Gastro Intestinal Secretions
Types
of Glands
Several types of glands provide the different types
of secretions.
1. On the surface of the epithelium
in most parts of the gastrointestinal tract are billions of single cell mucous
glands called simply mucous cells or sometimes goblet cells because they look
like goblets. They function mainly in response to local irritation of the
epithelium: They extrude mucus directly onto the epithelial surface to act as a
lubricant that also protects the surfaces from excoriation and digestion.
2. Many surface areas of the
gastrointestinal tract are lined by pits that represent invaginations of the
epithelium into the submucosa. In the small intestine, these pits, called
crypts of Lieberkühn, are deep and contain specialized secretory cells.
3. In the stomach and upper
duodenum are large numbers of deep tubular glands.
4. Also associated with the
alimentary tract are several complex glands—the salivary glands, pancreas, and
liver—that provide secretions for digestion or emulsification of food.
The salivary glands and the pancreas are compound
acinous glands. These glands lie outside the walls of the alimentary tract and,
in this, differ from all other alimentary glands. They contain millions of
acini lined with secreting glandular cells; these acini feed into a system of
ducts that finally empty into the alimentary tract itself.
Basic
Mechanisms of Stimulation of the Alimentary Tract Glands
Functions
of Enteric Nervous Stimuli
The mechanical presence of food in a particular
segment of the gastrointestinal tract usually causes the glands of that region
and adjacent regions to secrete moderate to large quantities of juices.
Some part of this local effect, especially the
secretion of mucus by mucous cells, results from direct contact stimulation of
the surface glandular cells by the food. In addition, local epithelial
stimulation also activates the enteric nervous system of the gut wall. The
types of stimuli that do this are –
·
Tactile stimulation
·
Chemical irritation
·
Distention of the gut wall
The resulting nervous reflexes stimulate both the
mucous cells on the gut epithelial surface and the deep glands in the gut wall
to increase their secretion.
Autonomic
Stimulation of Secretion
Parasympathetic Stimulation Increases Alimentary
Tract Glandular Secretion Rate. Stimulation of the parasympathetic nerves to
the alimentary tract almost invariably increases the rates of alimentary
glandular secretion.
Like the glands in the upper portion of the tract
(innervated by the Glossopharyngeal and vagus parasympathetic nerves) such as
the salivary glands, esophageal glands, gastric glands, pancreas, and Brunner’s
glands in the duodenum as also of some glands in the distal portion of the
large intestine, innervated by pelvic parasympathetic nerves. Secretion in the
remainder of the small intestine and in the first two thirds of the large
intestine occurs mainly in response to local neural and hormonal stimuli in
each segment of the gut.
Sympathetic Stimulation has a dual effect on
alimentary tract glandular Secretion rate. Stimulation of the sympathetic
nerves going to the gastrointestinal tract causes a slight to moderate increase
in secretion by some of the local glands. But sympathetic stimulation also
results in constriction of the blood vessels that supply the glands. Therefore,
sympathetic stimulation can have a dual effect:
(1) Sympathetic stimulation alone
usually slightly increases secretion
(2) If parasympathetic or hormonal
stimulation is already causing copious secretion by the glands, superimposed
sympathetic stimulation usually reduces the secretion, mainly because of
vasoconstrictive reduction of the blood supply.
Regulation
of Glandular Secretion by Hormones
In the stomach and intestine, several different
gastrointestinal hormones help regulate the volume and character of the
secretions. These hormones are secreted from the gastrointestinal mucosa in
response to the presence of food in the lumen of the gut. The hormones are then
absorbed into the blood and carried to the glands, where they stimulate
secretion.
This type of stimulation is particularly important
to increase the output of gastric juice and pancreatic juice when food enters
the stomach or duodenum. Chemically, the gastrointestinal hormones are
polypeptides or polypeptide derivatives.
Basic
Mechanism of Secretion by Glandular Cells
Secretion
of Organic Substances
Although all the basic mechanisms by which glandular
cells function are not known, experimental evidence points to the following
principles of secretion.
1.The nutrient material needed for
formation of the secretion must first diffuse or be actively transported by the
blood in the capillaries into the base of the glandular cell.
2. Many mitochondria located inside
the glandular cell near its base use oxidative energy to form adenosine
triphosphate (ATP).
3. Energy from the ATP, along with
appropriate substrates provided by the nutrients, is then used to synthesize
the organic secretory substances; this synthesis occurs almost entirely in the
endoplasmic reticulum and Golgi complex of the glandular cell. Ribosomes
adherent to the reticulum are specifically responsible for formation of the
proteins that are secreted.
4. The secretory materials are
transported through the tubules of the endoplasmic reticulum, passing in about
20 minutes all the way to the vesicles of the Golgi complex.
5. In the Golgi complex, the
materials are modified, added to, concentrated, and discharged into the
cytoplasm in the form of secretory vesicles, which are stored in the apical
ends of the secretory cells.
6. These vesicles remain stored
until nervous or hormonal control signals cause the cells to extrude the
vesicular contents through the cells’ surface. The control signal first
increases the cell membrane permeability to calcium ions, and calcium enters
the cell. The calcium in turn causes many of the vesicles to fuse with the
apical cell membrane. Then the apical cell membrane breaks open, thus emptying
the vesicles to the exterior; this process is called exocytosis.
Water
and Electrolyte Secretion
A second necessity for glandular secretion is
secretion of sufficient water and electrolytes to go along with the organic
substances, washing the organic substances through the secretory border of the
cells at the same time. Hormones acting on the cell membrane of some glandular
cells are believed also to cause secretory effects similar to those caused by
nervous stimulation.
Importance
of Mucus in the Gastrointestinal Tract
Mucus is a
thick secretion composed mainly of water, electrolytes, and a mixture of
several glycoproteins, which themselves are composed of large polysaccharides
bound with much smaller quantities of protein. Mucus is slightly different in
different parts of the gastrointestinal tract, but everywhere it has several
important characteristics that make it both an excellent lubricant and a
protectant for the wall of the gut as follows.
1. Mucus has adherent qualities
that make it adhere tightly to the food or other particles and to spread as a
thin film over the surfaces.
2. Mucus has sufficient body that
it coats the wall of the gut and prevents actual contact of most food particles
with the mucosa.
3. Mucus has a low resistance for
slippage, so the particles can slide along the epithelium with great ease.
4. Mucus causes fecal particles to
adhere to one another to form the feces that are expelled during a bowel
movement.
5. Mucus is strongly resistant to
digestion by the gastrointestinal enzymes.
6. The glycoproteins of mucus have
amphoteric properties, which mean they are capable of buffering small amounts
of either acids or alkalies.
7. Mucus often contains moderate
quantities of bicarbonate ions, which specifically neutralize acids.
In short mucus has the ability to allow easy
slippage of food along the gastrointestinal tract and to prevent excoriation or
chemical damage to the epithelium. When the salivary glands fail to secrete
saliva, it is difficult to swallow solid food even when it is taken along with
large amounts of water.
Secretion
of Saliva
The principal glands of salivation are the parotid,
submandibular, and sublingual glands; in addition, there are many tiny buccal
glands. Daily secretion of saliva normally ranges between 800 and 1500
milliliters, as shown by the average value of 1000 milliliters in Table.
Saliva contains two major types of protein
secretion:
(1) A serous secretion that
contains ptyalin (an α-amylase), which is an enzyme for digesting starches
(2) Mucus secretion that contains
mucin for lubricating and for surface protective purposes
The parotid glands secrete almost entirely the
serous type of secretion, whereas the submandibular and sublingual glands
secrete both serous secretion and mucus. The buccal glands secrete only mucus.
Saliva has a pH between 6.0 and 7.0, a favorable range for the digestive action
of ptyalin.
Secretion
of Ions in Saliva
Saliva contains especially large quantities of
potassium and bicarbonate ions. Conversely, the concentrations of both sodium
and chloride ions are several times less in saliva than in plasma. Salivary secretion
is a two-stage operation-
·
The first stage involves the acini
·
The second stage involves salivary ducts
The acini secrete a primary secretion that contains
ptyalin and/or mucin in a solution of ions in concentrations not greatly
different from those of typical extracellular fluid.
As the primary secretion flows through the ducts,
two major active transport processes take place that markedly modify the ionic
composition of the fluid in the saliva.
1. Sodium ions are actively
reabsorbed from all the salivary ducts and potassium ions are actively secreted
in exchange for the sodium. Therefore, the sodium ion concentration of the
saliva becomes greatly reduced, whereas the potassium ion concentration becomes
increased. However, there is excess sodium reabsorption over potassium
secretion, and this creates electrical negativity of about −70 millivolts in
the salivary ducts; this in turn causes chloride ions to be reabsorbed
passively. Therefore, the chloride ion concentration in the salivary fluid
falls to a very low level, matching the ductal decrease in sodium ion
concentration.
2. Bicarbonate ions are secreted by
the ductal epithelium into the lumen of the duct. This is at least partly
caused by passive exchange of bicarbonate for chloride ions, but it may also
result partly from an active secretory process. The net result of these
transport processes is, that under resting conditions, the concentrations of
sodium and chloride ions in the saliva are about 15 mEq/L each, about
one-seventh to one-tenth their concentrations in plasma. Conversely, the
concentration of potassium ions is about 30 mEq/L, seven times as great as in
plasma, and the concentration of bicarbonate ions is 50 to 70 mEq/L, about two
to three times that of plasma.
During maximal salivation, the salivary ionic
concentrations change considerably because the rate of formation of primary
secretion by the acini can increase as much as 20-fold. This acinar secretion
then flows through the ducts so rapidly that the ductal reconditioning of the
secretion is considerably reduced. Therefore, when copious quantities of saliva
are being secreted, the sodium chloride concentration is about one-half or
two-thirds that of plasma, and the potassium concentration rises to only four
times that of plasma.
Function
of Saliva for oral hygiene
Under basal awake conditions, about 0.5 milliliter
of saliva, almost entirely of the mucous type, is secreted each minute; but
during sleep, little secretion occurs. This secretion plays an exceedingly
important role for maintaining healthy oral tissues. The mouth is loaded with
pathogenic bacteria that can easily destroy tissues and cause dental caries.
Saliva helps prevent the deteriorative processes in several ways.
1. The flow of saliva itself helps
wash away pathogenic bacteria, as well as food particles that provide their
metabolic support.
2. Saliva contains several factors
that destroy bacteria. One of these is thiocyanate ions and another is several
proteolytic enzymes, most importantly, Lysozyme, that
(a) Attack the bacteria
(b) Aid the thiocyanate ions in
entering the bacteria where these ions in turn become bactericidal
(c) Digest food particles, thus
helping further to remove the bacterial metabolic support.
3. Saliva often contains
significant amounts of protein antibodies that can destroy oral bacteria,
including some that cause dental caries. In the absence of salivation, oral
tissues often become ulcerated and otherwise infected, and caries of the teeth
can become rampant.
Nervous
Regulation of Salivary Secretion
The salivary
glands are controlled mainly by parasympathetic nervous signals all the way
from the superior and inferior salivatory nuclei in the brain stem. The
salivatory nuclei are located approximately at the juncture of the medulla and
pons and are excited by both taste and tactile stimuli from the tongue and
other areas of the mouth and pharynx.
Many taste stimuli, especially the sour taste
(caused by acids), elicit copious secretion of saliva—often 8 to 20 times the
basal rate of secretion. Also, certain tactile stimuli, such as the presence of
smooth objects in the mouth (e.g., a pebble), cause marked salivation, whereas
rough objects cause less salivation and occasionally even inhibit salivation.
Salivation can also be stimulated or inhibited by
nervous signals arriving in the salivatory nuclei from higher centers of the
central nervous system. For instance, when a person smells or eats favorite
foods, salivation is greater than when disliked food is smelled or eaten. The
appetite area of the brain, which partially regulates these effects, is located
in proximity to the parasympathetic centers of the anterior hypothalamus, and
it functions to a great extent in response to signals from the taste and smell
areas of the cerebral cortex or amygdala.
Salivation also occurs in response to reflexes
originating in the stomach and upper small intestines particularly when
irritating foods are swallowed or when a person is nauseated because of some
gastrointestinal abnormality. The saliva, when swallowed, helps to remove the
irritating factor in the gastrointestinal tract by diluting or neutralizing the
irritant substances.
Sympathetic stimulation can also increase salivation
a slight amount, much less so than done by parasympathetic stimulation. The
sympathetic nerves originate from the superior cervical ganglia and travel
along the surfaces of the blood vessel walls to the salivary glands.
A secondary factor that also affects salivary
secretion is the blood supply to the glands because secretion always requires
adequate nutrients from the blood. The parasympathetic nerve signals that
induce copious salivation also moderately dilate the blood vessels.
In addition, salivation itself directly dilates the
blood vessels, thus providing increased salivatory gland nutrition as needed by
the secreting cells. Part of this additional vasodilator effect is caused by
kallikrein secreted by the activated salivary cells, which in turn acts as an
enzyme to split one of the blood proteins, an alpha2-globulin, to form
bradykinin, a strong vasodilator.
Esophageal
Secretion
The esophageal secretions are entirely mucous and
mainly provide lubrication for swallowing. The main body of the esophagus is
lined with many simple mucous glands. At the gastric end and to a lesser extent
in the initial portion of the esophagus, there are also many compound mucous
glands. The mucus secreted by the compound glands in the upper esophagus
prevents mucosal excoriation by newly entering food, whereas the compound
glands located near the esophagogastric junction protect the esophageal wall from
digestion by acidic gastric juices that often reflux from the stomach back into
the lower esophagus.
Gastric
Secretions
Characteristics of the Gastric Secretions In
addition to mucus-secreting cells that line the entire surface of the stomach,
the stomach mucosa has two important types of tubular glands:
·
Oxyntic glands (also called gastric
glands)
·
Pyloric glands.
The oxyntic (acid-forming) glands secrete
hydrochloric acid, Pepsinogen, intrinsic factor, and mucus. The oxyntic glands
are located on the inside surfaces of the body and fundus of the stomach,
constituting the proximal 80 percent of the stomach.
The pyloric glands secrete mainly mucus for
protection of the pyloric mucosa from the stomach acid. They also secrete the
hormone gastrin. The pyloric glands are located in the antral portion of the
stomach, the distal 20 percent of the stomach.
Secretions
from the Oxyntic (Gastric) Glands
A typical stomach oxyntic gland is composed of three
types of cells:
(1) Mucous neck cells, which secrete mainly mucus
(2) Peptic (or chief) cells, which secrete large quantities of Pepsinogen
(3) Parietal (or oxyntic) cells, which secrete hydrochloric acid and
intrinsic factor
Basic
Mechanism of Hydrochloric Acid Secretion
When stimulated, the parietal cells secrete an acid
solution that contains about 160 mmol/L of hydrochloric acid, which is nearly
isotonic with the body fluids. The pH of this acid is about 0.8, demonstrating
its extreme acidity. At this pH, the hydrogen ion concentration is about 3
million times that of the arterial blood. To concentrate the hydrogen ions this
tremendous amount requires more than 1500 calories of energy per liter of
gastric juice.
At the same time that hydrogen ions are secreted,
bicarbonate ions diffuse into the blood so that gastric venous blood has a
higher pH than arterial blood when the stomach is secreting acid.
The hydrochloric acid is formed at the villus-like
projections inside these canaliculi and is then conducted through the
canaliculi to the secretory end of the cell. The main driving force for
hydrochloric acid secretion by the parietal cells is a hydrogen-potassium pump
(H+-K+ ATPase).
The chemical mechanism of hydrochloric acid
formation consists of the following steps:
1. Water inside the parietal cell
becomes dissociated into H+ and OH− in the cell
cytoplasm. The H+ is then actively secreted into the canaliculus in exchange
for K+, an active exchange process that is catalyzed by H+K+
ATPase. Potassium ions transported into the cell by the Na+-K+
ATPase pump on the basolateral (extracellular) side of the membrane tend to
leak into the lumen but are recycled back into the cell by the H+-K+
ATPase. The basolateral Na+-K+ ATPase creates low
intracellular Na+, which contributes to Na+ reabsorption from the
lumen of the canaliculus. Thus, most of the K+ and Na+ in
the canaliculus are reabsorbed into the cell cytoplasm, and hydrogen ions take
their place in the canaliculus.
2. The pumping of H+ out
of the cell by the H+-K+ ATPase permits OH− to
accumulate and form HCO3− from CO2, either formed during metabolism
in the cell or entering the cell from
the blood. This reaction is catalyzed by carbonic anhydrase. The HCO3−
is then transported across the basolateral membrane into the extracellular
fluid in exchange for chloride ions, which enter the cell and are secreted
through chloride channels into the canaliculus, giving a strong solution of
hydrochloric acid in the canaliculus. The hydrochloric acid is then secreted
outward through the open end of the canaliculus into the lumen of the gland.
3. Water passes into the
canaliculus by osmosis because of extra ions secreted into the canaliculus.
Thus, the final secretion from the canaliculus contains water, hydrochloric
acid at a concentration of about 150 to 160 mEq/L, potassium chloride at a
concentration of15 mEq/L, and a small amount of sodium chloride. To produce a
concentration of hydrogen ions as great as that found in gastric juice requires
minimal back leak into the mucosa of the secreted acid.
A major part of the stomach’s ability to prevent
back leak of acid can be attributed to the gastric barrier due to the formation
of alkaline mucus and to tight junctions between epithelia cells. If this
barrier is damaged by toxic substances, as occurs with excessive use of aspirin
or alcohol, the secreted acid does leak down an electrochemical gradient into
the mucosa, causing stomach mucosal damage.
Basic
Factors That Stimulate Gastric Secretion
Basic factors that stimulate gastric secretion are
·
Acetylcholine
·
Gastrin
·
Histamine
Acetylcholine released by parasympathetic
stimulation excites secretion of pepsinogen by peptic cells, hydrochloric acid
by parietal cells, and mucus by mucous cells.
In comparison, both gastrin and histamine strongly
stimulate secretion of acid by parietal cells but have little effect on the
other cells.
Several slightly different types of Pepsinogen are
secreted by the peptic and mucous cells of the gastric glands. All types of the
pepsinogens perform the same functions. When Pepsinogen is first secreted, it
has no digestive activity. However, as soon as it comes in contact with
hydrochloric acid, it is activated to form active pepsin.
In this process, the Pepsinogen molecule, having a
molecular weight of about 42,500, is split to form a pepsin molecule, having a
molecular weight of about 35,000. Pepsin functions as an active proteolytic
enzyme in a highly acid medium (optimum pH 1.8 to 3.5), but above a pH of about
5 it has almost no proteolytic activity and becomes completely inactivated in a
short time. Hydrochloric acid is as necessary as pepsin for protein digestion
in the stomach.
Secretion
of Intrinsic Factor by Parietal Cells
The substance intrinsic factor, essential for
absorption of vitamin B12 in the ileum, is secreted by the parietal cells along
with the secretion of hydrochloric acid. When the acid-producing parietal cells
of the stomach are destroyed, which frequently occurs in chronic gastritis, the
person develops not only achlorhydria (lack of stomach acid secretion) but
often also pernicious anemia because of failure of maturation of the red blood
cells in the absence of vitamin B12 stimulation of the bone marrow.
Pyloric
Glands—Secretion of Mucus and Gastrin
The pyloric glands are structurally similar to the
oxyntic glands but contain few peptic cells and almost no parietal cells.
Instead, they contain mostly mucous cells that are identical with the mucous
neck cells of the oxyntic glands. These cells secrete a small amount of
pepsinogen, and an especially large amount of thin mucus that helps to
lubricate food movement, as well as to protect the stomach wall from digestion
by the gastric enzymes. The pyloric glands also secrete the hormone gastrin,
which plays a key role in controlling gastric secretion.
Surface
Mucous Cells
The entire surface of the stomach mucosa between
glands has a continuous layer of a special type of mucous cells called simply
surface mucous cells. They secrete large quantities of viscid mucus that coats
the stomach mucosa with a gel layer of mucus often more than 1 millimeter
thick, thus providing a major shell of protection for the stomach wall, as well
as contributing to lubrication of food transport.
Another characteristic of this mucus is that it is
alkaline. Therefore, the normal underlying stomach wall is not directly exposed
to the highly acidic, proteolytic stomach secretion. Even the slightest contact
with food or any irritation of the mucosa directly stimulates the surface
mucous cells to secrete additional quantities of this thick, alkaline, viscid
mucus.
Stimulation
of Gastric Acid Secretion
The parietal cells, located deep in the oxyntic
glands of the main body of the stomach, are the only cells that secrete
hydrochloric acid. The acidity of the fluid secreted by these cells can be
great, with pH as low as 0.8. However, secretion of this acid is under
continuous control by both endocrine and nervous signals.
The parietal cells operate in close association with
another type of cell called Enterochromaffin like cells (ECL cells), the
primary function of which is to secrete histamine. The ECL cells lie in the
deep recesses of the oxyntic glands and therefore release histamine in direct
contact with the parietal cells of the glands. The rate of formation and secretion
of hydrochloric acid by the parietal cells is directly related to the amount of
histamine secreted by the ECL cells.
In turn, the ECL cells are stimulated to secrete
histamine by the hormonal substance gastrin, which is formed almost entirely in
the antral portion of the stomach mucosa in response to proteins in the foods
being digested.
The ECL cells may also be stimulated by hormonal
substances secreted by the enteric nervous system of the stomach wall. Let us
discuss first the gastrin mechanism for control of the ECL cells and their
subsequent control of parietal cell secretion of hydrochloric acid.
Stimulation
of Acid Secretion by Gastrin
Gastrin is itself a hormone secreted by gastrin
cells, also called G cells. These cells are located in the pyloric glands in
the distal end of the stomach. Gastrin is a large polypeptide secreted in two
forms: a large form called G-34, which contains 34 amino acids, and a smaller
form, G-17, which contains 17 amino acids. Although both of these are important,
the smaller is more abundant.
When meats or other protein-containing foods reach
the antral end of the stomach, some of the proteins from these foods have a
special stimulatory effect on the gastrin cells in the pyloric glands to cause
release of gastrin into the blood to be transported to the ECL cells of the
stomach.
The vigorous mixing of the gastric juices transports
the gastrin rapidly to the ECL cells in the body of the stomach, causing
release of histamine directly into the deep oxyntic glands. The histamine then
acts quickly to stimulate gastric hydrochloric acid secretion.
Regulation
of Pepsinogen Secretion
Regulation of Pepsinogen secretion by the peptic
cells in the oxyntic glands occurs in response to two main types of signals:
(1) Stimulation of the peptic cells
by acetylcholine released from the vagus nerves or from the gastric enteric
nervous plexus
(2) Stimulation of peptic cell
secretion in response to acid in the stomach.
The acid probably does not stimulate the peptic
cells directly but instead elicits additional enteric nervous reflexes that
support the original nervous signals to the peptic cells. Therefore, the rate
of secretion of Pepsinogen, the precursor of the enzyme pepsin that causes
protein digestion, is strongly influenced by the amount of acid in the stomach.
In people who have lost the ability to secrete
normal amounts of acid, secretion of Pepsinogen is also decreased, even though
the peptic cells may otherwise appear to be normal.
Phases
of Gastric Secretion
Gastric secretion is said to occur in three phases-
·
cephalic phase
·
gastric phase
·
intestinal phase
Cephalic
Phase
The cephalic phase of gastric secretion occurs even
before food enters the stomach, especially while it is being eaten. It results
from the sight, smell, thought, or taste of food, and the greater the appetite,
the more intense is the stimulation.
Neurogenic signals that cause the cephalic phase of
gastric secretion originate in the cerebral cortex and in the appetite centers
of the amygdala and hypothalamus. They are transmitted through the dorsal motor
nuclei of the vagi and thence through the vagus nerves to the stomach. This
phase of secretion normally accounts for about 30 percent of the gastric
secretion associated with eating a meal.
Gastric
Phase
Once food enters the stomach, it excites
(1) Long vagovagal reflexes from
the stomach to the brain and back to the stomach
(2) Local enteric reflexes
(3) The gastrin mechanism
All of these in turn cause secretion of gastric
juice during several hours while food remains in the stomach. The gastric phase
of secretion accounts for about 60 percent of the total gastric secretion
associated with eating a meal and therefore accounts for most of the total
daily gastric secretion of about 1500 milliliters.
The presence of food in the upper portion of the
small intestine, particularly in the duodenum, will continue to cause stomach
secretion of small amounts of gastric juice, probably partly because of small
amounts of gastrin released by the duodenal mucosa. This accounts for about 10
percent of the acid response to a meal.
Inhibition
of Gastric Secretion by other Post-Stomach Intestinal Factors
Although intestinal chyme slightly stimulates
gastric secretion during the early intestinal phase of stomach secretion, it
paradoxically inhibits gastric secretion at other times. This inhibition
results from at least two influences.
1. The presence of food in the
small intestine initiates a reverse enterogastric reflex, transmitted through
the myenteric nervous system and extrinsic sympathetic and vagus nerves that
inhibits stomach secretion. This reflex can be initiated by distending the
small bowel, by the presence of acid in the upper intestine, by the presence of
protein breakdown products, or by irritation of the mucosa. This is part of the
complex mechanism for slowing stomach emptying when the intestines are already
filled.
2. The presence of acid, fat, protein
breakdown products, hyper osmotic or hypo osmotic fluids, or any irritating
factor in the upper small intestine causes release of several intestinal
hormones. One of these is secretin, which is especially important for control
of pancreatic secretion. However, secretin opposes stomach secretion.
Three other hormones namely gastric inhibitory
peptide (glucose-dependent insulinotropic peptide), vasoactive intestinal
polypeptide, and somatostatin—also have slight to moderate effects in
inhibiting gastric secretion.
The functional purpose of intestinal factors that
inhibit gastric secretion is presumably to slow passage of chyme from the
stomach when the small intestine is already filled or already overactive. In
fact, the enterogastric inhibitory reflexes plus inhibitory hormones usually
also reduce stomach motility at the same time that they reduce gastric
secretion.
Gastric
Secretion during the Inter digestive Period
The stomach secretes a few milliliters of gastric
juice each hour during inter digestive period, when little or no digestion is
occurring anywhere in the gut. The secretion that does occur is usually almost
entirely of the non-oxyntic type, composed mainly of mucus but little pepsin
and almost no acid.
Emotional stimuli frequently increase inter
digestive gastric secretion (highly peptic and acidic) to 50 milliliters or
more per hour, in much the same way that the cephalic phase of gastric
secretion excites secretion at the onset of a meal. This increase of secretion
in response to emotional stimuli is believed to be one of the causative factors
in development of peptic ulcers.
Chemical
Composition of Gastrin and Other Gastrointestinal Hormones
Gastrin, cholecystokinin (CCK), and secretin are all
large polypeptides with approximate molecular weights, respectively, of 2000,
4200, and 3400. The terminal five amino acids in the gastrin and CCK molecular
chains are the same. The functional activity of gastrin resides in the terminal
four amino acids, and the activity for CCK resides in the terminal eight amino
acids. All the amino acids in the secretin molecule are essential.
Pancreatic
Secretion
The pancreas, which lies parallel to and beneath the
stomach, is a large compound gland with most of its internal structure similar
to that of the salivary glands. The pancreatic digestive enzymes are secreted
by pancreatic acini, and large volumes of sodium bicarbonate solution are
secreted by the small ductules and larger ducts leading from the acini.
The combined product of enzymes and sodium
bicarbonate then flows through a long pancreatic duct that normally joins the
hepatic duct immediately before it empties into the duodenum through the
papilla of Vater, surrounded by the sphincter of Oddi. Pancreatic juice is
secreted most abundantly in response to the presence of chyme in the upper
portions of the small intestine, and the characteristics of the pancreatic
juice are determined to some extent by the types of food in the chyme.
It is added that the pancreas also secretes insulin,
but insulin is secreted directly into the blood, not into the intestine, by the
islets of Langerhans that occur in islet patches throughout the pancreas.
Pancreatic
Digestive Enzymes
Pancreatic secretion contains multiple enzymes for
digesting all of the three major types of food: proteins, carbohydrates, and
fats. It also contains large quantities of bicarbonate ions, which play an
important role in neutralizing the acidity of the chyme emptied from the
stomach into the duodenum.
The most important of the pancreatic enzymes for
digesting proteins are trypsin, chymotrypsin, and carboxypolypeptidase. The most common of these is trypsin. Trypsin
and chymotrypsin split whole and partially digested proteins into peptides of
various sizes but do not cause release of individual amino acids. However,
carboxypolypeptidase splits some peptides into individual amino acids, thus
completing digestion of some proteins all the way to the amino acid state.
The pancreatic enzyme for digesting carbohydrates is
pancreatic amylase, which hydrolyzes starches, glycogen, and most other carbohydrates
(except cellulose) to form mostly disaccharides and a few polysaccharides.
The main enzymes for fat digestion are-
(1) Pancreatic lipase, which is
capable of hydrolyzing neutral fat into fatty acids and monoglycerides
(2) Cholesterol esterase, which
causes hydrolysis of cholesterol esters
(3) Phospholipase, which splits
fatty acids from phospholipids.
When first synthesized in the pancreatic cells, the
proteolytic digestive enzymes are in the inactive forms i.e. trypsinogen,
chymotrypsinogen, and procarboxypolypeptidase. They become activated only after
they are secreted into the intestinal tract. Trypsinogen is activated by an
enzyme called enterokinase, which is secreted by the intestinal mucosa when
chyme comes in contact with the mucosa. Also, trypsinogen can be auto
catalytically activated by trypsin that has already been formed from previously
secreted trypsinogen.
Chymotrypsinogen is activated by trypsin to form
chymotrypsin, and procarboxypolypeptidase is activated in a similar manner.
Secretion of Trypsin Inhibitor Prevents Digestion of the Pancreas Itself.
It is important that the proteolytic enzymes of the
pancreatic juice not become activated until after they have been secreted into
the intestine because the trypsin and the other enzymes would digest the
pancreas itself.
The same cells that secrete proteolytic enzymes into
the acini of the pancreas secrete simultaneously another substance called
trypsin inhibitor. This substance is formed in the cytoplasm of the glandular
cells, and it prevents activation of trypsin both inside the secretory cells
and in the acini and ducts of the pancreas. And, because it is trypsin that
activates the other pancreatic proteolytic enzymes, trypsin inhibitor prevents activation
of the others as well.
Secretion
of Bicarbonate Ions
Although the enzymes of the pancreatic juice are
secreted entirely by the acini of the pancreatic glands, the other two
important components of pancreatic juice, bicarbonate ions and water, are
secreted mainly by the epithelial cells of the ductules and ducts that lead
from the acini.
When the pancreas is stimulated to secrete copious
quantities of pancreatic juice, the bicarbonate ion concentration can be as
high as 145mEq/L; a value about five times that of bicarbonate ions in the
plasma. This provides a large quantity of alkali in the pancreatic juice that
serves to neutralize the hydrochloric acid emptied into the duodenum from the
stomach.
The basic steps in the cellular mechanism for
secreting sodium bicarbonate solution into the pancreatic ductules and ducts
are the following:
1. Carbon dioxide diffuses to the interior of the
cell from the blood and, under the influence of carbonic anhydrase, combines
with water to form carbonic acid (H2CO3). The carbonic acid in turn
dissociates into bicarbonate ions and hydrogen ions (HCO3− and H+).
Then the bicarbonate ions are actively transported in association with sodium
ions (Na+) through the luminal border of the cell into the lumen of
the duct.
2. The hydrogen ions formed by dissociation of
carbonic acid inside the cell are exchanged for sodium ions through the blood
border of the cell by a secondary active transport process. This supplies the
sodium ions (Na+) that are transported through the luminal border into the
pancreatic duct lumen to provide electrical neutrality for the secreted
bicarbonate ions.
3. The overall movement of sodium and bicarbonate
ions from the blood into the duct lumen creates an osmotic pressure gradient
that causes osmosis of water also into the pancreatic duct, thus forming an almost
completely isosmotic bicarbonate solution.
Regulation
of Pancreatic Secretion
Basic
Stimuli That Cause Pancreatic Secretion
Three basic stimuli are important in causing
pancreatic secretion:
1. Acetylcholine, which is released from the
parasympathetic vagus nerve endings and from other cholinergic nerves in the
enteric nervous system
2. Cholecystokinin, which is secreted by the
duodenal and upper jejunal mucosa when food enters the small intestine
3. Secretin, which is also secreted by the duodenal
and jejunal mucosa when highly acidic food enters the small intestine
The first two of these stimuli, acetylcholine and
cholecystokinin, stimulate the acinar cells of the pancreas, causing production
of large quantities of pancreatic digestive enzymes but relatively small
quantities of water and electrolytes to go with the enzymes. Without the water,
most of the enzymes remain temporarily stored in the acini and ducts until more
fluid secretion comes along to wash them into the duodenum. Secretin, in contrast
to the first two basic stimuli, stimulates secretion of large quantities of
water solution of sodium bicarbonate by the pancreatic ductal epithelium.
Multiplicative
Effects of Different Stimuli
When all the different stimuli of pancreatic
secretion occur at once, the total secretion is far greater than the sum of the
secretions caused by each one separately. Therefore, the various stimuli are
said to multiply or potentiate one another. Thus, pancreatic secretion normally
results from the combined effects of the multiple basic stimuli, not from one
alone.
Phases
of Pancreatic Secretion
Pancreatic
secretion occurs in three phases, the same as for gastric secretion: the
cephalic phase, the gastric phase, and the intestinal phase. Their
characteristics are as follows.
Cephalic
Phase
During the cephalic phase of pancreatic secretion,
the same nervous signals from the brain that cause secretion in the stomach
also cause acetylcholine release by the vagal nerve endings in the pancreas.
This causes moderate amounts of enzymes to be secreted into the pancreatic
acini, accounting for about 20 percent of the total secretion of pancreatic
enzymes after a meal. But little of the secretion flows immediately through the
pancreatic ducts into the intestine because only small amounts of water and
electrolytes are secreted along with the enzymes.
Gastric
Phase
During the gastric phase, the nervous stimulation of
enzyme secretion continues, accounting for another 5 to 10 percent of
pancreatic enzymes secreted after a meal. But, again, only small amounts reach
the duodenum because of continued lack of significant fluid secretion.
Intestinal
Phase
After chyme leaves the stomach and enters the small
intestine, pancreatic secretion becomes copious, mainly in response to the
hormone secretin. Secretin stimulates copious secretion of bicarbonate Ions,
which neutralizes acidic stomach chyme. Secretin is a polypeptide, containing
27 amino acids (molecular weight about 3400), present in an inactive form,
prosecretin, in S cells in the mucosa of the duodenum and jejunum.
When acid chyme with pH less than 4.5 to 5.0 enters the
duodenum from the stomach, it causes duodenal mucosal release and activation of
secretin, which is then absorbed into the blood. The one truly potent
constituent of chyme that causes this secretin release is the hydrochloric acid
from the stomach. Secretin in turn causes the pancreas to secrete large
quantities of fluid containing a high concentration of bicarbonate ion up to
145mEq/L but a low concentration of chloride ion.
The secretin mechanism is especially important for
two reasons-
(1) Secretin begins to be released
from the mucosa of the small intestine when the pH of the duodenal contents
falls below 4.5 to 5.0, and its release increases greatly as the pH falls to
3.0. This immediately causes copious secretion of pancreatic juice containing abundant
amounts of sodium bicarbonate. The net result is then the following reaction in
the duodenum:
HCl + NaHCO3 =NaCl + H2CO3
Then the carbonic acid immediately
dissociates into carbon dioxide and water. The carbon dioxide is absorbed into
the blood and expired through the lungs, thus leaving a neutral solution of
sodium chloride in the duodenum. This way, the acid contents emptied into the
duodenum from the stomach become neutralized, so further peptic digestive
activity by the gastric juices in the duodenum is immediately blocked.
(2) Because the mucosa of the small
intestine cannot withstand the digestive action of acid gastric juice, this is
an essential protective mechanism to prevent development of duodenal ulcers.
Bicarbonate ion secretion by the pancreas provides
an appropriate pH for action of the pancreatic digestive enzymes, which
function optimally in a slightly alkaline or neutral medium, at a pH of 7.0 to
8.0. The pH of the sodium bicarbonate secretion averages 8.0.
Cholecystokinin
The presence of food in the upper small intestine
also causes a second hormone, CCK, a polypeptide containing 33 amino acids, to
be released from yet another group of cells, I cells, in the mucosa of the
duodenum and upper jejunum. This release of CCK results especially from the
presence of proteoses and peptones (products of partial protein digestion) and
long-chain fatty acids in the chyme coming from the stomach. CCK, like
secretin, passes by way of the blood to the pancreas but instead of causing
sodium bicarbonate secretion causes mainly secretion of still much more
pancreatic digestive enzymes by the acinar cells.
This effect is similar to that caused by vagal
stimulation but even more pronounced, accounting for 70 to 80 percent of the
total secretion of the pancreatic digestive enzymes after a meal.
The difference of action between the pancreatic
stimulatory effects of secretin and CCK are-
(1) Intense sodium bicarbonate
secretion in response to acid in the duodenum, stimulated by secretin
(2) A dual effect in response to soap (a fat)
(3) Intense digestive enzyme
secretion (when peptones enter the duodenum) stimulated by CCK.
Secretion
of Bile by the Liver
One of the many functions of the liver is to secrete
bile, normally between 600 and 1000 ml/day. Bile serves two important
functions.
1. Bile plays an important role in
fat digestion and absorption, not because of any enzymes in the bile that cause
fat digestion, but because bile acids in the bile do two things:
(a) They help to emulsify the large
fat particles of the food into many minute particles, the surface of which can
then be attacked by lipase enzymes secreted in pancreatic juice
(b) They aid in absorption of the
digested fat end products through the intestinal mucosal membrane.
2. Bile serves as a means for
excretion of several important waste products from the blood. These include
especially bilirubin, an end product of hemoglobin destruction, and excesses of
cholesterol.
Biliary
Secretion
Bile is secreted in two stages by the liver-
First
stage
The initial portion is secreted by the principal
functional cells of the liver, the hepatocytes; this initial secretion contains
large amounts of bile acids, cholesterol, and other organic constituents. It is
secreted into minute bile canaliculi that originate between the hepatic cells.
Second
stage
Next, the bile flows in the canaliculi toward the
interlobular septa, where the canaliculi empty into terminal bile ducts and
then into progressively larger ducts, finally reaching the hepatic duct and
common bile duct.
From these the bile either empties directly into the
duodenum or is diverted for minutes up to several hours through the cystic duct
into the gallbladder.
In its course through the bile ducts, a second
portion of liver secretion is added to the initial bile. This additional
secretion is a watery solution of sodium and bicarbonate ions secreted by
secretory epithelial cells that line the ductules and ducts. This second
secretion sometimes increases the total quantity of bile by as much as an
additional 100 percent.
The second secretion is stimulated especially by
secretin, which causes release of additional quantities of bicarbonate ions to
supplement the bicarbonate ions in pancreatic secretion (for neutralizing acid
that empties into the duodenum from the stomach).
Storing
and Concentrating Bile in the Gallbladder
Bile is secreted continually by the liver cells, but
most of it is normally stored in the gallbladder until needed in the duodenum.
The maximum volume that the gallbladder can hold is only 30 to 60 milliliters. As
much as 12 hours of bile secretion (usually about 450 milliliters) can be
stored in the gallbladder because water, sodium, chloride, and most other small
electrolytes are continually absorbed through the gallbladder mucosa,
concentrating the remaining bile constituents that contain the bile salts,
cholesterol, lecithin, and bilirubin.
Most of this gallbladder absorption is caused by
active transport of sodium through the gallbladder epithelium, and this is
followed by secondary absorption of chloride ions, water, and most other
diffusible constituents. Bile is normally concentrated in this way about
5-fold, but it can be concentrated up to a maximum of 20-fold.
In the concentrating process in the gallbladder,
water and large portions of the electrolytes (except calcium ions) are
reabsorbed by the gallbladder mucosa; essentially all other constituents,
especially the bile salts and the lipid substances cholesterol and lecithin,
are not reabsorbed and, therefore, become highly concentrated in the
gallbladder bile.
Emptying
of the Gallbladder
Stimulatory
Role of Cholecystokinin
When food begins to be digested in the upper
gastrointestinal tract, the gallbladder begins to empty, especially when fatty
foods reach the duodenum about 30 minutes after a meal. The mechanism of
gallbladder emptying is rhythmical contractions of the wall of the gallbladder,
but effective emptying also requires simultaneous relaxation of the sphincter
of Oddi, which guards the exit of the common bile duct into the duodenum.
By far the most potent stimulus for causing the
gallbladder contractions is the hormone CCK. This is the same CCK that causes
increased secretion of digestive enzymes by the acinar cells of the pancreas.
The stimulus for CCK entry into the blood from the duodenal mucosa is mainly
the presence of fatty foods in the duodenum.
The gallbladder is also stimulated less strongly by
acetylcholine-secreting nerve fibers from both the vagi and the intestinal
enteric nervous system. They are the same nerves that promote motility and
secretion in other parts of the upper gastrointestinal tract.
In summary, the gallbladder empties its store of
concentrated bile into the duodenum mainly in response to the CCK stimulus that
itself is initiated mainly by fatty foods.
When fat is not in the food, the gallbladder empties
poorly, but when significant quantities of fat are present, the gallbladder
normally empties completely in about 1 hour.
The liver cells synthesize about 6 grams of bile
salts daily. The precursor of the bile salts is cholesterol, which is either
present in the diet or synthesized in the liver cells during the course of fat
metabolism. The cholesterol is first converted to cholic acid or
chenodeoxycholic acid in about equal quantities.
These acids in turn combine principally with glycine
and to a lesser extent with taurine to form glyco- and tauro-conjugated bile
acids. The salts of these acids, mainly sodium salts, are then secreted in the
bile.
The bile salts have two important actions in the
intestinal tract:
1. They have a detergent action on
the fat particles in the food. This decreases the surface tension of the
particles and allows agitation in the intestinal tract to break the fat
globules into minute sizes. This is called the emulsifying or detergent
function of bile salts.
2. Bile salts help in the
absorption of following
·
Fatty acids
·
Monoglycerides
·
Cholesterol
·
Other lipids from the intestinal tract
They do this by forming small physical complexes
with these lipids; the complexes are called micelles, and they are semi soluble
in the chyme because of the electrical charges of the bile salts. The
intestinal lipids are ferried in this form to the intestinal mucosa, where they
are then absorbed into the blood. Without the presence of bile salts in the
intestinal tract, up to 40 percent of the ingested fats are lost into the feces
and the person often develops a metabolic deficit because of this nutrient
loss.
Enterohepatic
Circulation of Bile Salts
About 94 percent of the bile salts are reabsorbed
into the blood from the small intestine, about one half of this by diffusion
through the mucosa in the early portions of the small intestine and the
remainder by an active transport process through the intestinal mucosa in the
distal ileum. They then enter the portal blood and pass back to the liver.
On reaching the liver, on first passage through the
venous sinusoids these salts are absorbed almost entirely back into the hepatic
cells and then re secreted into the bile.
In this way, about 94 percent of all the bile salts
are recirculated into the bile, so on the average these salts make the entire
circuit some 17 times before being carried out in the feces. The small
quantities of bile salts lost into the feces are replaced by new amounts formed
continually by the liver cells. This recirculation of the bile salts is called
the enterohepatic circulation of bile salts.
The quantity of bile secreted by the liver each day
is highly dependent on the availability of bile salts, the greater the quantity
of bile salts in the enterohepatic circulation (usually a total of only about
2.5 grams), the greater the rate of bile secretion. Indeed, ingestion of
supplemental bile salts can increase bile secretion by several hundred
milliliters per day.
In fact daily rate of liver bile salt secretion is
actively controlled by the availability or non availability of bile salts in
the enterohepatic circulation.
In addition to the strong stimulating effect of bile
acids to cause bile secretion, the hormone secretin that also stimulates
pancreatic secretion increases bile secretion, sometimes more than doubling its
secretion for several hours after a meal. This increase in secretion is almost
entirely secretion of a sodium bicarbonate–rich watery solution by the
epithelial cells of the bile ductules and ducts, and not increased secretion by
the liver parenchymal cells themselves.The bicarbonate in turn passes into the
small intestine and joins the bicarbonate from the pancreas in neutralizing the
hydrochloric acid from the stomach.
Thus, the secretin feedback mechanism for
neutralizing duodenal acid operates not only through its effects on pancreatic
secretion but also to a lesser extent through its effect on secretion by the
liver ductules and ducts.
Liver
Secretion of Cholesterol
Bile salts are formed in the hepatic cells from
cholesterol in the blood plasma. In the process of secreting the bile salts,
about 1 to 2 grams of cholesterol are removed from the blood plasma and
secreted into the bile each day. Cholesterol is almost completely insoluble in
pure water, but the bile salts and lecithin in bile combine physically with the
cholesterol to form ultramicroscopic micelles in the form of a colloidal
solution. When the bile becomes concentrated in the gallbladder, the bile salts
and lecithin become concentrated along with the cholesterol, which keeps the
cholesterol in solution.
Under abnormal conditions, the cholesterol may precipitate
in the gallbladder, resulting in the formation of cholesterol gallstones. The
amount of cholesterol in the bile is determined partly by the quantity of fat
that the person eats, because liver cells synthesize cholesterol as one of the
products of fat metabolism in the body. For this reason, people on a high-fat
diet over a period of years are prone to the development of gallstones.
Inflammation of the gallbladder epithelium, often
resulting from low-grade chronic infection, may also change the absorptive
characteristics of the gallbladder mucosa, sometimes allowing excessive
absorption of water and bile salts but leaving behind the cholesterol in the
gallbladder in progressively greater concentrations. Then the cholesterol
begins to precipitate, first forming many small crystals of cholesterol on the
surface of the inflamed mucosa, but then progressing to large gallstones.
Secretions
of the Small Intestine
Secretion
of Mucus by Brunner’s Glands in the Duodenum
An extensive array of compound mucous glands, called
Brunner’s glands, is located in the wall of the first few centimeters of the
duodenum, mainly between the pylorus of the stomach and the papilla of Vater,
where pancreatic secretion and bile empty into the duodenum. These glands
secrete large amounts of alkaline mucus in response to following
(1) Tactile or irritating stimuli
on the duodenal mucosa;
(2) Vagal stimulation, which causes
increased Brunner’s glands secretion concurrently with increase in stomach
secretion;
(3) Gastrointestinal hormones,
especially secretin.
The function of the mucus secreted by Brunner’s
glands is to protect the duodenal wall from digestion by the highly acidic
gastric juice emptying from the stomach. In addition, the mucus contains a
large excess of bicarbonate ions, which add to the bicarbonate ions from
pancreatic secretion and liver bile in neutralizing the hydrochloric acid
entering the duodenum from the stomach.
Brunner’s glands are inhibited
by sympathetic stimulation; therefore, such stimulation in very excitable
persons is likely to leave the duodenal bulb unprotected and is perhaps one of
the factors that cause this area of the gastrointestinal tract to be the site
of peptic ulcers in about 50 percent of ulcer patients.
Secretion
of Intestinal Digestive Juices by the Crypts of Lieberkühn
Located over the entire surface of the small
intestine are small pits called crypts of Lieberkühn. These crypts lie between
the intestinal villi. The surfaces of both the crypts and the villi are covered
by an epithelium composed of two types of cells:
(1) A moderate number of goblet
cells, which secrete mucus that lubricates and protects the intestinal
surfaces
(2) A large number of enterocytes,
which, in the crypts, secrete large quantities of water and electrolytes and,
over the surfaces of adjacent villi, reabsorb the water and electrolytes along
with end products of digestion.
The intestinal secretions are formed by the
enterocytes of the crypts at a rate of about 1800 ml/day. These secretions are
almost pure extracellular fluid and have a slightly alkaline pH in the range of
7.5 to 8.0. The secretions are also rapidly reabsorbed by the villi. This flow
of fluid from the crypts into the villi supplies a watery vehicle for
absorption of substances from chyme when it comes in contact with the villi.
Thus, the primary function of the small intestine is
to absorb nutrients and their digestive products into the blood.
Mechanism
of Secretion of the Watery Fluid
The mechanism that controls the marked secretion of
watery fluid by the crypts of Lieberkühn is believed to involve at least two
active secretory processes:
(1) Active secretion of chloride ions
into the crypts
(2) Active secretion of bicarbonate
ions.
The secretion of both ions causes electrical drag of
positively charged sodium ions through the membrane and into the secreted fluid
as well. Finally, all these ions together cause osmotic movement of water.
Digestive
Enzymes in the Small Intestinal Secretion
When secretions of the small intestine are collected
without cellular debris, they have almost no enzymes. The enterocytes of the
mucosa, especially those that cover the villi, contain digestive enzymes that
digest specific food substances while they are being absorbed through the
epithelium. These enzymes are the following:
(1) Several peptidases for
splitting small peptides into amino acids;
(2) Four enzymes for splitting
disaccharides into monosaccharides namely
·
sucrase
·
maltase
·
Isomaltase
·
lactase
(3) Small amounts of intestinal
lipase for splitting neutral fats into glycerol and fatty acids.
The epithelial cells deep in the crypts of
Lieberkühn continually undergo mitosis, and new cells migrate along the
basement membrane upward out of the crypts toward the tips of the villi, thus
continually replacing the villus epithelium and also forming new digestive
enzymes.
As the villus cells age, they are finally shed into
the intestinal secretions. The life cycle of an intestinal epithelial cell is
about 5 days. This rapid growth of new cells also allows rapid repair of
excoriations that occur in the mucosa.
Regulation
of Small Intestine Secretion
Local
Stimuli
The most important means for regulating small
intestine secretion are local enteric nervous reflexes, especially reflexes
initiated by tactile or irritative stimuli from the chyme in the intestines.
Secretion
by the Large Intestine
Mucus
Secretion
The mucosa of the large intestine, like that of the
small intestine, has many crypts of Lieberkühn; however, unlike the small
intestine, there are no villi. The epithelial cells secrete almost no digestive
enzymes. Instead, they contain mucous cells that secrete only mucus.
This mucus contains moderate amounts of bicarbonate
ions secreted by a few non-mucus-secreting epithelial cells. The rate of
secretion of mucus is regulated principally by direct, tactile stimulation of
the epithelial cells lining the large intestine and by local nervous reflexes
to the mucous cells in the crypts of Lieberkühn.
Stimulation of the pelvic nerves from the spinal
cord, which carry parasympathetic innervation to the distal one half to two
thirds of the large intestine, also can cause marked increase in mucus
secretion. This occurs along with increase in peristaltic motility of the
colon.
During extreme parasympathetic stimulation, often
caused by emotional disturbances, so much mucus can occasionally be secreted
into the large intestine that the person has a bowel movement of rope like
mucus as often as every 30 minutes; this mucus often contains little or no
fecal material.
Mucus in the large intestine has the following
functions-
1. It protects the intestinal wall
against excoriation,
2. It provides an adherent medium
for holding fecal matter together
3. It protects the intestinal wall
from the great amount of bacterial activity that takes place inside the feces
4. The mucus plus the alkalinity of
the secretion (pH of 8.0 caused by large amounts of sodium bicarbonate)
provides a barrier to keep acids formed in the feces from attacking the
intestinal wall.
Whenever a segment of the large intestine becomes
intensely irritated, as occurs in bacterial infection, the mucosa secretes
extra large quantities of water and electrolytes in addition to the normal
viscid alkaline mucus. This acts to dilute the irritating factors and to cause
rapid movement of the feces toward the anus. The result is diarrhea, with loss
of large quantities of water and electrolytes.
No comments:
Post a Comment