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FROM MOUTH TO RECTUM
BIOLOGY
The organs in the digestive system are presented
in the order food passes through them. In the
mindmaps I have chosen to concentrate my
description of the different organs to include
their different parts, sections and occasionally
cells, motility processes, the exocrine and in some
cases endocrine secretions, elements that trigger
secretions and the visual characteristics and
microscopic structures of the lumen.
But first, an explanation of some expressions used
in this part of my research.
The digestive system refers to the gastrointestinal
tract/digestive tract and the accessory digestive
organs.1
The gastrointestinal tract consists of the following
organs as shown in the illustration on the next
page; the oral cavity (1), pharynx (2), esophagus (3),
stomach(12), small intestine with its three parts;
duodenum (5), jejunum (13) and ileum (9) and
the large intestine with its four colons (7,8,14,15),
cecum (10,11), rectum (16) and anus.
The accessory digestive organs refers to the teeth,
tongue, salivary glands, liver (4), gallbladder and
pancreas (6).
The lumen refers to the area inside the gastrointestinal tract.
Epithelium refers to a type of animal tissue,
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generally described as the outermost layer of cells
that line the walls of cavities and surfaces in our
body (e.g. the inner walls of the digestive tract).2
Mucosa and sub-mucosa refers to the different
layers of epithelial cells that the inner walls of the
digestive tract consists of.3
Digestion refers to the breakdown of large food
molecules into their smallest subunits (monomers).
Absorption refers to the passage of monomers
through the walls of the lumen of the small and
large intestine.
Motility refers to the movement of food through
the digestive tract which is driven by a number
of different processes including ingestion (putting
food into our mouth) mastication (chewing),
deglutition (swallowing) and peristalsis (a type of
muscle contractions).4
Secretions is the general expression for all
substances that are secreted into the lumen of
the gastrointestinal tract (exocrine secretions)
and all the substances that are secreted from the
gastrointestinal tract (endocrine secretions).5
MIRJAM MITTERNACHT
SEROSA
SUBMUCOSA
LUMEN
MUCOSA
EPITHELIUM
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FROM MOUTH TO RECTUM
ORAL CAVITY AND PHARYNX
The oral cavity and its accessory digestive organs;
teeth, tongue and salivary glands is the first organ
in the digestive system.
The oral cavity is covered by a mucous membrane.
An adult human has 32 permanent teeth (including
the wisdom teeth) which breaks down food into
smaller parts by mastication.6
Our tongue is our primary taste organ and is a
mucus covered muscular organ. In certain glands
on the tongue a fat digesting enzyme (lingual lipase)
is produced, an enzyme especially important for
small children with a not yet fully functioning
pancreas.
We have 6 major salivary glands in our mouth,
three on each side. The secretion of saliva into
the oral cavity is a direct reaction to mastication
but also a central mechanism triggered by the
thought of something tasty.7 Saliva consists of 98%
water and 2% mixed substances, among them the
salivary enzyme amylase that breaks down starch
(carbohydrate) in our food into sugars.8
Deglutition is a voluntary activity in which the
larynx is raised so that the epiglottis covers the
entrance to the respiratory tract and prevents
food from entering. 25 muscles in the pharynx are
involved in the act of swallowing.9 The soft pallet
in the “roof” of the oral cavity closes the nasal
passages while we swallow. From the oral cavity
and pharynx, food, now called bolus, continues its
journey into the esophagus (gullet).
ESOPHAGUS
The esophagus is a 25 cm long muscular tube
through which food is transported by peristalsis,
a type of muscular movement in which the
esophagus’s smooth muscles contracts behind
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the bolus while the muscles in front relaxes. This
type of muscular contraction is also found in other
parts of the digestive tract. The esophagus ends
in the gastroesophageal sphincter through which
food enters the stomach. The gastroesophageal
sphincter is closed until food is pushed through by
peristalsis.10
STOMACH
The stomach is the most extendible organ of the
digestive tract. It has two main parts, the cardiac
region (consisting of the fundus and the lower
body) and the pyloric region (consisting of the
antrum and the pyloric sphincter). The main
functions of our stomach are to store food (now
called chyme), break it down by mixing it with
gastric juice through muscle contractions, kill
ingested bacteria and start digesting proteins
with the help from the enzyme pepsin. The inner
structure of the stomach is characterized by long
folds called rugae. The gastric mucosa is however
also folded and forms gastric pits. In the bottom of
these pits you find the gastric glands.11
In the gastric glands groups of secreting cells
are positioned. These cells secrete mucus,
hydrochloric acid, intrinsic factor and pepsinogen
into the lumen of the stomach. Through a reaction
between pepsinogen and hydrochloric acid, the
active enzyme pepsin is produced. Together with
water these secretions form the acidic solution:
gastric juice. Cells in the stomach also produce a
small amount of the fat digesting enzyme gastric
lipase.12
Since our body is built up by different proteins,
protein-digesting enzymes that are secreted into
the digestive tract form a significant risk to our
tissues. Our body has two main defenses against
this. First of all, these enzymes are secreted in an
inactive form (as pro-enzymes) and react with
other substances in the lumen to become active
MIRJAM MITTERNACHT
enzymes. Secondly, epithelial cells along the
digestive tract produces a thick layer of mucus that
prevents the activated enzymes from injuring the
inner walls.
In the stomach, as well as in the duodenum, the
mucus produced by the epithelial cells also contains
bicarbonate that neutralizes the hydrochloric acid
in the gastric juice.13
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FROM MOUTH TO RECTUM
THE SMALL INTESTINE
Duodenum
The duodenum stretches from the pyloric
sphincter, which connects it to the stomach, into
the jejunum which is the next part of the small
intestine.
Through the pyloric sphincter food is portioned
out into the duodenum by the muscle contractions
in the lower part of the stomach. This, so that
sufficient amounts of chyme can be mixed
with the secretions in duodenum. The mucosa
in duodenum, as well as in the rest of the small
intestine is folded into fingerlike forms called villi.
Each villus is covered by a brush border called
microvilli that consists of folded epithelial cell
membranes. Stuck on the microvilli you find brush
border enzymes and through the villi nutrients
are absorbed into the capillary network (blood)
and lymph system (via the lacteal). Thanks to the
folds the area of absorption in the small intestine
increases considerably.14
Even though you find villi and microvilli in the
duodenum, very little absorption takes place here.
Instead the duodenum together with the accessory
digestive organs; the liver, gallbladder and pancreas
is responsible for most of the enzyme driven
digestion in the gastrointestinal tract.15
The chyme that reaches duodenum is strongly
acidic and needs to be neutralized to protect
the mucosa. The duodenum has, apart from its
bicarbonate and mucus producing epithelial cells
also a type of exocrine glands (Brunner’s glands)
that produce bicarbonate to add protection against
the acidic chyme. In the pancreatic juice (explained
further down) you also find bicarbonate that adds
to the neutralization of chyme.16
When chyme enters the duodenum a chain of
reactions starts. Cells in the epithelium detect
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the chyme and thereby free the hormone
cholecystokinin (among others). This hormone
gives signals to the pancreas and liver (gallbladder)
to secrete bile and pancreatic juice into the
duodenum through the common bile duct which
has its opening on the right middle part of the
duodenum.17
Bile, a yellowish/green secretion concentrated in
the gallbladder and originating from the liver has
many components, e.g. bile salt that works as a
emulsifier and splits fat molecules in the chyme
into smaller fat drops so that enzymes later can
break them down. Bile also contains the pigment
bilirubin that is responsible for the colour of our
feces. The gallbladder is a saclike organ attached to
the liver. When the liver produces bile it is stored
and concentrated in the gallbladder and later
ejected through the cystic duct and common bile
duct into the duodenum.18
Pancreatic juice, produced in the pancreas,
contains a mix of bicarbonate, enzymes and proenzymes that can break down carbohydrates,
fats and proteins into smaller parts. One of the
protein-digesting pro-enzymes in pancreatic juice
(trypsinogen) reacts with the brush border enzyme
enteropeptidase, and becomes the active enzyme
trypsin. This enzyme then activates other proenzymes in the pancreatic juice so that proteins
can be digested.19
The pancreas is a soft glandular organ with both
endocrine and exocrine functions. While its
endocrine secretions; the hormones insulin and
glucagon; are produced by the pancreatic islets
(the islets of Langerhans), its exocrine secretions;
the components in pancreatic juice; are produced
by cells in the exocrine secretory units, the acini.20
MIRJAM MITTERNACHT
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FROM MOUTH TO RECTUM
Jejunum and ileum
Jejunum and Ileum stretch from the duodenum
to the ileocecal valve and is the longest part of the
digestive tracts, approximately 3-5 meters.21
In the jejunum and ileum most nutrients are
absorbed. Proteins as well as carbohydrates do
however need to go through one last step of
digestion before they can be absorbed into the
capillary network in the villi. The brush border
enzymes, stuck on the inner walls of the small
intestine are responsible for breaking apart the
final bounds. Fat has been broken down by bile
salts and pancreatic lipase and it’s now absorbed
into the lymphatic system through the lacteal.22
All along the jejunum and ileum nutrients are
absorbed through the villi. The activity however
decreases in ileum. When chyme enters the large
intestine through the ileocecal valve only a few
components are left.
THE LARGE INTESTINE
The large intestine is the last part of the digestive
tract, ending in the anus. The mucosa differs from
the one in the small intestine. The epithelial cells
still produce mucus but instead of fingerlike folds
resulting in villi, you find folds that form crypts
(the crypts of Lieberkühn).23
The large intestine has little or no digestive function
but through the epithelial cells’ membranes, water
is absorbed from the food (now called chyme or
feces) as a result of the osmoregulation in our
body. This absorption is closely linked to the
absorption of salts (electrolytes) that also occurs
in the large intestine. It should be mentioned that
water and salt also are absorbed in ileum, yet the
large intestine gets rid of the last water leaving less
than approx 200 ml to be excreted in the feces.24
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The large intestine is also home to a large amount
of bacteria, called the intestinal microbiota. Over
400 different species populate our intestines and
many of them are found in the large intestine.
The relationship between us and our intestinal
microbiota can be described as either mutual;
we both benefit, and/or as commensal; bacteria
benefit and we are neither harmed or benefit. The
bacteria in our intestines are among other things
responsible for the production of vitamin K and
B vitamins and for the production of short chain
fatty acids (which give energy to the epithelial cells)
and ferment indigestible molecules in our food, i.e.
different types of fibres (complex carbohydrates).
While fibres are fermented the gas methane is also
produced.25 It can also be noted that nearly a third
of the dry weight of our feces consists of bacteria.26
The cecum and the appendix are two parts of the
large intestine that you find in the beginning of
the ascending colon below its connection to ileum.
Humans do not necessarily need the appendix but
this small part of the large intestine has functions
in the immune system.27
The chyme/feces travels slowly through the large i
ntestine’s four colons driven forward by peristalsis
and periodic mass movements and in the end,
what’s left of the food we once ate, exits through
our anus.
TIME FRAME
Foods journey through our body takes about 3648 hours. The stomach portions out food into the
Duodenum continuously during 3-4 hours after a
meal and this food will reach the ileocecal valve
after approximately 8 hours. The time frame for
the journey through the large intestine varies
depending on the human but should normally
take between 24-48 hours.28
MIRJAM MITTERNACHT
AFTER ABSORPTION
After being absorbed through the walls of the small and large intestine, monomers from carbohydrates and
proteins go from the capillary networks into the portal vein and continues into the liver.
The liver is a complex organ responsible for many regulating processes29 and can e.g. remove the carbohydrate
monomer glucose from the blood in case of a surplus and store it as glycogen. Later when needed, the liver
can release it as glucose or triglycerides. Many of the processes in the liver are driven by hormones.30
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FROM MOUTH TO RECTUM
NUTRIENTS
All nutrients we eat serve different purposes in
our body and are all needed. Here follows a short
description of the different nutrients.
Proteins
The smallest building unit (monomer) in protein
molecules is the amino acid. There are 20 different
amino acids and their characteristics and the order
in which they are placed forms the functions and
characteristics of a protein. In the human body you
find about 100 000 different proteins. Eleven types
of amino acids can be built by our body but the
remaining nine are essential and need to be a part
of our diet. Especially babies need many of these
essential amino acids since they are important for
the growth of tissue.31
Proteins have many essential functions in our body
and are used:
- For transportation of nutrients (oxygen, lipids,
iron etc) and digesting products in our blood and
tissues
- For storage and release of oxygen in our muscle
cells
- As receptors on cell membranes to, e.g. give cells
signals from hormones
- As structure builders e.g. as collagen (unit in bone
tissue and connective tissue) and/or keratin (unit
in hair, nails and skin)
- For contractions in muscles
- As enzymes
- As defenders in our immune system
- As regulators of activities in our cells.32
Proteins are found in most food types except in
pure carbohydrate or fat products. Proteins and
amino acids are not stored in our body, an excess
of proteins will be used as energy or turned into
fat, why we must have proteins in our diet.33
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Carbohydrates
Carbohydrates is one of our body’s most important
sources of energy and the most common organic
compounds on earth.34
The smallest building units in carbohydrates are
monosaccharides. Glucose is the most common
monosaccharide and the monomer in starch and
maltose. Glucose is transported by our blood to
all cells in our body and the hormone insulin is
responsible for the consumption level of glucose
in our muscle- and fat- cells. Glucose is first of all
an energy source but can also be transformed into
glycogen or triglyceride. Glycogen can be stored in
our body (e.g. in our liver, as earlier mentioned).
Our brain needs 100-150 g of glucose each day
to function and has no own storage. Thereby it
is crucial that the liver can free glucose from its
glycogen storage between meals to provide energy
to our brain.35
Health
The amount of glucose in our blood is regulated by
three hormones, insulin - that lowers the levels, and
glucagon and adrenaline that both rase the level.
A disturbance in the hormone levels can result in
hyperglycemia (high blood sugar) or hypoglycemia
(low blood sugar) depending on which hormone
that is disturbed. The disease diabetes is the
result of low insulin levels and causes a state of
hyperglycemia with its following effects.36
There is a huge difference in the speed, in which
different carbohydrate products are digested and
absorbed after a meal. To use the glycemic index
(GI) is a way to measure how much the blood
glucose level raises and thereby measure how fast
the carbohydrates are absorbed. Low GI indicates
a slow absorption resulting in a low blood glucose
level while high GI indicates fast absorption and
a high blood glucose level. The type of product
as well as how it is prepared has an effect on the
glycemic index.37
MIRJAM MITTERNACHT
Fibres and dietary fibres
Fibres are complex carbohydrates from plants,
which can’t be digested by the enzymes in the
digestive tract. Instead some of them (soluble) are
fermented by the microbiota in our large intestine
while others (insoluble) travels through our
digestive system unaffected. The most important
types are cellulose, hemicellulose, pectin and
lignin.38 Soluble and insoluble fibres have different
effects on the large intestine. Insoluble fibres can
absorb water and thereby ease the feces’ passage
through the large intestine (defecation). Because
the feces then travels faster, constipation is
prevented. Soluble fibres also ease the defecation
because the microbiotic mass increases.39
A diet rich of carbohydrates, especially dietary
fibres automatically lowers the amount of fat
absorbed. This because, the fermenting microbiota
in our intestines will be more active and thereby
produce more acids which will change the pH-level
in the large intestine. This, in the next step, lowers
the levels of bile salts secreted from the liver into
the duodenum and the amount of fat that can be
absorbed (there are, in other words, hormones
secreted by the cells in the large intestine that
affect the functions in the liver).40
Fats (lipids)
Lipids are important for our body for a number of
reasons and they come in different forms;
Triglycerides are rich energy providers for our cells
and can also be stored in our fat cells (lipocytes).
Triglycerides consist of the alcohol glycerol and
three fatty acids.
Fatty acids
Fatty acids have a number of different characteristics; they can be saturated, polyunsaturated or
just unsaturated. Some of them are also essential
and must be a part of our diet. For humans linoleic
acid and alpha-linoleic acid are essential.
Fatty acids are structural parts of our cell
membranes and are important in the production
of arachidonic acid and different eicosanoids.
Eicosanoids are hormone like substances that play
an important role in different regulating processes
in our body e.g. blood pressure and the secretion
of gastric juice.
Phospholipids, another type of fat is a building unit
in our cell membranes.41
Cholesterol is not only a part of the food we eat but
also produced by our liver by digestive products
such as glucose, other lipids and amino acids.
The livers production is regulated by the levels of
cholesterol in the food we eat. High value equals
lower production and vice versa. Cholesterol is
also an important component in some hormones,
in our cell membranes, in lipoproteins and helps in
the production of vitamin D and bile salts.42
The digestion, absorption and transport of lipids in
our body is a complex matter. The main digestion
takes place in duodenum where bile salts and
different pancreatic lipases split lipids into smaller
parts. However lingual lipase and gastric lipase;
active enzymes in earlier parts of the digestive tract,
helps in this process in an perhaps unusual way. To
understand how lipids are digested it is important
to first of all understand that lipids’ chemical
structure makes them insoluble in water. Since
they latter must travel through blood and lymph,
both water based fluids, their chemical structure
must be changed so that this transport is possible.
This is done through emulsion and the production
of micelles; a type of molecules with a hydrophobic
(water rejecting) center and a hydrophilic (water
friendly) surface. With the help from different
emulsifiers e.g. bile salts and free fatty acids, the
digested lipids are rearranged into micelles. Free
fatty acids that are involved in this process are
digested products broken down by the lingual and
gastric lipase enzymes. To create fatty acids that are
part of this process is the main purpose of lingual
and gastric lipase.43
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FROM MOUTH TO RECTUM
Micelles are molecules that can be absorbed into
the lymphatic system through the cell membranes
in the villi and glands because of their water friendly
surface. They travel through the lymphatic system
and are later transferred to the blood where they
continue their journey as lipoproteins (a molecule
consisting of lipids and proteins).44
Sodium chloride (NaCl) or salt is an ionic
compound. The sodium ion is essential and has
several important functions in our body. Sodium
ions does e.g. play an important role in the
osmoregulation. To eat too much salt can raise the
blood pressure and thereby our heartbeat.48
Permeability describes the rate in which molecules
and ions can pass through the membranes of
the epithelial cells. A number of factors effect
permeability and an increased permeablilty can
lead to an increased risk for different diseases.
Our body consists of 50 - 75% water and most
of it is found in our cells. Water is essential for
all metabolic processes in our body and helps
maintaining the physiological balance, transporting
nutrients to our cells, transporting waste, producing
digestive fluids, by being the medium in which
biochemical reactions can happen as well as act as
a reactive substance in them, prevent over-heating
and contribute to the form and structure of an
organism. 49
It has been shown that eating fat increases the
permeability of LPS (lipo-polysaccharides found
in the cell membranes of gram-negative cells). LPS
can cause inflammations that are behind several
metabolic diseases and the insulin-resistance
syndrome.45
Vitamins
Vitamins are organic substances important for our
body that must be added to our diet since we can’t
synthesize them ourselves (apart from at certain
conditions). They are a heterogenic group divided
into two main subgroups; water soluble vitamins
(B-vitamins and vitamin C) and fat soluble vitamins
(A, D, E and K).46
Minerals
Minerals have three main functions in our body.
First of all they have structural functions, secondly,
regulating functions and thirdly, they are involved
in the transmission of signals between cells and
tissues. The range in which they are needed in
our body divides minerals into different groups.
While calcium, magnesium, potassium, sodium,
sulphur, phosphate and chlorine occurs frequently
and are highly needed; iron, zinc, copper, cobalt,
manganese, iodine, selenium and molybdenum
are essential but only needed in utterly small
concentrations.47
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Water
DIGESTION & ABSORPTION
Nutrients and water are absorbed in the small
and large intestine once they have reached their
absorbable form. In the map on the right page, the
digestion and absorption of proteins, carbohydrates,
lipids, salt and fermentable fibers can be seen.
Water is a component in most nutrients, separated
from them as they split and is absorbed in the small
and large intestine.
MIRJAM MITTERNACHT
37
FROM MOUTH TO RECTUM
SUMMARY
Nutrients are digested and mechanically broken
down in a sequence of processes during their
journey through our body.
Carbohydrates are digested in three steps, first
by salivary amylase in the mouth, secondly by
pancreatic enzymes in duodenum and thirdly
by brush border enzymes in jejunum and ileum.
When they have become monosaccharides they
can be absorbed into the blood through the walls
of jejunum and ileum.
Proteins are digested in three steps as well, first by
pepsin in our stomach, secondly by four different
pancreatic enzymes in duodenum and thirdly by
brush border enzymes in jejunum and ileum. They
can be absorbed as free amino acids, dipeptides or
as tripeptides into the blood through the walls of
jejunum and ileum.
Fats are digested in four steps (the first two are of less
relevance). First by lingual lipase in the oral cavity,
secondly by gastric lipase in our stomach, thirdly
by bile salts in duodenum and finally by pancreatic
enzymes in duodenum. They are rearranged into
micelles and absorbed into the lymphatic system
through the walls of jejunum and ileum.
Water and salt (NaCl) is primarily absorbed into
the blood through the epithellial cell menbranes in
the walls of the large intestine.
Food is mechanically broken down in two main
areas of the digestive system. In the oral cavity
through chewing and in the stomach through
muscle contractions.
There are, in other words, two types of processes
in which food is broken down into smaller parts by
our body; mechanical processes and biochemical
processes.
38
TEXTURES, STRUCTURES
AND COLOURS
To investigate the microscopic structures of the
walls and lumen in the digestive tract I used the
Science Photo Library Database.50 To investigate
the colours and structures I used an endoscopic
journey through the digestive system made
by Bengt Jeppsson. To get an idea of the visual
textures of the different organs, I primarily used
images from Henry Gray’s Anatomy of the human
body.51