for nutrition, is absorbed less than half as rapidly as either galactose or glucose; also, its mechanism of absorption is different, as will be explained below.
4. There is competition between certain sugars for the respective carrier system. For instance, if large amounts of galactose are being transported, the amount of glucose that can be transported simultaneously is considerably reduced.
Mechanism of Glucose and Galactose Absorption.
Glucose and galactose transport either ceases or is greatly reduced wherever active sodium transport is blocked. Therefore, it is assumed that the energy required for transport of these two monosaccharides is actually provided by the sodium transport system. A theory that attempts to explain this is the following: It is known that the carrier protein for transport of glucose (which is the carrier for galactose as well) is present in the brush border of the epithelial cell. However, this carrier will not transport the glucose in the absence of sodium transport. Therefore, it is believed that the carrier protein has receptor sites for both a glucose molecule and a sodium ion, and that it will not transport either of these to the interior of the epithelial cell until both receptor sites are simultaneously filled. The energy to cause movement of the carrier from the exterior of the membrane to the interior is derived from the difference in sodium concentration between the outside and inside. That is, as sodium diffuses to the inside of the cell it „drags“ the glucose along with it, thus providing the energy for transport of the glucose. For obvious reasons, this explanation is called the sodium cotransport theory for glucose transport; it is also called secondary active transport of glucose. This sodium cotransport of glucose obviously moves the glucose only to the interior of the cell. However, this increases the intracellular glucose concentration to a higher than normal level, and the glucose then diffuses, probably by facilitated diffusion, through the basolateral membrane of the epithelial cell into the extracellular fluid.
Subsequently, we will see that sodium transport is also required for transport of many if not all amino acids, suggesting a similar „carrier-drag“ mechanism for ammo acid transport.
Absorption of Fructose.
Transport of fructose is slightly different from that of most other monosaccharides. It is not blocked by some of the same metabolic poisons – specifically, phlorhizin – and it does not require metabolic energy for transport, even though it does require a specific carrier. Therefore, it is transported by facilitated diffusion rather than active transport. Also, it is mainly converted into glucose inside the epithelial cell before entering the portal blood, the fructose first becoming phosphorylated, then converted to glucose, and finally released from the epithelial cell into the blood.
Products of fats hydrolyses
As fats are digested to form monoglycerides and free fatty acids, both of these digestive end-products become dissolved in the lipid portion of the bile acid micelles. Because of the molecular dimensions of these micelles, only 2,5 nanometers in diameter, and also because of their highly charged exterior, they are soluble in the chyme. In this form the monoglycerides and the fatty acids are transported to the surfaces of the brush border microvilli, even penetrating into the recesses among the moving, agitating microvilli. On coming in contact with these surfaces, both the monoglycerides and the fatty acids immediately diffuse through the epithelial membrane, because they are equally as soluble in this membrane as in the micelles. This leaves the bile acid micelles still in the chyme. The micelles then diffuse back through the chyme and absorb still more monoglycerides and fatty acids, and similarly transport these also to the epithelial cells. Thus, the bile acids perform a „ferrying“ function, which is highly important for fat absorption. In the presence of an abundance of bile acids, approximately 97 per cent of the fat is absorbed; in the absence of bile acids, only 50 to 60 per cent is normally absorbed.
The mechanism for absorption of the monoglycerides and fatty acids through the brush border is based entirely on the fact that both these substances are highly lipid-soluble. Therefore, they become dissolved in the membrane and simply diffuse to the interior of the cell.
The undigested triglycerides and the diglycerides are both also highly soluble in the lipid membrane of the epithelial cell. However, only small quantities of these are normally absorbed because the bile acid micelles will not dissolve either triglycerides or diglycerides and therefore will not ferry them to the epithelial membrane.
After entering the epithelial cell, the fatty acids and monoglycerides are taken up by the smooth endoplasmic reticulum, and here they are mainly recombined to form new triglycerides. However, a few of the monoglycerides are further digested into glycerol and fatty acids by an epithelial cell lipase. Then, the free fatty acids are reconstituted by the smooth endoplasmic reticulum into triglycerides. Most of the glycerol that is utilized for this purpose is synthesized de novo from alpha-glycerophosphate, this synthesis requiring both energy from ATP and a complex of enzymes to catalyze the reactions.
Once formed, the triglycerides aggregate within the endoplasmic reticulum into globules along with absorbed cholesterol, absorbed phospholipids, and small amounts of newly synthesized cholesterol and phospholipids. The phospholipids arrange themselves in these globules with the fatty portion of the phospholipid toward the center and the polar portions located on the surface. This provides an electrically charged surface that makes these globules miscible with the fluids of the cell. In addition, small amounts of beta-lipoprotein, also synthesized by the endoplasmic reticulum, coat part of the surface of each globule. In this form the globule diffuses to the side of the epithelial cell and is excreted by the process of cellular exocytosis into the space between the cells; from there it passes into the lymph in the central lacteal of the villus. These globules are then called chylomicrons.
The beta-lipoprotein is essential for cellular exocytosis of the chylomicrons to occur, because this protein provides a means for attaching the fatty globule to the cell membrane before it is extruded. In persons who have a genetic inability to form this (3-lipoprotein, the epithelial cells become engorged with fatty products that cannot proceed the rest of the way to be absorbed.
Transport of the Chylomicrons in the Lymph.
From the sides of the epithelial cells the chylomicrons wend their way into the central lacteals of the villi and from here are propelled, along with the lymph, by the lymphatic pump upward through the thoracic duct to be emptied into the great veins of the neck. Between 80 and 90 per cent of all fat absorbed from the gut is absorbed in this manner and is transported to the blood by way of the thoracic lymph in the form of chylomicrons.
Direct Absorption of Fatty Acids into the Portal Blood.
Small quantities of short chain fatty acids, such as those from butterfat, are absorbed directly into the portal blood rather than being converted into triglycerides and absorbed into the lymphatics. The cause of this difference between short and long chain fatty acid absorption is that the shorter chain fatty acids are more water-soluble and are not reconverted into triglycerides by the endoplasmic reticulum. This allows direct diffusion of these fatty acids from the epithelial cells into the capillary blood of the villus.
FUNCTIONAL TYPES OF MOVEMENTS IN THE GASTROINTESTINAL TRACT.
Two basic types of movements occur in the gastrointestinal tract: (1) mixing movements, which keep the intestinal contents thoroughly mixed at all times, and (2) propulsive movements, which cause food to move forward along the tract at an appropriate rate for digestion and absorption.
The mixing movements in most parts of the alimentary tract are caused by either peristaltic contractions or local constrictive contractions of small segments of the gut wall. These movements are modified in different parts of the gastrointestinal tract for proper performance of the respective activities of each part.
The basic propulsive movement of the gastrointestinal tract is peristalsis. A contractile ring appears around the gut and then moves forward; this is analogous to putting one’s fingers around a thin distended tube, then constricting the fingers and moving forward along the tube. Obviously, any material in front of the contractile ring is moved forward.
The usual stimulus for peristalsis is distension. That is, if a large amount
of food collects at any point in the gut, the distension stimulates the gut
wall 2 to
Function of the Myenteric Plexus in Peristalsis. Peristalsis occurs only weakly, if at all, in portions of the gastrointestinal tract that have congenital absence of the my enteric plexus. It is greatly depressed or completely blocked in the entire gut when the person is treated with atropme to paralyze the cholinergic nerve endings of the myenteric plexus. Therefore, effectual peristalsis requires an active myenteric plexus.
Analward Peristaltic Movements. Peristalsis, theoretically, can occur in either direction from a stimulated point, but it normally dies out rapidly in the orad direction while continuing for a considerable distance analward. The exact cause of this directional transmission of peristalsis has never been ascertained, though it probably results mainly from the fact that the myenteric plexus itself is "polarized" in the anal direction, which can be explained as follows:
The Peristaltic Reflex and the „Law of the Gut“. When a segment of the intestinal tract is excited by distension and thereby initiates peristalsis, the contractile ring for causing the peristalsis begins slightly on the oral side of the distended segment; then it moves toward the distended segment, thus pushing the intestinal contents in the anal direction. At the same time, the gut sometimes relaxes several centimeters downstream toward the anus, which is called „receptive relaxation“, thus allowing the food to be propelled more easily analward than in the oral direction.
This complex pattern, consisting of contraction of the gut above the point of distension and relaxation below, does not occur in the absence of the myenteric plexus. Therefore, the complex is frequently called the myenteric reflex, or it is also called simply the peristaltic reflex. And the peristaltic reflex plus the analward direction of movement of the peristalsis is called the „law of the gut“.
Peristalsis In the Fasting Human Being – The Migrating Myoelectric Motor Complex
After a person eats a meal, the nature of the gastrointestinal motor functions is determined mainly by the stimulating effects of the food in the gastrointestinal tract itself. Late at night, many hours after a meal, or at other times when the human being is fasting, a distinct pattern of activity occurs in the stomach and small intestine called the migrating myoelectric motor complex. The most important feature of this migrating complex is that it causes peristaltic waves to sweep slowly and rhythmically downward along the stomach and small intestine approximately once every two hours, sweeping the excess digestive secretions into the colon and therefore preventing their accumulation in the upper gastrointestinal tract.
The migrating complex begins in the body of the
stomach and spreads all the way through the ileum. At any one time only about
The active phase of the migrating complex in any
given segment of the intestine usually lasts for 10 to 15 minutes. During this
time, about 50 peristaltic waves move through this portion of the intestine,
that is, through this distance of about
INGESTION OF FOOD (actual mechanical aspects of food ingestion including mastication and swallowing).
The teeth are admirably designed for chewing, the
anterior teeth (incisors) providing a strong cutting action and the posterior
teeth (molars) a grinding action. All the jaw muscles working together can
close the teeth with a force as great as
Most of the muscles of chewing are innervated by the motor branch of the 5th cranial nerve, and the chewing process is controlled by nuclei in the hindbrain. Stimulation of the reticular formation near the hindbrain centers for taste can cause continual rhythmic chewing movements. Also, stimulation of areas in the hypothalamus, amygdala, and even in the cerebral cortex near the sensory areas for taste and smell can cause chewing.
Much of the chewing process is caused by the chewing reflex, which may be explained as follows: The presence of a bolus of food in the mouth causes reflex inhibition of the muscles of mastication, which allows the lower jaw to drop. The drop in turn initiates a stretch reflex of the jaw muscles that leads to rebound contraction. This automatically raises the jaw to cause closure of the teeth, but it also compresses the bolus again against the linings of the mouth, which inhibits the jaw muscles once again, allowing the jaw to drop and rebound another time, and this is repeated again and again.
Chewing of the food is important for digestion of all foods, but it is especially important for most fruits and raw vegetables, because these have undigestible cellulose membranes around their nutrient portions which must be broken before the food can be utilized. Chewing aids in the digestion of food for the following simple reason: Since the digestive enzymes act only on the surfaces of food particles, the rate of digestion is highly dependent on the total surface area exposed to the intestinal secretions. Also, grinding the food to a very fine particulate consistency prevents excoriation of the gastrointestinal tract and increases the ease with which food is emptied from the stomach into the small intestine and thence into all succeeding segments of the gut.
SWALLOWING (DEGLUTITION). Swallowing is a complicated mechanism, principally because the pharynx most of the time subserves several other functions besides swallowing and is converted for only a few seconds at a time into a tract for propulsion of food. Especially is it important that respiration not be seriously compromised during swallowing.
In general, swallowing can be divided into (1) the voluntary stage, which initiates the swallowing process, (2) the pharyngeal stage, which is involuntary and constitutes the passage of food through the pharynx into the esophagus, and (3) the esophageal stage, another involuntary phase which promotes passage of food from the pharynx to the stomach.
Voluntary Stage of Swallowing. When the food is ready for swallowing, it is „voluntarily“ squeezed or rolled posteriorly in the mouth by pressure of the tongue upward and backward against the palate. Thus, the tongue forces the bolus of food into the pharynx. From here on, the process of swallowing becomes entirely, or almost entirely, automatic and ordinarily cannot be stopped.
Pharyngeal Stage of Swallowing. When the bolus of food is pushed backward in the mouth, it stimulates swallowing receptor areas all around the opening of the pharynx, especially on the tonsillar pillars, and impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscular contractions as follows:
1. The soft palate is pulled upward to close the posterior nares, in this way preventing reflux of food into the nasal cavities.
2. The palatopharyngeal folds on either side of the pharynx are pulled medialward to approximate each other. In this way these folds form a sagittal slit through which the food must pass into the posterior pharynx. This slit performs a selective action, allowing food that has been masticated properly to pass with ease while impeding the passage of large objects. Since this stage of swallowing lasts less than 1 second, any large object is usually impeded too much to pass through the pharynx into the esophagus.
3. The vocal cords of the larynx are strongly approximated, and the hyoid bone and larynx are pulled upward and anteriorly by the neck muscles, causing the epiglottis to swing backward over the superior opening of the larynx. Both these effects prevent passage of food into the trachea. Especially important is the approximation of the vocal cords, but the epiglottis helps to prevent food from ever getting as far as the vocal cords. Destruction of the vocal cords or of the muscles that approximate them can cause strangulation. On the other hand, removal of the epiglottis usually does not cause serious debility in swallowing.
4. The upward movement of the larynx also
stretches the opening of the esophagus. At the same time, the upper 3 to
5. At the same time that the larynx is raised and the pharyngoesophageal sphincter is relaxed, the superior constrictor muscle of the pharynx contracts, giving rise to a rapid peristaltic wave passing downward over the middle and inferior pharyngeal muscles and into the esophagus, which also propels the food into the esophagus.
To summarize the mechanics of the pharyngeal stage of swallowing – the trachea is closed, the esophagus is opened, and a fast peristaltic wave originating in the pharynx then forces the bolus of food into the upper esophagus, the entire process occurring in 1 to 2 seconds.
Nervous Control of the Pharyngeal Stage of Swallowing. The most sensitive tactile areas of the pharynx for initiation of the pharyngeal stage of swallowing lie in a ring around the pharyngeal opening, with greatest sensitivity in the tonsillar pillars. Impulses are transmitted from these areas through the sensory portions of the trigeminal and glossopharyngeal nerves into a region of the medulla oblongata closely associated with the tractus solitarius which receives essentially all sensory impulses from the mouth.
The successive stages of the swallowing process are then automatically controlled in orderly sequence by neuronal areas distributed throughout the reticular substance of the medulla and lower portion of the pons. The sequence of the swallowing reflex is the same from one swallow to the next, and the timing of the entire cycle also remains constant from one swallow to the next. The areas in the medulla and lower pons that control swallowing are collectively called the deglutition or swallowing center.
The motor impulses from the swallowing center to the pharynx and upper esophagus that cause swallowing are transmitted by the 5th, 9th, 10th, and 12th cranial nerves and even a few of the superior cervical nerves.
In summary, the pharyngeal stage of swallowing is principally a reflex act. It is almost never initiated by direct stimuli to the swallowing center from higher regions of the central nervous system. Instead, it is almost always initiated by voluntary movement of food into the back of the mouth, which, in turn, elicits the swallowing reflex.
Effect of the Pharyngeal Stage of Swallowing on Respiration. The entire pharyngeal stage of swallowing occurs in less than 1 to 2 seconds, thereby interrupting respiration for only a fraction of a usual respiratory cycle. The swallowing center specifically inhibits the respiratory center of the medulla during this time, halting respiration at any point in its cycle to allow swallowing to proceed. Yet, even while a person is talking, swallowing interrupts respiration for such a short time that it is hardly noticeable.
Esophageal Stage of Swallowing. The esophagus functions primarily to conduct food from the pharynx to the stomach, and its movements are organized specifically for this function.
Normally the esophagus exhibits two types of peristaltic movements – primary peristalsis and secondary peristalsis. Primary peristalsis is simply a continuation of the peristaltic wave that begins in the pharynx and spreads into the esophagus during the pharyngeal stage of swallowing. This wave passes all the way from the pharynx to the stomach in approximately 8 to 10 seconds. However, food swallowed by a person who is in the upright position is usually transmitted to the lower end of the esophagus even more rapidly than the peristaltic wave itself, in about 5 to 8 seconds, because of the additional effect of gravity pulling the food downward. If the primary peristaltic wave fails to move all the food that has entered the esophagus into the stomach, secondary peristaltic waves, generated by the enteric nervous system of the esophagus, result from distension of the esophagus by the retained food. These waves are essentially the same as the primary peristaltic waves, except that they originate in the esophagus itself rather than in the pharynx. Secondary peristaltic waves continue to be initiated until all the food has emptied into the stomach.
The peristaltic waves of the esophagus are initiated by vagal reflexes that are part of the overall swallowing mechanism. These reflexes are transmitted through vagal afferent fibers from the esophagus to the medulla and then back again to the esophagus through vagal afferent fibers.
The musculature of the pharynx and the upper quarter of the esophagus is striated muscle, and, therefore, the peristaltic waves in these regions are controlled only by skeletal nerve impulses in the glossopharyngeal and vagus nerves. In the lower two thirds of the esophagus, the musculature is smooth, but this portion of the esophagus is also strongly controlled by the vagus nerves acting through their connections with the enteric nervous system. However, when the vagus nerves to the esophagus are sectioned, the myenteric nerve plexus of the esophagus becomes excitable enough after several days to cause secondary peristaltic waves even without support from the vagal reflexes. Therefore, following paralysis of the swallowing reflex, food forced into the upper esophagus and then pulled by gravity to the lower esophagus still passes readily into the stomach.
Receptive Relaxation of the Stomach. As the esophageal peristaltic wave passes toward the stomach, a wave of relaxation, transmitted through myenteric inhibitory neurons, precedes the constriction. Furthermore, the entire stomach and, to a lesser extent, even the duodenum become relaxed as this wave reaches the lower end of the esophagus. Especially important, also, is relaxation of the gastroesophageal sphincter at the juncture between the esophagus and the stomach. In other words, the constrictor and the stomach are prepared ahead of time to receive food being propelled down the esophagus during the swallowing act.
FUNCTION OF THE LOWER ESOPHAGEAL SPHINCTER (GASTROESOPHAGEAL SPHINCTER)
At the lower end of the esophagus, extending from
about 2 to
A principal function of the lower esophageal sphincter is to prevent reflux of stomach contents into the esophagus. The stomach contents are highly acidic and contain many proteolytic enzymes The esophageal mucosa, except in the lower eighth of the esophagus, is not capable of resisting for long the digestive action of gastric secretions. Fortunately, the tonic constriction of the lower esophageal sphincter prevents significant reflux of stomach contents into the esophagus except under abnormal conditions. Indeed, increased intragastric pressure, except during retching or vomiting, causes a vagal reflex that further constricts the sphincter to add extra insurance against reflux.
Prevention of Reflux by Flutter-Valve Closure of the Distal End of the Esophagus. Another factor that prevents reflux is a valvelike mechanism of that portion of the esophagus that lies immediately beneath the diaphragm. Greatly increased intra-abdommal pressure caves the esophagus inward at this point at the same time that the abdominal pressure also increases the mtragastnc pressure. This flutter-valve closure of the lower esophagus therefore prevents the high pressure in the stomach from forcing stomach contents into the esophagus. Otherwise, every time we should walk, cough, or breath hard, we might expel acid into the esophagus.
MOTOR FUNCTIONS OF THE STOMACH
The motor functions of the stomach are three-fold. (1) storage of large quantities of food until it can be accommodated in the lower portion of the gastrointestmal tract, (2) mixing of this food with gastric secretions until it forms a semifluid mixture called chyme, and (3) slow emptying of the food from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.
Physiologically, the stomach can be divided into two major parts: (1) the corpus, or body, and (2) the antrum. The fundus, located at the upper end of the body of the stomach, is considered by some anatomists to be a separate entity from the body, but physiologically the fundus functions mainly as part of the body.
STORAGE FUNCTION OF THE STOMACH
As food enters the stomach, it forms concentric
circles in the body and fundus of the stomach, the
newest food lying closest to the esophageal opening and the oldest food lying
nearest the wall of the stomach. Normally, when food enters the stomach, a vagal reflex greatly reduces the tone in the muscular wall
of the body of the stomach so that the wall can bulge progressively outward,
accommodating greater and greater quantities of food up to a limit of about
THE BASIC ELECTRICAL RHYTHM OF THE STOMACH
The digestive juices of the stomach are secreted by the gastric glands, which cover almost the entire outer wall of the body of the stomach. These secretions come immediately into contact with that portion of the stored food lying against the mucosal surface of the stomach; when the stomach is filled, weak peristaltic constrictor waves, also called mixing waves, move toward the antrum along the stomach wall approximately once every 20 seconds These waves are controlled by the basic electrical rhythm (called BER) consisting of electrical „slow waves“ that occur spontaneously in the stomach wall. As the waves move down the stomach, they not only cause the secretions to mix with the stored food but also provide weak propulsion to move these mixed contents into the antrum When the stomach is full, these mixing waves usually begin near the midpoint of the stomach, but, as the stomach empties, the waves become stronger and also originate farther back up the stomach wall, thus propelling the last vestiges of stored food into the stomach antrum.
As the constrictor waves progress from the body of the stomach into the antrum, they become more intense, some becoming extremely intense and providing powerful peristaltic constrictor rings that force the antral contents under high pressure toward the pylorus. These constrictor rings also play an exceedingly important role in mixing the stomach contents in the following way: Each time a peristaltic wave passes over the antrum toward the pylorus, it digs deeply into the contents of the antrum. Yet the opening of the pylorus is small enough that only a few milliliters of antral contents are expelled into the duodenum with each peristaltic wave. Also, as each peristaltic wave approaches the pylorus, the pyloric muscle itself contracts, which further impedes emptying through the pylorus. Therefore, most of the antral contents are squirted backward through the peristaltic ring toward the body of the stomach. Thus, the moving peristaltic constrictive ring, combined with this squirting action, called „retropulsion“, is an exceedingly important mixing mechanism of the stomach.
Chyme. After the food has become mixed with the stomach secretions, the resulting mixture that passes on down the gut is called chyme. The degree of fluidity of chyme depends on the relative amounts of food and stomach secretions and on the degree of digestion that has occurred. The appearance of chyme is that of a murky, milky semifluid or paste.
Hunger Contractions. Besides the peristaltic contractions that occur when food is present in the stomach, another type of intense contractions, called hunger contractions, often occurs when the stomach has been empty for a long time. These are usually rhythmic peristaltic contractions probably representing exacerbated peristaltic mixing waves in the body of the stomach. However, when they become extremely strong, they often fuse together to cause a continuing tetanic contraction lasting for as long as two to three minutes.
Hunger contractions are usually most intense in young healthy persons with high degrees of gastrointestinal tonus, and they are also greatly increased by a low level of blood sugar.
When hunger contractions occur in the stomach, the person sometimes experiences a sensation of pain in the pit of the stomach, called hunger pangs. Hunger pangs usually do not begin until 12 to 24 hours after the last ingestion of food; in starvation they reach their greatest intensity in three to four days, and then gradually weaken in succeeding days.
Hunger contractions are often associated with a feeling of hunger and therefore are perhaps an important means by which the alimentary tract intensifies the animal drive to acquire food when a person is in a state of incipient starvation.
Basically, stomach emptying is opposed by resistance of the pylorus to the passage of food, and it is promoted by peristaltic waves in the antrum of the stomach.
Role of the Pylorus in Stomach Emptying.
The pylorus normally remains almost, but not completely, closed because of tonic contraction of the pyloric muscle. The closing force is weak enough that water and other fluids empty from the stomach with ease. On the other hand, it is great enough to prevent movement of semi-solid chyme into the duodenum except when a strong antral peristaltic wave forces the chyme through. However, the degree of constriction of the pyloric sphincter can increase or decrease under the influence of signals both from the stomach and from the duodenum, as we shall discuss subsequently. In this way, the pylorus plays an important role in the control of stomach emptying.
Role of Antral Peristalsis in Stomach Emptying – The Pyloric Pump.
The intensity of antral peristalsis changes markedly
under different conditions, especially in response to signals both from the
stomach and from the duodenum. Most of the time the antral peristaltic contractions are weak and function
mainly to cause mixing of the food and gastric secretions, thus increasing the
fluidity of the chyme. However, about 20 per
cent of the time while food is in the stomach, these peristaltic contractions
become very intense at the incisura angularis of the stomach and spread through the antrum no longer as weak mixing waves, but instead as
strong peristaltic, ringlike constrictions. As the
stomach becomes progressively more and more empty, these constrictions begin
farther and farther up the body of the stomach, gradually pinching off the
lowermost portions of the stored food and adding this food to the chyme in the antrum. These
intense peristaltic waves often create as much as 50 to
When pyloric tone is normal, each strong antral peristaltic wave forces several milliliters of chyme into the duodenum. Thus, the peristaltic waves provide a pumping action that is frequently called the „pyloric pump“.
Regulation of Stomach Emptying. The rate at which the stomach empties is regulated by signals both from the stomach and from the duodenum. The stomach signals are mainly twofold: (1) nervous signals caused by distension of the stomach by food, and (2) the hormone gastrin released from the antral mucosa in response to the presence of certain types of food in the stomach. Both these signals increase pyloric pumping force and at the same time inhibit the pylorus, thus promoting stomach emptying.
On the other hand, signals from the duodenum depress the pyloric pump and usually increase pyloric tone at the same time. In general, when an excess volume of chyme or excesses of certain types of chyme enter the duodenum, strong negative feedback signals, both nervous and hormonal, depress the pyloric pump and enhance pyloric sphincter tone. Obviously, these feedback signals allow chyme to enter the duodenum only as rapidly as it can be processed by the small intestine.
Effect of Gastric Food Volume on Rate of Emptying. It is very easy to see how increased food volume in the stomach could promote increased emptying from the stomach. However, this increased emptying does not occur for the reasons that one would expect. It is not increased pressure in the stomach that causes the increased emptying because, in the usual normal range of volume, the increase in volume does not increase the pressure significantly. On the other hand, stretch of the stomach wall does elicit vagal and local myenteric reflexes in the wall that increase the activity of the pyloric pump and at the same time inhibit the pylorus. In general, the rate of food emptying from the stomach is approximately proportional to the square root of the volume of food remaining in the stomach at any given time.
Effect of the Hormone Gastrin on Stomach Emptying. In the following chapter we shall see that stretch, as well as the presence of certain types of foods in the stomach –particularly meat – elicits release of a hormone called gastrin from the antral mucosa, and this has potent effects on causing secretion of highly acidic gastric juice by the stomach fundic glands. However, gastrin also has moderate stimulatory effects on motor functions of the stomach as well. Most important, it enhances the activity of the pyloric pump while at the same time relaxing the pylorus. Thus, it is an important influence for promoting stomach emptying. It also has a slight constrictor effect on the gastroesophageal sphincter at the lower end of the esophagus for preventing reflux of gastric contents into the esophagus during the enhanced gastric activity.
The Inhibitory Effect of the Enterogastric Reflex from the Duodenum on Pyloric Activity. Reflex nervous signals are transmitted from the duodenum back to the stomach most of the time when the stomach is emptying food into the duodenum These signals play an especially important role in controlling both the pyloric pump and the pylorus and, therefore, also in determining the rate of emptying of the stomach. The nervous reflexes are mediated partly through the extrinsic nerves, some going to the prevertebral sympathetic ganglia and then returning through inhibitory sympathetic nerve fibers to the stomach, and others transmitted in the vagus nerves to the brain stem and then back through the same vagi to the stomach. However, local signals are probably also transmitted from the upper portion of the duodenum to the pylorus directly through the enteric nervous system within the gut wall itself.
The types of factors that are continually monitored in the duodenum and that can elicit the enterogastric reflex include:
1 The degree of distension of the duodenum.
2. The presence of any degree of irritation of the duodenal mucosa.
3. The degree of acidity of the duodenal chyme.
4. The degree ofosmolality of the chyme.
5. The presence of certain breakdown products in the chyme, especially breakdown products of proteins and perhaps to a lesser extent of fats.
The enterogastric reflex is especially sensitive to the presence of irritants and acids in the duodenal chyme. For instance, whenever the pH of the chyme in the duodenum falls below approximately 3.5 to 4, this reflex is immediately elicited, which inhibits the pyloric pump and increases pyloric constriction, thus reducing or even blocking further release of acidic stomach contents into the duodenum until the duodenal chyme can be neutralized by pancreatic and other secretions.
Breakdown products of protein digestion will also elicit this reflex; by slowing the rate of stomach emptying, sufficient time is insured for adequate protein digestion in the upper portion of the small intestine.
Finally, either hypo- or hypertonic fluids (especially hypertonic) will elicit the enterogastric reflex. This effect prevents too rapid flow of nonisotonic fluids into the small intestine, thereby preventing rapid changes in electrolyte balance of the body fluids during absorption of the intestinal contents.
Hormonal Feedback from the Duodenum in Inhibiting Gastric Emptying – Role of Fats. Not only do nervous reflexes from the duodenum to the stomach inhibit stomach emptying, but hormones released from the upper intestine do so as well. The stimulus for producing the hormones is mainly fats entering the duodenum, though other types of foods can stimulate the hormones to a lesser degree. On entering the duodenum, the fats extract several different hormones from the duodenal andjejunal epithelium, and these in turn are carried by way of the blood to the stomach where they (a) inhibit the activity of the pyloric pump and at the same time (b) increase slightly the strength of contraction of the pyloric sphincter. These effects are important because fats are much slower to be digested than are most other foods; this inhibitory feedback mechanism allows the necessary time before the fats pass deeper into the intestinal tract where they are to be absorbed.
Unfortunately, the precise hormones that cause the hormonal feedback inhibition of the stomach are not fully clear. In the past, this mixture of hormones has been called enterogastrone, but such a single hormone has never been identified as a specific entity. On the other hand, several different hormones released by the mucosa of the upper small intestine are known to inhibit stomach emptying. One of these is cholecystokinin, which is released from the mucosa of the jejunum in response to fatty substances in the chyme. This hormone acts as a competitive inhibitor to block the increased stomach motility caused by gastrin. Another is the hormone secretin, which is released mainly from the duodenal mucosa in response to gastric acid released from the stomach through the pylorus. This hormone has the general but only weak effect of decreasing gastrointestinal motility. Finally, a hormone called gastric inhibitory peptide, which is released from the upper small intestine in response mainly to fat in the chyme but to carbohydrates as well, is known also to inhibit gastric motility under some conditions. (However, its effect at physiological concentrations is probably mainly to stimulate the secretion of insulin by the pancreas.) All these hormones will be discussed at greater length elsewhere in this text, especially in the following chapter where both cholecystokinin and secretin will be discussed in detail.
In summary, several different hormones are known that could serve as hormonal mechanisms for inhibiting gastric emptying when excess quantities of chyme, especially acidic or fatty chyme, enter the duodenum from the stomach.
Emptying of the stomach is controlled to a moderate degree by stomach factors, such as the degree of filling in the stomach and the excitatory effect of gastrin on antral peristalsis. However, probably the more important control of stomach emptying resides in feedback signals from the duodenum, including both the enterogastric feedback reflex and hormonal feedback. These two feedback inhibitory signals work together to slow the rate of emptying when (a) too much chyme is already in the small intestine or (b) the chyme is excessively acid, contains too much protein or fat, is hypotonic or hypertonic, or is irritating. In this way the rate of stomach emptying is limited to that amount of chyme that the small intestine can process.
The movements of the small intestine, as elsewhere in the gastrointestinal tract, can be divided into the mixing contractions and the propulsive contractions. However, to a great extent this separation is artificial because essentially all movements of the small intestine cause at least some degree of both mixing and propulsion. Yet, the usual classification of these processes is the following:
MIXING CONTRACTIONS (SEGMENTATION CONTRACTIONS)
When a portion of the small intestine becomes
distended with chyme, the stretch of the intestinal
wall elicits localized concentric contractions spaced at intervals along the
intestine. The longitudinal length of each one of the contractions is only
The maximum frequency of the segmentation contractions in the small intestine is determined by the frequency of the slow waves in the intestinal wall, which is the basic electrical rhythm (BER) as explained earlier. Since this frequency is about 12 per minute in the duodenum, the maximum frequency of the segmentation contractions in the duodenum is also about 12 per minute. However, in the ileum, the maximum frequency is usually 8 to 9 contractions per minute.
The segmentation contractions are exceedingly weak when the excitatory activity of the enteric nervous system is blocked by atropine. Therefore, even though it is the slow waves in the smooth muscle itself that control the segmentation contractions, these contractions are not really effective without background excitation by the enteric nervous system, especially by the myenteric plexus.
PROPULSIVE MOVEMENTS. Peristalsis in the Small
Intestine. Chyme is
propelled through the small intestine by by peristaltic waves. These can occur in any
part of the small intestine, and they move analward
at a velocity of 0.5 to
Peristaltic activity of the small intestine is greatly increased after a meal This is caused partly by the beginning entry of chyme into the duodenum but also by the so-called gastroenteric reflex that is initiated by distension of the stomach and conducted principally through the myenteric plexus from the stomach down along the wall of the small intestine. This reflex increases the overall degree of excitability of the small intestine, including both increased motility and secretion.
The function of the peristaltic waves in the small intestine is not only to cause progression of the chyme toward the ileocecal valve but also to spread out the chyme along the intestinal mucosa. As the chyme enters the intestine from the stomach and causes initial distension of the proximal intestine, the elicited peristaltic waves begin immediately to spread the chyme along the intestine, and this process intensifies as additional chyme enters the intestine. On reaching the ileocecal valve the chyme is sometimes blocked for several hours until the person eats another meal, when a new gastroenteric (also called gastroileal) reflex intensifies the peristalsis in the ileum and forces the remaining chyme through the ileocecal valve into the cecum.
The Propulsive Effect of the Segmentation Movements. The segmentation movements, though they last for only a few seconds, also travel in the analward direction and help propel the food down the intestine. Therefore, the difference between the segmentation and the peristaltic movements is not as great as might be implied by their separation into these two classifications.
The Peristaltic Rush. Though peristalsis in the small intestine is normally very weak, intense irritation of the intestinal mucosa, as occurs in some severe cases of infectious diarrhea, can cause both very powerful and rapid peristalsis called the peristaltic rush. This is initiated mainly by extrinsic nervous reflexes to the brain stem and back again to the gut. The powerful peristaltic contractions then travel long distances in the small intestine within minutes, sweeping the contents of the intestine into the colon and thereby relieving the small intestine of either irritative chyme or excessive distension.
Movements Caused by the Muscularis Mucosae and Muscle Fibers of the Villi
The muscularis mucosae, which is stimulated by local nervous reflexes in the submucosal plexus, can cause short or long folds to appear in the intestinal mucosa and also cause the folds to move to progressively newer areas of mucosa. Individual fibers from this muscle extend into the intestinal villi and cause them to contract intermittently. The mucosal folds increase the surface area exposed to the chyme, thereby increasing the rate of absorption. The contractions of the villi – shortening, elongating, and shortening again – „milk“ the villi so that lymph flows freely from the central lacteals into the lymphatic system. Both these types of contraction also agitate the fluids surrounding the villi so that progressively new areas of fluid become exposed to absorption.
These mucosal and villous contractions are initiated by local nervous reflexes that occur in response to chyme in the small intestine.
function of the ileocecal valve is to prevent
backflow of fecal contents from the colon into the small intestine. The lips of
the ileocecal valve protrude into the lumen of the cecum and therefore are forcefully closed when the cecum fills. Usually the valve can resist reverse pressure
of as much as 50 to
The wall of the ileum for several centimeters immediately preceding the ileocecal valve has a thickened muscular coat called the ileocecal sphincter. This normally remains mildly constricted and slows the emptying of ileal contents into the cecum except immediately following a meal when a gastroileal reflex (described earlier) intensifies the peristalsis in the ileum. Also, the hormone gastrin, which is liberated from the stomach mucosa in response to food in the stomach, has a relaxant effect on the ileocecal sphincter, thus allowing increased emptying. Even so, only about 1500 ml of chyme empty into the cecum each day. The resistance to emptying at the ileocecal valve prolongs the stay of chyme in the ileum and, therefore, facilitates absorption.
Feedback Control of the Ileocecal Sphincter. The degree of contraction of the ileocecal sphincter, as well as the intensity of peristalsis in the terminal ileum, is also controlled strongly by reflexes from the cecum. Whenever the cecum is distended, the degree of contraction of the ileocecal sphincter is intensified while ileal peristalsis is inhibited, which greatly delays emptying of additional chyme from the ileum. Also, any irritant in the cecum delays emptying. For instance, when a person has an inflamed appendix, the irritation of this vestigial remnant of the cecum can cause such intense spasm of the ileocecal sphincter and paralysis of the ileum that this completely blocks emptying of the ileum. These reflexes from the cecum to the ileocecal sphincter and ileum are mediated both by way of the myenteric plexus and through extrinsic nerves, especially reflexes by way of the prevertebral sympathetic ganglia.
MOVEMENTS OF THE
The functions of the colon are (1) absorption of water and electrolytes from the chyme and (2) storage of fecal matter until it can be expelled. The proximal half of the colon is concerned principally with absorption, and the distal half with storage. Since intense movements are not required for these functions, the movements of the colon are normally sluggish. Yet in a sluggish manner, the movements still have characteristics similar to those of the smal
l intestine and can be divided once again into mixing movements and propulsive movements.
Movements – Haustrations.
In the same manner that segmentation movements occur in the small intestine,
large circular constrictions also occur in the large intestine. At each of
these constriction points, about
Propulsive Movements – „Mass Movements“. Peristaltic waves of the type seen in the small intestine only rarely occur in most parts of the colon. Instead, most propulsion occurs by (1) the slow analward movement of the haustral contractions just discussed and (2) mass movements.
Most of the propulsion in the cecum and ascending colon results from the slow but persistent haustral contractions, requiring as many as 8 to 15 hours to move the chyme only from the ileocecal valve to the transverse colon, while the chyme itself becomes fecal in quality and also becomes a semisolid slush instead of a semifluid.
From the transverse colon to the sigmoid, mass movements mainly take over the propulsive role. These movements usually occur only a few times each day, most abundantly for about 15 minutes during the first hour after eating breakfast.
A mass movement is a modified type of peristalsis characterized by the following sequence of events: First, a constrictive ring occurs at a distended or irritated point in the colon, usually in the transverse colon, and then rapidly thereafter the 20 or more centimeters of colon distal to the constriction contract almost as a unit, forcing the fecal material in this segment en masse down the colon. During this process, the haustrations disappear completely. The initiation of contraction is complete in about 30 seconds, and relaxation then occurs during the next 2 to 3 minutes before another mass movement occurs, this time perhaps farther along the colon. But the whole series of mass movements will usually persist for only 10 minutes to half an hour, and they will then return perhaps a half day or even a day later.
Mass movements can occur in any part of the colon, though most often they occur in the transverse or descending colon. When they have forced a mass of feces into the rectum, the desire for defecation is felt.
Initiation of Mass Movements by the Gastrocolic and Duodenocolic Reflexes. The appearance of mass movements after meals is facilitated by gastrocohc and duodenocohc reflexes. These reflexes result from distension of the stomach and duodenum. They can take place only weakly when the extrinsic nerves are removed; therefore, it is probable that weak reflexes are basically transmitted through the myenteric plexus, though reflexes conducted through the extrinsic nerves of the autonomic nervous system probably provide most of the intensity of the gastrocolic and duodenocohc reflexes.
Irritation in the colon can also initiate intense mass movements. For instance, a person who has an ulcerated condition of the colon (ulcerative colitis) frequently has mass movements that persist almost all the time.
Mass movements can also be initiated by intense stimulation of the parasympathetic nervous system or simply by overdistension of a segment of the colon.
of the time, the rectum is empty of feces. This results partly from the fact
that a weak functional sphincter exists approximately
Continual dribble of fecal matter through the anus is prevented by tonic constriction of (1) the internal anal sphincter, a circular mass of smooth muscle that lies immediately inside the anus, and (2) the external anal sphincter, composed of striated voluntary muscle that both surrounds the internal sphincter and also extends distal to it; the external sphincter is controlled by nerve fibers in the pudendal nerve, which is part of the somatic nervous system and therefore is under voluntary, conscious control.
OTHER AUTONOMIC REFLEXES AFFECTING BOWEL ACTIVITY
Aside from the duodenocolic, gastrocolic, gastroileal, enterogastric, and defecation reflexes, several other important nervous reflexes can affect the overall degree of bowel activity. These are the peritoneointestinal reflex, reno-intestinal reflex, vesicointestinal reflex, and somato-intestinal reflex. All these reflexes are initiated by sensory signals that pass to the prevertebral sympathetic ganglia or to the spinal cord and then are transmitted through the sympathetic nervous system back to the gut. And they all inhibit gastrointestinal activity.
The peritoneointestinal reflex results from irritation of the peritoneum, it strongly inhibits the excitatory enteric nerves and thereby causes intestinal paralysis. The renointestinal and vesicointestinal reflexes inhibit intestinal activity as a result of kidney or bladder irritation. Finally, the somato-intestinal reflex causes intestinal inhibition when the skin over the abdomen is irritatingly stimulated.
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