G A
S T R O I N T E S T I N A L
A N
A T O M Y A N D P H Y S I O L O G Y.
Course
Overview
Introduction
The
digestive system, also referred to as the gastrointestinal or alimentary tr=
act,
contains the organs involved in the ingestion and processing of food. The
digestive system plays a role in four major functions: ingestion, digestion,
absorption, and elimination. This course describes the anatomy and physiolo=
gy
of the gastrointestinal system, and explains its involvement in acid related
disorders and inflammatory diseases.
Transcript
The
first section, ANATOMY, introduces the organs that make up the digestive tr=
act
including the mouth, pharynx, esophagus, stomach, small intestine, and large
intestine, and a number of accessory organs that assist in the processing of
foods and the extraction of nutrients.
The
second section, PHYSIOLOGY, describes the sub‐structure of the alimentary canal and its relationshi=
p to
function, especially acid secretion. The production and regulation of gastr=
ic
acid secretion is also covered.
The
third section, GI TRACT AND DISEASE, gives an overview of how the
gastrointestinal tract is involved in disease, particularly acid related
disorders and inflammatory diseases. These areas are covered in detail in
separate courses that are part of the gastrointestinal therapeutic area
curriculum.
Anatomy
Introduction
Anat=
omically,
the digestive system consists of a 30R=
08;foot long, mucous membrane‐lined tube beginning at the mouth, where food enters =
the
body, and ending at the anus, where solid waste is excreted. This section
introduces each of the key organs of the digestive system and a number of
accessory organs that assist in the processing of foods and the extraction =
of
nutrients.
Transcript
These
are the objectives for this section.
After
you finish this section, you should be able to:
v &nb=
sp;
List the four main impor=
tant
functions in GI system
v &nb=
sp;
List the main structures=
of
the digestive tract, and discuss their functions.
v &nb=
sp;
List the accessory organ=
s of
the digestive tract, and discuss their functions.
The
digestive consists of the following components
Mouth
(buccal cavity; includes salivary glands, mucosa, teeth and tongue)
Phar=
ynx
: The upper expanded portion of the digestive tube, between the esophagus b=
elow
and the mouth and nasal cavities .
Esop=
hagus
and cardia
Stom=
ach,
which includes the antrum and pylorus and pyloric sphincter
Bowe=
l or
intestine:
Small
intestine, which has three parts:
Large
intestine, which has three parts:
Rela=
ted
organs
The
liver secretes bile into the small intestine via the biliary system, employ=
ing
the gallbladder as a reservoir. The pancreas secretes an isosmotic fluid
containing bicarbonate and several enzymes, including trypsin, chymotrypsin,
lipase, and pancreatic amylase, as well as nucleolytic enzymes, into the sm=
all
intestine. Both these secretory organs aid in digestion.
Immu=
ne
function
The
gastrointestinal tract is also a prominent part of the immune system. The l=
ow
pH (ranging from 1 to 4) of the stomach kills many microorganisms that enter
it. Similarly, mucus (containing IgA antibodies) neutralizes many of these
microorganisms. Other factors in the GI tract help with immune function as
well, including enzyme in the saliva and bile enhancing intestinal bacteria
serve to prevent the overgrowth of potentially harmful bacteria in the gut.=
Oral
Cavity
Introduction
Food=
is
ingested through the mouth, or oral cavity. This lesson describes the struc=
ture
of the oral cavity, and explains how the salivary glands begin the digestio=
n of
food, and how the pharynx directs food to the next step in the alimentary
canal.
Transcript
The
boundaries of the oral cavity include the lips, the cheeks, the palate, and=
the
tongue. The back of the mouth is continuous with the pharynx, or throat.The
digestion of food begins in the mouth with the mechanical breakdown by chew=
ing.
Chewing helps break down food and mixes saliva with food. Saliva is secreted
from the salivary glands andcontains an enzyme, salivary amylase, which bre=
aks
complex carbohydrates such as starch and glycogen down into smaller fragmen=
ts.
Saliva also lubricates food, making it easier to swallow. The mixture of ch=
ewed
food and saliva is called a bolus. The tongue compacts the bolus of food and
initiates swallowing. During swallowing, the bolus of food first passes thr=
ough
the pharynx, a transition area between the mouth and the esophagus. The pha=
rynx
is a passageway for food, fluids, and air, but food is prevented from enter=
ing
the respiratory pathways during swallowing by a flap of tissue known as the
epiglottis.
Esop=
hagus
Introduction
Once=
a
chewed bolus of food leaves the oral cavity, it enters the esophagus. This
lesson describes the structure of the esophagus and the role of two sphinct=
ers
found at either end of the esophagus. It also introduces the process of
peristalsis.
Transcript
The
esophagus is a hollow tube that leads from the throat to the stomach. Altho=
ugh
most of this 10‐inch long tube is located in the thorax, the lower en=
d of
the esophagus passes through the diaphragm and enters the abdomen at the
esophageal hiatus. Within the abdomen, the esophagus joins the stomach at t=
he
cardiac orifice. The esophagus is separated from the pharynx and stomach by
valve‐like
rings of circular muscle tissue known as sphincters. The sphincter that
separates the upper end of the esophagus from the throat is called =
the
upper esophageal sphincter (UES), and the sphincter that separates the lower end of the
esophagus from the stomach is known variously as the lower esophageal sphin=
cter
(LES), the cardiac sphincter, or the gastroesophageal sphincter.<=
span
style=3D'font-size:10.0pt;font-family:Verdana;mso-bidi-font-family:Arial;
mso-bidi-language:HI'> Under resting
conditions, when there is no food to be delivered from the mouth to the
stomach, the esophagus is collapsed and both sphincters are closed. When fo=
od
is moved to the back of the mouth by contractions of the pharyngeal muscles,
the upper esophageal sphincter relaxes and the bolus of food is propelled i=
nto
the esophagus by the tongue. Food is moved through the esophagus by success=
ive
waves of muscular contraction known as peristalsis. The time required for a
bolus of food=
to
pass through the length of the esophagus is normally less than 10 seconds. =
At
the lower end of the esophagus, the combined effect of the peristaltic wave=
and
the weight of the bolus causes the lower esophageal sphincter to open, allo=
wing
the swallowed food to enter the stomach.
In
addition to regulating the passage of food into the stomach, the lower
esophageal sphincter plays an important role in protecting the esophagus fr=
om
the damage that would otherwise result from exposure to the highly acidic
gastric fluids of the stomach.
Esop=
hageal
diseases and conditions:
The
following are diseases and conditions that affect the esophagus:
Stomach
Introduction
Once=
a
bolus of food leaves the esophagus, it enters the stomach. This lesson
describes the structure of the stomach and its dual role in the digestive
process.
Transcript
The
stomach is a sac‐like organ found just under the diaphragm. It is
typically “J‐shaped” and is located high in the abdomen, usu=
ally
on the left side. The inner, concave surface of the stomach is referred to =
as
the lesser curvature, while the outer, convex surface is referred to as the
greater curvature. Anatomically, the stomach is divided into four regions. =
The
small area surrounding the lower esophageal sphincter is called the cardia.=
The
rounded, dome‐like area at the top of the stomach is known as the
fundus, and the narrow, funnel‐
Diseases
Of stomach
Small
Intestine
Introduction
Chyme
from the stomach enters the small intestine, which is the primary organ for
digestion and absorption. This lesson introduces the three sections that fo=
rm
the small intestine, the duodenum, the jejunum and the ileum, and their rol=
e in
digestion. The small intestine is the site where most of the nutrients from
ingested food are absorbed. There are microscopic finger‐like projections called villi covering the small
intestinal walls, which increase surface area for absorption. Each villus
contains a lacteal and capillaries. The lacteal absorbs the digested fat in=
to
the lymphatic system, which will eventually drain into the circulatory syst=
em.
The capillaries absorb all other digested nutrients
Transcript
Part=
ially
digested food leaves the stomach through the pyloric sphincter and enters t=
he
small intestine. The small intestine is the primary organ of digestion and =
absorption.
It consists of three segments: a relatively short 10‐inch initial segment known as the duodenum, an 8‐foot long central segment known as the jejunum, and a=
12‐foot long terminal segment known as the ileum, which
connects to the large intestine. The first two inches of the duodenum are
slightly enlarged and are referred to as the duodenal bulb. Acidic chyme fr=
om
the stomach is delivered to the duodenal bulb. The presence of chyme in the
small intestine triggers the release of mucus and a number of digestive
enzymes. These secretions combine to form the intestinal juice, a watery
mixture that serves as a vehicle for the absorption of nutrients from the
chyme. During absorption, nutrients cross the highly specialised epithelial
lining of the small intestine to enter the bloodstream and lymph ducts. In =
the
small intestine, proteins are changed into amino acids; fats are changed in=
to
fatty acids; and carbohydrates are changed into sugars. The small intestine=
is
also where most of the nutrients from ingested food are absorbed
Small
Intestine Disorders
DUODENUM
In
anatomy of the digestive system, the duodenum is a hollow jointed tube
connecting the stomach to the jejunum. It is the first part of the small
intestine. It starts with the duodenal bulb and ends at the ligament of Tre=
itz.
Two very important ducts open into the duodenum, namely the bile duct and t=
he
pancreatic duct.
The
duodenum is largely responsible for the breakdown of food in the small
intestine. Brunner’s glands are only found in the duodenum and they
secrete mucus.The duodenum is divided into four sections for the purposes of
description. The duodenum is almost all retroperitoneal.
JEJUNUM
In
anatomy of the digestive system, the jejunum is the central of the three
divisions of the small intestine and lies between the duodenum and the ileu=
m.
In adult humans, it is usually between 2‐8m (06' 07"=
‐26' 03") long. The pH in the jejunum is usually
between 7 and 8 (neutral or slightly alkaline). The jejunum and the ileum a=
re
suspended by mesentery which gives the bowel great mobility within the
abdomen.The inner surface of the jejunum, its mucous membrane, is covered in
projections called villi, which increase the surface area of tissue availab=
le
to absorb nutrients from the gut contents. It differs from the duodenum due=
to
lack of Brunner's glands. It is also different from the ileum due to less
goblet cells and generally lacks Peyer's patches
ILEUM<= o:p>
In
anatomy of the digestive system, the ileum (not to be confused with the ili=
um,
a pelvic bone), is the final section of the small intestine. It is about 4m
long in humans, follows the jejunum and duodenum, and is separated from the
cecum by the ileocecal valve (ICV). The pH in the ileum is usually between 7
and 8 (neutral or slightly alkaline).Its function is to absorb vitamin B12 =
and
bile salts.
Large
Intestine
Introduction
The
final part of the alimentary canal is the large intestine. This lesson
introduces the three segments of the large intestine, the caecum, the colon,
and the rectum, and explains their role in completing absorption, and in
elimination.
Transcript
The
large intestine, or large bowel, is a 5‐foot long tube t=
hat
extends from the small intestine to the end of the gastrointestinal tract. =
The
large intestine is divided into three sections: the caecum, a pouch<=
span
style=3D'font-size:10.0pt;font-family:Calibri;mso-ascii-font-family:Verdana;
mso-bidi-font-family:Calibri;mso-bidi-language:HI'>‐like structure that connects the small and large
intestines, the colon, which comprises the main length of the large intesti=
ne,
and the rectum.
The
colon is divided into four sections: the ascending colon, which extends upw=
ard
from the
caec=
um
to the upper abdominal area on the right side of the body; the transverse
colon, which passes from the right side of the abdomen to the left; the
descending colon, which extends from the upper to lower abdominal area on t=
he
left side of the body; and the sigmoid colon, the distal end of the colon t=
hat
extends to the rectum.
As f=
luid
waste from the small intestine passes through the various sections of the
colon, water is reabsorbed into the body, and the waste is converted to a s=
olid
form known as faeces or stool. At the distal end of the large intestine, so=
lid
waste material passes into the sigmoid colon, where it is stored. When it
enters the rectum an urge to defaecate is sensed, and the faeces are expell=
ed
from the body through the anus.
Diseases
of the colon
CECUM
In
anatomy of the digestive system, the cecum or caecum is a pouch connected to
the large intestine between the ileum. It is separated from the ileum by the
ileocecal valve (ICV) or Bauhin's valve, and is considered to be the beginn=
ing
of the large intestine and part of the colon.Its primary function is to abs=
orb
water and salts from undigested food. It has a muscular wall that can knead=
the
contents to enhance absorption. The appendix is a branch of the cecum. Like=
the
appendix, the cecum was once believed to have no function.
RECTUM
The
rectum (from the Latin verb regere "to straighten, correct, rule"
hence the rectum is "that which is ruled, controlled") is the fin=
al
straight portion of the large intestine in some mammals, and the gut in oth=
ers,
terminating in the anus. For the diagnosis of certain ailments, a rectal ex=
am
may be done. Suppositories may be inserted into the rectum as a route of
administration for medicine. Body temperature can also be taken in the rect=
um.
Rectal temperature can be taken by inserting a mercury thermometer for 3 to=
5 minutes,
ANUS
In
anatomy, the anus (from Latin ānus "ring, anus") is the exte=
rnal
opening of the rectum. Closure is controlled by sphincter muscles. Feces are
expelled from the body through the anus during the act of defecation, which=
is
the primary function of the anus.
Role=
in
defecation
When=
the
rectum is full the increase in intrarectal pressure forces the walls of the
anal canal apart allowing the fecal matter to enter the canal. The rectum
shortens as material is forced into the anal canal and peristaltic waves pr=
opel
the feces out of the rectum. The internal and external sphincters of the an=
us
allow the feces to be passed by muscles pulling the anus up over the exiting
feces.
Path=
ology
Anal
cancer, abscess, warts, fistula, fissure, itching and hemorrhoids are among=
the
diseases of the anus that benefit from medical intervention. Birth defects =
of
the anus include stenosis and imperforation. The anus is also a frequent si=
te
of sexually transmitted infections.
Liver
Introduction
In
addition to the structures that make up the alimentary canal, the digestive
system includes a number of accessory organs. This lesson introduces the li=
ver.
Transcript
Loca=
ted
in the upper right quadrant of the abdominal cavity, the liver is a large
glandular organ that performs many different metabolic functions, including=
the
manufacture of blood proteins, filtration of the blood, and the regulation =
of
glucose metabolism. As an accessory digestive organ, the liver manufactures
bile, a thick, greenish‐brown fluid that aids in the digestion of fat=
s.
As the liver manufactures it, bile passes through the hepatic and cystic du=
cts
into the gall bladder for storage. The adult human liver normally weighs
between 1.0 ‐ 2.5 kilograms, and is a soft, pinkish=
‐brown "boomerang shaped" organ. It is the
second largest organ (the largest organ being the skin) Its anatomical posi=
tion
in the body is : immediately under the diaphragm on the right side of the u=
pper
abdomen. The liver lies on the right of the stomach and makes a kind of bed=
for
the gallbladder (which stores bile). The liver is supplied by two major blo=
od
vessels: the hepatic artery and the portal vein. The hepatic artery normally
comes off the celiac trunk. The portal vein brings venous blood from the
spleen, pancreas, and small intestines, so that the liver can process the n=
utrients
and byproducts of food digestion. The hepatic veins drain directly into the
inferior vena cava.
The =
bile
produced in the liver is collected in bile canaliculi, which merge to form =
bile
ducts. These eventually drain into the right and left hepatic ducts, which =
in
turn merge to form the common hepatic duct. The cystic duct (from the
gallbladder) joins with the common hepatic duct to form the common bile duc=
t.
Bile can either drain directly into the duodenum via the common bile duct o=
r be
temporarily stored in the gallbladder via the cystic duct. The common bile =
duct
and the pancreatic duct enter the duodenum together at the ampulla of Vater.
The branchings of the bile ducts resemble those of a tree, and indeed the t=
erm
"biliary tree" is commonly used in this setting. The liver is one=
of
the only internal human organs capable of natural regeneration of lost tiss=
ue;
as little as 25% of remaining liver can regenerate into a whole liver again.
This is predominantly due to the hepatocytes acting as unipotential stem ce=
lls.
The liver produces and excretes bile required for food digestion. Some of t=
he
bile drains directly into the duodenum, and some is stored in the gallbladd=
er.
The
functions of the liver are
Diseases
of the liver
Many
diseases of the liver are accompanied by jaundice caused by increased level=
s of
bilirubin in the system. The bilirubin results from the breakup of the
hemoglobin of dead red blood cells; normally, the liver removes bilirubin f=
rom
the blood and excretes it through bile.
Prim=
ary
biliary cirrhosis, autoimmune disease of small bile ducts
A nu=
mber
of liver function tests are available to test the proper function of the li=
ver.
These are enzymes that are most abundant in liver tissue, metabolites or
products.
Gall
Bladder
Introduction
In a=
ddition
to the structures that make up the alimentary canal, the digestive system
includes a number of accessory organs. This lesson introduces the gall blad=
der.
Transcript
The =
gall
bladder is a small muscular sac located next to the liver. The gall bladder
plays a role in digestion by storing and concentrating the bile produced by=
the
liver. When nutrients containing fatreach the duodenum, the hormone
cholecystokinin is produced by cells in the intestinal wall and carried to =
the
gall bladder via the bloodstream. This hormone causes the gall bladder to
contract, forcing bile into the bile ducts that connect the gall bladder to=
the
duodenum. Within the duodenum, bile aids in the emulsification of fats.
Pancreas
Introduction
In
addition to the structures that make up the alimentary canal, the digestive
system includes a number of accessory organs. This lesson introduces the
pancreas.
Transcript
The
pancreas, a spongy, elongated organ located just behind the stomach, produc=
es
digestive juices that contain a variety of enzymes responsible for the
breakdown of proteins, starches, and fats. These enzymes are delivered to t=
he
duodenum through the pancreatic duct.
In
addition to digestive enzymes, the pancreas secretes sodium bicarbonate into
the duodenum.
Bica=
rbonate
acts as a buffer to neutralise the acidity of the gastric contents as they =
move
from the stomach into the small intestine. The buffering action of bicarbon=
ate
is important in protecting tissues of the small intestine, especially the
duodenal bulb, from the potentially damaging effects of gastric acid. It is
important to note that the duodenal wall also secretes bicarbonate
Dise=
ases
of the pancreas
•Benign tumors
•Carcinoma of
pancreas
•Cystic fibrosis<=
/span>
•Diabetes<=
span
style=3D'font-size:10.0pt;font-family:Verdana;mso-fareast-font-family:Symbo=
lMT;
mso-bidi-font-family:SymbolMT;mso-bidi-language:HI'>
•Pancreatitis
•Acute pancreatit=
is
•Chronic pancreat=
itis
•Pancreatic
pseudocyst .
Mucosa
Introduction
The
walls of the alimentary canal have the same basic structure, and consist of
four basic layers referred to as the mucosa, the submucosa, the muscularis
externa, and the serosa. This lesson describes the mucosa and explains how =
it
is specialised to perform a specific function.
Transcript
Thro=
ughout
the length of the digestive tract, from the esophagus to the anus, the wall=
s of
the alimentary canal have the same basic structure. Extending outward from =
the
central cavity, or lume=
n,
the four basic layers of the alimentary wall are: the mucosa, the submucosa,
the muscularis externa, and the serosa (or adventitia, in the esophagus). E=
ach
layer of the alimentary wall contains tissue that is specialised to perform=
a
specific function.
The
mucosa, or mucous membrane, is a moist layer of epithelial cells that lines=
the
lumen of the digestive tract. The three main functions of the gastrointesti=
nal
mucosa are: the production and secretion of varying amounts of mucus, diges=
tive
enzymes, and hormones, the absorption of nutrients from digested food into =
the
blood, and the protection of the underlying structures and the gastric muco=
sa
from the damage due to mechanical or chemical forces (such as the hydrochlo=
ric acid
and pepsin in gastric juice). The
microscopic structure of the gastrointestinal mucosa varies from one part of
the digestive system to the next. In the esophagus, the mucosa is classifie=
d as
a stratified squamous epithelium, which is made up of a collection of flat
cells stacked atop one another in a plate‐like fashion, an=
d is
typical in parts of the body that are subjected to mechanical wear and tear=
. At
the border between the esophagus and the stomach, the mucosa abruptly shift=
s to
a columnar epithelium, a type of epithelium more commonly associated with s=
ecretory
organs. In
addition to the epithelium, the mucosa includes a thin layer of connective
tissue called the lamina propria, and a layer of smooth muscle cells known =
as
the muscularis mucosae. The lamina propria houses the capillaries that supp=
ly
the endothelium and lymphatic nodules, which protect against bacterial
infection, while the muscularis mucosae produce localized movements of the
mucosa.
Submucosa
Introduction
This
lesson describes the submucosa and explains how it is specialised to perfor=
m a
specific function.
Transcript
The
submucosa is a thin layer of connective tissue lying just below the mucosa.
Collagen and elastin are key proteins forming the structure of the submucos=
a.
The submucosa is rich in blood vessels, lymph nodes and nerve endings. The
vasculature of the submucosa supports the tissues around the wall of the
digestive tract.
Muscularis
Externa
Introduction
This
lesson describes the muscularis externa and explains how it is specialised =
to
perform a specific function.
Transcript
The
muscularis externa, also known as the muscularis propria or simply as the
muscularis, is a layer of muscular tissue responsible for peristalsis. Alth=
ough
the muscularis varies depending on its location within the digestive tract,=
it
can consist of up to three layers: an inner oblique layer, a layer of circu=
lar
muscle, and an outer layer of longitudinal muscle.
Serosa
Introduction
This
lesson describes the serosa and explains how it is specialised to perform a
specific function.
Transcript
The
serosa, or visceral peritoneum, is the outer membrane that envelops all of =
the
organs of the abdomen. The serosa is formed of connective tissue that holds=
the
organs in place within the abdominal cavity; it also serves as a protective
layer.
Esophageal
Secretions
Introduction
Secr=
etions
from the gastrointestinal mucosa have two main functions: to provide
lubrication and protection for the structures of the digestive tract, and to
deliver the enzymes and other substances required for the digestion of inge=
sted
food. This lesson defines and describes the esophageal secretions.
Transcript
The =
only
secretory product of the oesophageal mucosa is mucus, a slimy, viscous solu=
tion
made up of water, electrolytes, and a mixture of mucopolysaccharides which,
acts to lubricate the passage of food through the digestive tract and as a
protection layer from the effects of digestive enzymes and pathogens.
In t=
he
esophagus, mucus is produced in great abundance by glandular goblet cells t=
hat
lie partially buried in the submucosa. Mucus is released when the mechanical
pressure exerted by a bolus of food compresses these glandular cells, causi=
ng
them to secrete their contents onto the surface of the mucosa.
Stomach
Secretory Cells
Introduction
This
lesson defines and describes the secretory cells found in the stomach.
Transcript
In t=
he
stomach, deep indentations on the mucosal surface, known as gastric pits, h=
ouse
the gastric secretory glands. Depending on their location in the stomach,
gastric glands contain varying numbers of four different types of secretory
cells. Mucus neck cells are found in the upper region of most glands and are
responsible for producing mucus. Although the composition of this mucus is
somewhat different from that produced by goblet cells of the epithelial sur=
face,
its primary function remains protection of the gland from the corrosive eff=
ects
of digestive juices. Parietal cells, also known as oxyntic cells, are
responsible for the production of hydrochloric or gastric acid, which is
released into the lumen of the stomach. Parietal cells are also responsible=
for
the manufacture of a glycoprotein known as intrinsic factor, which is requi=
red
for absorption of vitamin B12 in the small intestine. Chief cells, also kno=
wn
as zymogenic cells, are responsible for the production of pepsinogen, the
precursor to a protein‐digesting enzyme called pepsin.
Ente=
roendocrine
cells are responsible for the production of gastric hormones involved in the
regulation of acid secretion. Gastrin is produced by a family of
enteroendocrine cells known as G cells, and histamine is produced by the fa=
mily
of enteroendocrine cells known as enterochromaffin‐like or ECL cells.
Secretory
Cell Distribution
Introduction
The
relative proportion of mucus neck cells, parietal cells, chief cells, and
enteroendocrine cells depends on the region of the stomach. This lesson
associates the different secretory cells with different areas of the stomac=
h.
Transcript
The
relative proportion of mucus neck cells, parietal cells, chief cells, and
enteroendocrine cells depend on the region of the stomach. For example, gas=
tric
glands in the cardia and fundus tend to have an unusually high proportion of
mucus neck cells, reflecting their greater role in generating the thick lay=
er
of mucus that protects the walls of the stomach from acid‐related damage. By comparison, glands in the body of =
the
stomach — where most of the chemical breakdown of ingested food occurs
— tend to be rich in the parietal cells responsible for the productio=
n of
gastric acid. Gastric glands in the antrum tend to contain more of the
enteroendocrine cells involved in the overall regulation of the digestive
process.
These
cells, their secretions and their functions are summarised in this table.
pH
Review
Introduction
This
section provides a review of pH in order to introduce the next section about
gastric acid.
Transcript
The
acidity of any liquid is determined by the concentration of hydrogen ions (=
H+)
it contains. The relative concentration of hydrogen ions in a solution is
measured using the pH scale, which runs from 0 to 14. Neutral solutions are
assigned a pH of 7. If a solution has more hydrogen ions than hydroxyl ions=
, it
is classified as an acid, and has a pH lower than 7. Conversely, a solution=
in
which the number of hydroxyl ions exceeds the number of hydrogen ions is
referred to as a base, and has a pH greater than 7. The greater the acidity=
of
a solution, the lower its pH will be. Because pH is measured on a logarithm=
ic
scale, a one‐point change in pH represents a ten
Gastric
Acid Production
Introduction
The
gastric secretion responsible for the chemical breakdown of food within the
stomach is hydrochloric or gastric acid, which is produced by parietal cells
within the gastric mucosa. This lesson explains how gastric acid is produce=
d.
Transcript
The
digestive juices produced by the gastric mucosa are extremely acidic, with =
a pH
that normally ranges between 1.5 and 3.5. The outer membrane of parietal ce=
lls
contains a series of small intracellular channels, called canaliculi, which
penetrate deeply into the interior of the parietal cell and increase the
surface area available for secretion. Within the membranes that line the
canaliculi, protein based proton pumps actively transfer hydrogen ions acro=
ss
the parietal cell membrane, into the lumen of the canaliculi, in exchange f=
or
potassium ions. Negatively charged chloride ions diffuse across the parietal
cell membrane, and combine with the protons to form a solution of hydrochlo=
ric
acid. The human stomach contains approximately one billion parietal cells, =
each
secreting more than 3 billion protons per second. During the course of a da=
y,
parietal cells of the gastric mucosa produce between 2 and 3 litres of
hydrochloric acid.
Regulation
of Acid Production
Introduction
The
activity of the parietal proton pump responsible for gastric acid productio=
n is
controlled by the parasympathetic arm of the nervous system through
acetylcholine, and the hormones, gastrin and histamine. This lesson describ=
es
these components and their role in regulation. An understanding of this
regulation is critical in comprehending how pharmacological agents can be
effective in the treatment of acid related disorders, such as gastroesophag=
eal
reflux disease and peptic ulcer disease.
Transcript
The
activity of the parietal cell proton pump is regulated by three different
chemicals:
R=
26;
Acetylcholine, a neurotransmitter substance released by the parasympathetic
nervous system in response to a wide range of gustatory (eating‐related) stimuli
R=
26;
Gastrin, a hormone released by antral G cells in response to the presence of
partially digested proteins in the stomach and/or changes in the pH of the
gastric fluid
R=
26;
Histamine, a locally acting hormone released by ECL cells in response to
stimulation by either acetylcholine or gastrin. When the stomach is empty, =
the
secretion of gastric acid occurs at a rate of about 0.5 mL/min. This rate of
acid production is known as the basal secretion rate. The basal secretion r=
ate
fluctuates during the course of the day; the lowest levels of basal secreti=
on
normally occur during the early morning hours (5:00 a.m. to 11 a.m.), and t=
he
highest levels occur during the late afternoon and evening hours (2:00 p.m.=
to
1:00 a.m.). Histamine appears to be the primary stimulant for gastric acid
release under basal conditions.
Cephalic
Phase
Introduction
Thro=
ugh
a variety of neural and hormonal mechanisms, the ingestion of food causes t=
he
production of gastric acid to increase dramatically. The three stages of ac=
id
secretion stimulation are referred to as the cephalic, gastric, and intesti=
nal
phases, respectively. The cephalic phase is the body’s first response=
to
food, originating in the central nervous system.
Transcript
Thro=
ugh
a variety of neural and hormonal mechanisms, the ingestion of food causes t=
he
production of gastric acid to increase dramatically. Because these mechanis=
ms
involve the brain, stomach, and small intestine, the three stages of acid
secretion stimulation are referred to as the cephalic, gastric, and intesti=
nal
phases, respectively. During the cephalic phase of gastric secretion, senso=
ry
stimuli related to the sight, taste, smell, or thought of food trigger area=
s of
the central nervous system responsible for preparing the stomach to receive
food. Electrical impulses transmitted from the brain to the stomach by way =
of
the vagus nerve cause the release of acetylcholine from nerve endings locat=
ed
in the gastric mucosa. By binding to cholinergic receptors on the surface of
the parietal cell, acetylcholine stimulates the increased production of gas=
tric
acid. In addition to its direct effects on acid production, acetylcholine a=
lso
sensitises the parietal cell to the effects of gastrin and histamine, and m=
ay
promote the release of gastrin from G cells of the antrum.
Gastric
Phase
Introduction
Thro=
ugh
a variety of neural and hormonal mechanisms, the ingestion of food causes t=
he
production of gastric acid to increase dramatically. The gastric phase of a=
cid
secretion stimulation involves multiple systems and feedback loops.
Transcript
The
gastric phase of secretion begins when food actually reaches the stomach.
Distention of the stomach activates stretch receptors within the stomach wa=
ll,
which in turn trigger the activation of the vagal nerve and a network of ne=
ural
circuits located in the digestive system. Both of these cause acetylcholine=
to
be released in the gastric mucosa, which stimulates the activity of the pro=
ton
pump, causing gastric acid to be released into the stomach. During the gast=
ric
phase, the effects of acetylcholine on gastric acid secretion are amplified
many times over by the release of gastrin from antral G cells. The release =
of
gastrin is stimulated by the presence of food in the stomach, particularly
partially digested proteins, and by increases in the pH of the gastric flui=
d.
The more protein in the ingested food, the greater the amount of gastrin
secreted. Since gastrin also stimulates the activity of the parietal proton
pump, the production of gastric acid increases dramatically. In addition to
their direct effects on acid secretion, acetylcholine and gastrin also
stimulate the release of histamine from ECL cells. The histamine released f=
rom
ECL cells binds with histamine receptors on the surface of nearby parietal
cells. Although a relatively weak stimulator of gastric acid secretion in i=
ts
own right, histamine strongly potentiates the stimulatory effect of gastrin=
and
acetylcholine. The gastric phase of secretion continues as long as food is
present in the stomach.
Intestinal
Phase
Introduction
Thro=
ugh
a variety of neural and hormonal mechanisms, the ingestion of food causes t=
he
production of gastric acid to increase dramatically. The intestinal phase of
acid secretion stimulation is regulated by the presence of chyme in the int=
estine.
Transcript
The
intestinal phase of gastric secretion begins when partially digested food
enters the small intestine. As chyme begins to be delivered to the small
intestine, glandular cells of the intestinal mucosa secrete a hormone that
promotes the continued secretion of gastric acid. Because of its similarity=
to
gastrin, this hormone is called intestinal or enteric gastrin. Once the
duodenum is filled with chyme, however, further secretion of gastric acid is
inhibited by a series of events known collectively as the enterogastric ref=
lex.
As a result of this reflex, neural activity in the vagus and in local circu=
its
is inhibited, and the pyloric sphincter is stimulated to contract, temporar=
ily
blocking the flow of chyme from the stomach to the small intestine. In addi=
tion
to assuring that the duodenum contains no more chyme than can be efficiently
processed, the enterogastric reflex serves to protect the duodenal mucosa f=
rom
the damage that would otherwise be caused by prolonged exposure to the high=
ly
acidic chyme. These phases are summarised in this table.
Acid‐Related Disorders
Introduction
Gast=
ric
acid is a substance that is corrosive and damaging. Thus, its production is
restricted to the stomach and is tightly controlled. Normally, the speciali=
sed
walls of the stomach are protected from damage by acid, and the surrounding
structures are protected from acid exposure by sphincters that prevent
spillover and transfer. This lesson describes the types of disorders that o=
ccur
when these protective measures are lost, and the esophagus, stomach and
duodenum are exposed to above‐normal levels of gastric acid.
Transcript
In
healthy individuals, the lower esophageal sphincter prevents reflux of the
gastric contents. When a wave of peristalsis reaches the lower end of the
esophagus, the muscular activity within the esophageal wall combines with t=
he
weight of the bolus of food to cause the LES to relax, allowing food to ent=
er
the stomach. After the bolus of food passes into the stomach, the LES
constricts to prevent the contents of the stomach from regurgitating into t=
he
esophagus. If reflux of acid does occur, then a two‐step process known as the anti‐reflux mechanism occurs. In the first step, local
peristaltic contractions force the refluxate back into the stomach, leaving
only a small volume of acid in the esophagus. In the second step, the
bicarbonate ions in saliva in a series of swallows neutralise any remaining
acid. However, the ability of the esophageal mucosa to resist acid
Inflammatory
Bowel Disease
Introduction
Infl=
ammatory
Bowel Disease (IBD) is a term that refers to both ulcerative colitis and
Crohn's disease. Ulcerative colitis causes inflammation of the lining of the
large intestine. Crohn's disease causes inflammation of the lining and wall=
of
the large and/or small intestine. When inflamed, the lining of the intestin=
al
wall is red and swollen, becomes ulcerated, and bleeds.
Transcript
The
cause of IBD is not known. IBD does run in families, with about 15 to 30
percent of patients with IBD having a relative with the disease. Many chang=
es
in the body’s immune system have been discovered in patients with IBD,
and genes within the major histocompatibility complex have been associated =
with
IBD. There is little evidence that stress causes IBD. IBD occurs most
frequently in people in their late teens and twenties. Men and women have an
equal chance of getting the disease. Crohn’s disease is an inflammati=
on
and ulceration process that occurs in the deep layers of the intestinal wal=
l.
The most common areas affected are the ileum, and the first part of the col=
on.
Bowel strictures are common, leading to motility disturbance and abdominal
pain. Crohn’s disease can infrequently affect any part of the upper
gastrointestinal tract. Aphthous ulcers, which are similar to cold sores, a=
re
common. Ulcers can also occur in the esophagus, stomach, and duodenum. The =
most
common symptoms of Crohn’s disease are pain in the abdomen, often in =
the
lower right side, diarrhoea, and weight loss. There may also be rectal blee=
ding
and fever. Chronic bleeding may lead to anaemia. Ulcerative colitis occurs =
only
in the inner lining of the colon or rectum. When it is located only in the
rectum, it is called proctitis. Inflammation of the colon keeps water from
being absorbed into the bloodstream and results in diarrhoea. The most comm=
on
symptoms of ulcerative colitis are diarrhoea, abdominal cramps, and rectal
bleeding. Some people may be very tired and have weight loss, loss of appet=
ite,
abdominal pain, and loss of body fluids and nutrients. Bleeding may be seri=
ous,
leading to anaemia. Joint pain, redness and swelling of the eyes, and liver
problems can also occur.
Introduction
Irri=
table
bowel syndrome (IBS) is a common disorder of the intestines that leads to c=
ramp‐like pain, gassiness, bloating, and changes in bowel
habits. The cause of IBS is not known, and it is referred to as a functional
disorder because there is no sign of disease when the colon is examined. IBS
has been called by many names ‐ colitis, mucous colitis, spastic colon, spas=
tic
bowel, and functional bowel disease. However, IBS does not cause inflammati=
on
and should not be confused with ulcerative colitis, which is a more serious
disorder. This lesson gives an overview of IBS.
Transcript
Irri=
table
bowel syndrome (IBS) is a common disorder of the intestines that leads to
crampy pain, gassiness, bloating, and changes in bowel habits. Some people =
with
IBS have constipation, others have diarrhoea, and some people experience bo=
th.
Sometimes the person with IBS has an urge to move the bowels but cannot do =
so.
Through the years, IBS has been called by many names ‐ colitis, mucous colitis, spastic colon, spastic bowe=
l,
and functional bowel disease. Most of these terms are inaccurate. Colitis, =
for
instance, means inflammation of the colon. IBS, however, does not cause
inflammation and should not be confused with ulcerative colitis, which is a
more serious disorder. Because doctors have been unable to find an organic
cause, IBS has often been suggested to be caused by emotional conflict or
stress. While stress may worsen IBS symptoms, research suggests that other
factors are also important. For example, the colon muscle of a person with =
IBS
begins to spasm after only mild stimulation suggesting that IBS patients may
have a colon that is more sensitive and reactive than usual, so it responds
strongly to stimuli that would not bother most people. Finally, "visce=
ral
hypersensitivity"—the increased perception of pain, without
associated spasm, in response to normal gaseous distension or motility of t=
he
intestines has also been suggested as a cause.
Summary
Introduction
The
following lesson summarises the key concepts that have been covered in this
course. We advise you to review this lesson and the entire course.
Transcript
The
table shown here contains the key concepts for each section covered in this
course.
Roll
your cursor over the boxes to view the key concepts.
Anat=
omy
The
digestive system consists of a 30‐foot long, mucous
membrane‐
The
organs that make up the digestive tract include the mouth, pharynx, esophag=
us,
stomach, small intestine (the duodenum, the jejunum and the ileum), and lar=
ge
intestine (the caecum, the colon, the rectum). Accessory organs include the
liver, gall bladder, and pancreas.
The
esophagus is separated from the pharynx by the upper esophageal sphincter
(UES), and from the stomach by lower esophageal sphincter (LES). The pyloric
sphincter separates the stomach from the duodenum.
The
walls of the alimentary canal have the same basic structure, and consist of
four basic layers referred to as the mucosa, the submucosa, the muscularis
externa, and the serosa.
Phys=
iology
Secr=
etions
from the gastrointestinal mucosa provide lubrication and protection for the
structures of the digestive tract, and deliver the enzymes and other substa=
nces
required for the digestion of ingested food.
The
stomach plays an important role in the digestion of food by: secreting gast=
ric
acid and pepsin, physically by peristaltic contractions, and by the slowing=
of
gastric emptying, leading to a metering of gastric content into the duodenu=
m.
Gastric glands contain varying numbers of four different types of secretory
cells: mucus neck cells (produce mucus), parietal cells (gastric acid), chi=
ef
cells (pepsinogen), enteroendocrine cells (gastric hormones‐gastrin and histamine).
The
activity of the parietal proton pump responsible for gastric acid productio=
n is
controlled by the parasympathetic arm of the nervous system through
acetylcholine, and the hormones, gastrin and histamine. The three stages of=
acid
secretion stimulation are referred to as the cephalic, gastric, and intesti=
nal
phases.
GI T=
ract
and Disease
The
gastrointestinal tract is affected in acid related disorders such as peptic
ulcer disease and gastroesophageal reflux disease (GERD), and in inflammato=
ry
disorders such as Crohn’s disease and ulcerative colitis. In healthy
individuals, the lower esophageal sphincter prevents reflux of the gastric
contents. Transient relaxations of the LES are more frequent and prolonged =
in
patients with esophageal reflux disease.
Infl=
ammatory
Bowel Disease (IBD) is a term that refers to both ulcerative colitis and
Crohn’s disease. Ulcerative colitis causes inflammation of the lining=
of
the large intestine. Crohn’s disease causes inflammation of the lining
and wall of the large and/or small intestine.
Irritable bowel syndrome (IBS) is a common disorder of
the intestines that leads to cramp‐like pain, and
changes in bowel habits. It is referred to as a functional disorder because
there is no sign of disease when the colon is examined.