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The Digestive System
Lecture 1
Bill Sellers
mailto:[email protected]
November 30, 1998
Contents
1 Introduction 1
2 Objectives 1
3 General Characteristics 2
4 The Mouth 3
4.1 Teeth . . . . . . . . . . . . . . . 4
4.2 Tongue . . . . . . . . . . . . . . 4
4.3 Swallowing . . . . . . . . . . . . 5
4.4 Salivary Glands . . . . . . . . . . 5
5 Oesophagus 5
6 Stomach 6
7 The Small Intestine 8
7.1 Duodenum . . . . . . . . . . . . . 8
7.2 Jejunum and Ileum . . . . . . . . 8
8 Pancreas 9
9 Liver 10
10 Gall Bladder 10
11 Large Intestine 11
11.1 C aecum . . . . . . . . . . . . . . 11
11.2 Colon . . . . . . . . . . . . . . . 11
11.3 R ectum . . . . . . . . . . . . . . 12
12 Anus 12
13 Portal Circulation 12
1 Introduction
The purpose of these lectures is to describe the
structures involved in digestion and to relate these
structures to their functions. The information I will
provide will follow very closely what is given in
Hole and Koos, chapter 12, but because my re-
search interest is in the interaction of form and
function over the last 100 million years or so, Iwill also try to provide an evolutionary context for
some of these observations.
2 Objectives
The objectives of these three lectures are as follows
(taken straight from the textbook):
1. Name and describe the location of the organs
of the digestive system and their major parts.
2. Describe the structure and general function of
each digestive organ and the liver.
3. Describe the structure of the wall of the ali-
mentary canal.
4. Explain how the contents of the alimentary
canal are mixed and moved.
5. Describe the mechanism of swallowing.
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6. Explain how the products of digestion are ab-
sorbed.
3 General Characteristics
The major components of the digestive system that
we will cover can be seen in figure 1. As you
can see, the digestive system in humans is basi-
cally a long tube and associated glandular struc-
tures. Food is ingested cranially and processed in-
crementally as it passes caudally. This processing
is both mechanical and chemical, leading to the ab-sorption of some of the chemical components of
the food items whilst other, indigestible compo-
nents are passed straight through. You will proba-
bly already be familiar with the basic structure and
the location of the major components, but we will
cover each organ in detail.
The muscular tube through which the food
passes is called the alimentary canal. The exact
histological appearance of this tube varies down
the length of the tube, but it basically consists of
four concentric layers (see figure 2). The inner-most layer is called the mucosa. This itself con-
sists of three thin layers: an epithelium lining the
inner surface, some underlying connective tissue
(lamina propria), and a small amount of smooth
muscle, confusingly named the muscularis mu-
cosae. The next layer is the submucosa. This
is loose connective tissue containing blood and
lymph vessels and nerves. The next layer is a
thicker muscular layer called the muscularis. In
most regions this consists of two layers of smooth
muscle: the inner one with fibres arranged circu-larly, and the outer one with fibres arranged longi-
tudinally. Rhythmic contraction of these muscles
pushes the contents of the tube steadily caudally.
This movement is called peristalsis. This mechan-
ical arrangement of fibres running perpendicularly
is very common in biological tubes whether it is
hydraulic skeletons in roundworms or or armadillo
penises. The precise angulation of the fibres de-
pends on the exact mechanical properties required.
Figure 1: Overview of digestive tract
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Figure 2: Idealized cross-section of gut tube
The outermost layer of the the gut tube is a thin
covering of visceral peritoneum - very similar in
structure to the pleura round the lungs. It is called
the serosa and consists of an outer epithelium with
connective tissue underneath.
Movement in the gut tube is not continuous. At
points, food is not moved, but kept in one place
and mixed. Obvious places where this occurs are
in the mouth and stomach. Peristaltic movements
are largely intrinsic to the smooth muscle of the
gut tube, however there is both sympathetic and
parasympathetic nerve supply. The parasympa-
thetic supply (rest and digest) tends to increase
peristaltic frequency and increase secretion of en-
zymes and mucus. The sympathetic supply tends
to decrease inhibit this and also contracts the arteri-oles, reducing the local blood supply. The sympa-
thetic supply also controls directly specific muscu-
lar structures called sphincters. These are regions
of thickened circular muscle that block the tube
when contracted, preventing the movement of gut
contents. The anal sphincter is slightly more com-
plicated, with a contribution from skeletal muscle
allowing a degree of conscious control of defeca-
tion.
Figure 3: The mouth
4 The Mouth
The mouth (see figure 3) is extremely complicated
anatomically because it has to cope with a variety
of different functions. Speaking, eating, breathing
are the obvious three, but certainly in humans and
other primates it is also an important manipulation
and tactile organ. It is also important in non-verbal
communication for display and sexual behaviour.
Humans and other so-called haplorhine primates
are unusual among mammals in that the sides of
the upper lip have fused which means that we donot have the wet, dog-like noses present amongst
most mammals.
The most important digestive role of the mouth
is mastication. This is the physical breakdown of
food by chewing. It is a complex process requiring
interactions between the highly muscular tongue,
cheeks and lips to position the food bolus between
the teeth, and strong crushing and shearing move-
ments of the tooth surfaces controlled by the mus-
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cles of mastication pulling on the jaws. The exact
configuration of jaws and teeth is highly variableamongst mammals, and is often used as a means
of classifying different mammalian groups. Hu-
mans, as omnivores, do not have specializations
for specific food items. This means that they have
very loose temporomandibular joints, and a great
deal of movement flexibility (depression, eleva-
tion, protrusion, retraction, circumduction, left and
right deviation). Mammals with more specific di-
ets have more specialized dentition and restricted
jaw movements. Carnivora, for example, can only
depress and elevate to any great extent and havescissor like teeth for shearing food. Individual
teeth also vary in shape and function.
4.1 Teeth
Humans have a dental formula of 2 1 2 32 1 2 3
. This
means that an adult human has 2 incisors, 1 ca-
nine, 2 premolars and 3 molars on their up-
per jaw, and the same on their lower jaw. Com-
monly, the 3rd molar may be missing, or may never
erupt. Incisors are single cusped, sharp edged cut-ting teeth used for biting chunks off a larger food
item. They are also used for removing the skins
of fruit. Human canines are extremely small com-
pared to most other primates, although they are
still classically pointed. In carnivores, canines are
used to immobilize prey, but in most primates they
are more important for communication (aggression
and as a secondary sexual characteristic). In hu-
mans, their role is largely as an accessory incisor.
The premolars are bicuspid, with flattened surfaces
and the molars generally have 4 cusps on the up-per jaw and 5 on the lower jaw although there is a
certain amount of variation here. These so called
cheek-teeth are where most of the chewing takes
place. A combination of occlusive, puncture-crush
forces and side-to-side shearing forces reduces the
size of the food particles so that the surface area
is increased to help digestion, and to produce a
soft, deformable, moist, lubricated bolus that can
be easily swallowed and moved by peristalsis. This
formation into a bolus may be more important than
the classic explanation of increasing surface area.It is likely that our digestive system can cope with
large particle sizes, but we all know how difficult
eating dry cream-crackers is.
4.2 Tongue
The tongue has several important functions in eat-
ing. Most obviously, the tongue contains the tastereceptors. These help identify the food value of
what has been put into the mouth. Things that taste
pleasant are likely to be nutrition, and things that
taste unpleasant are likely to be poisonous. Things
that are eaten, whether they are plants or animals,
are involved in an escalating evolutionary arms
race which their predators. In the case of plants,
many species produce various toxic and otherwise
unpalatable compounds to reduce their desirability
as food items. Interestingly, at least two species,
humans and common chimpanzees have realizedthat these compounds can have valuable medicinal
effects and can be considerably more toxic to var-
ious parasitic organisms than to the host, and they
actively seek out certain plants when they feel un-
well. In addition, many of the chemicals that give
certain spices a hot taste are actually plant toxins.
The other major functions of the tongue are me-
chanical. It moves the bolus of food between the
teeth whilst chewing, and in swallowing it moves
the food to the back of the buccal cavity to ini-
tiate swallowing. The tongue contains three setsof intrinsic muscles with fibres that run orthogo-
nally in the three anatomical directions (proximo-
distally, transversely and supero-inferiorly). These
allow the tongue to flatten or elongate. There are
also extrinsic muscles that attach the tongue to the
lower jaw and throat which allow it to move around
within the mouth (pro- and retraction, lateral devi-
ation, elevation and depression). It can also roll
within the mouth.
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Figure 4: Sagittal section of mouth and pharynx
4.3 Swallowing
Swallowing is a complex series of movements that
moves food from the buccal to the oesophagus (see
figure 4). It is made more complicated by the dual
role (respiration and digestion) of a number of the
structures in this region it is important that food
does not fall into the trachea and the lungs and also
important that it does not get pushed up into the na-
sopharynx. During swallowing the tongue pushes
the bolus backwards into the oropharynx. Other
muscles raise the soft palate and this action closes
off the nasopharynx. The back of the tongue andmuscles in the walls of the pharynx start to push
the bolus downwards by peristalisis and this action
pushes the epiglottis, a sprung, cartilagenous flap,
downward so that it covers the entrance to the tra-
chea whilst the food passes downwards to the oe-
sophagus. The epiglottis does not produce a per-
fect seal and choking occurs as a reflex reaction of
particles of food do accidentally fall into the tra-
chea.
4.4 Salivary Glands
Salivary glands in the mouth secrete saliva (spit)
(see figure 5). This fluid moistens food and con-
tains enzymes that start off starch digestion. It
also contains a variety of other chemicals with an-
tibacterial and anti-inflammatory functions. There
are many small salivary glands lining the tongue,
cheeks and palate. In addition there are three pairs
of large, discrete glands: the submandibular,
parotid and sublingual glands. Salivary glands
contain two sorts of secretory cells: serous cells
that secrete a watery fluid containing digestive en-
zymes; and mucous cells that secrete a viscous
fluid called mucus whose major role is to stick
food particles together to form the bolus. In sec-
tion, the serous cells stain strongly with H&E, but
the mucous cells remain largely unstained (figure
6). The parotid and submandibular glands form
a branching tree structure around their ducts, and
saliva is secreted through a single duct per gland.
The parotid duct enters the buccal cavity near the
upper second molar and the submandibular duct
opens underneath the tongue quite close to the
midline. The sublingual glands have many, sepa-
rate small ducts. Problems with saliva production
can lead to the formation of salivary stones which
can block the ducts and need to be removed surgi-
cally.
5 Oesophagus
The oesophagus is a straight, muscular tube con-
necting between the pharynx and the stomach. It
descends through the thorax behind the trachea andanterior to the thoracic aorta. The vagus nerves
exit the thoracic cavity through the diaphragm next
to the oesophagus. The wall of the oesophagus is
muscular (figure 7), and mucous glands in the mu-
cosa serve to keep the stratified, squamous epithe-
lia moist and lubricated. In the resting state the
oesophagus is deeply folded and it distends greatly
to allow the passage of food.
At the oesophageal hiatus (the hole in the di-
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Figure 5: Diagram showing the locations of the
major salivary glands
Figure 6: Diagram of a salivary gland
Figure 7: Cross-section of the oesophagus
aphragm), the muscular wall of the oesophagus is
thickened. These fibres are usually contracted to
prevent reflux of stomach contents. This action is
helped by a local thickening of muscle in the di-
aphragm which acts externally on the oesophagus
to help prevent both regurgitation and the physical
movement of the top of the stomach into the tho-
rax.
6 Stomach
The stomach is a large, classically J-shaped, bag-
like organ that hangs under the diaphragm on the
left-hand side (figure 8). It has a capacity of about
one litre, although, this can increase markedly. In
humans the stomach is used to mix food with gas-
tric juices, initiate the digestive process and store
food before passage to the small intestine. In many
mammals, notably ruminants, but also some leaf
eating primates, it is also a site for the fermentationof vegetable matter. This means that the stomach
contains a large number of micro-organisms that
are used to help break down otherwise indigestible
vegetable matter and neutralize toxins. This role
can be enhanced by having a multi-chambered
stomach that helps maintain optimal fermentation
conditions, and in some cases allows controlled re-
flux so that food can be chewed repeatedly.
The stomach can be divided into a number of re-
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Figure 8: The stomach
gions. The cardiac region is near the oesophageal
opening. The fundus is the area just above this
opening that is often seen containing air on abdom-
inal X-rays. The pylorus is the area near the exit of
the duodenum. This contains the powerful pyloric
sphincter which is thickening of the muscle wall
that prevents reflux from the small intestine back
into the stomach. The rest (and largest part) of the
stomach is the body.The stomach wall consists of a thickmucosa that
contains gastric pits (figures 9 and 10). These
are tubular gastric glands that in general contain
three types of secretory cells. Mucous cells secrete
mucus, chief cells secrete digestive chemicals and
parietal cells secrete a strong acid. This com-
bined product is the aforementioned gastric juice.
The role of the acid is to kill bacteria present in
the food. The digestive chemicals start breaking
down large food molecules into smaller compo-
nent molecules that can be absorbed, and the mu-cus helps prevent damage to the stomach wall by
lubricating against mechanical damage, and neu-
tralizing the effects of both the acid and digestive
chemicals.
The stomach is a very muscular organ. It has
three layers of muscle in the muscularis rather than
the normal two. In addition to the normal exterior
longitudinal muscle, and the deeper circular mus-
cle, it has an inner layer of oblique muscle. These
Figure 9: Diagram showing the variations in the
stomach wall
Figure 10: Cross-section of stomach wall
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muscles are used to churn the food to help the mix-
ing of food with gastric juice which produces amixture called chyme. A small amount of this is
squirted at intervals through the pylorus into the
small intestine.
7 The Small Intestine
The small intestine is a tubular long, tubular struc-
ture running from the exit of the stomach to the
beginning of the large intestine. It is mostly con-
cerned with continued food digestion and absorp-
tion of the products of digestion. It is divided into
a number of sections.
7.1 Duodenum
The first section of the small intestine is the duode-
num. This is a short (about 25cm long) C-shaped
tube that runs horizontally to the right from the
exit of the stomach, wraps itself around the head
of the pancreas, passing over the top of the right
kidney and then passes to the left before becoming
the next section of the small intestine (figure 11).
It is attached to the posterior abdominal wall rather
that being suspended on a fold ofperitoneum like
most of the rest of the abdominal alimentary canal.
This condition is known as retroperitoneal.
The duodenum is histologically distinct from the
rest of the small intestine because it contains extra
(Brunners) glands that secrete an alkaline mucus
that is used to neutralize the acidic chyme that is
produced in the stomach. It is also the region that
receives large ducts from the major digestive or-
gans: the liver and the pancreas.
7.2 Jejunum and Ileum
The next two sections of the small intestine are the
jejunum and the ileum. There is little real distinc-
tion between the two parts the jejunum is sim-
ply the proximal 2/5ths of this section (figure 12).
The jejunum and ileum are suspended by a fold of
peritoneum. The parietal peritoneum is attached
Figure 11: The small intestine
Figure 12: Diagram showing the variation between
different areas of the small intestine
to the abdominal wall and the gut tube acts rather
like a hose-pipe left underneath a carpet. At in-
tervals the length if this hose is such that it loops
away from the floor taking the covering carpet withit. So, the jejunum and ileum, several metres long,
hang in loops in the abdominal cavity, suspended
by a mesentery which consists of a double layer of
peritoneum which is continuous with the visceral
peritoneum surrounding the tube and with the vis-
ceral peritoneum attached to the posterior abdom-
inal wall. The blood and nerve supply to and the
lymph drainage from the loops of gut runs between
these two folds.
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Figure 13: Cross section of ileum
The role of the small intestine is digestion and
absorption (figure 13). The gut wall consists ofmillions of small, finger-like projections called
villi. Between the villi are intestinal glands which
contain mucus secreting goblet cells. The villi
contain a rich blood supply for transporting away
digestion products, and also blind-ended lacteals
which drain into the lymph vessels which also
carry away digestion products. The internal wall of
the small intestine is raised in circular folds called
plicae circulares. These help to increase the sur-
face area available for absorption. The villi further
increase the surface area and the cells in the colum-nar epithelium that line the villi have microvilli on
their lumenal surface again increases the surface
area available for absorption.
The small intestine is an extremely active tissue.
The constant movement of intestinal contents and
the energetic constraints of food absorption mean
that cells are quickly lost from the tips of the villi.
There is a constant migration of cells from the in-
testinal glands (also known as crypts) towards the
Figure 14: Diagram showing the relations of the
liver and pancreas
tips of the villi, so that within the crypts, there are
often signs of active cell division.
The small intestine ends at the ileocaecal valve
which is a sphincter that separates the contents of
the small intestine from the large intestine.
8 Pancreas
The pancreas, in the context of this series of lec-
tures, is an organ for the production and secretion
of digestive enzymes (figure 14) this is its ex-
ocrine function. It also has an important endocrinerole which will be dealt with elsewhere. It is a
long, thin organ that sits underneath the stomach
with its head region closely associated to the inte-
rior of the C of the duodenum. The pancreatic
duct runs the length of the body of the pancreas
and merges with the common bile duct before en-
tering the duodenum. The contents of the duct is
pancreatic juice which is a secretion produced by
acinar cells (figure 15).
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Figure 15: Section of pancreas
9 Liver
The liver is the largest gland in the body (fig-
ure 14). This is a somewhat misleading statement
since many of its functions are not glandular. How-
ever, it is an extremely large and important organ.
You dont live long with no liver! The liver takes
up the space on the right-hand side that is occupied
by the stomach on the left. It is firmly pushed upagainst the right dome of the diaphragm where the
inferior vena cava, which passed through the liver,
enters the thoracic cavity. It is held in place by
folds of peritoneum. This peritoneal folding pro-
duces the falciform ligament anteriorly which at-
taches to the ventral abdominal wall and the coro-
nary ligament superiorly which attaches to the di-
aphragm. The liver itself is divided into four lobes,
with the left and right lobe being by far the largest,
with the caudate and quadrate lobes being small
lobes near the vena cava and the gall bladder re-spectively.
Microscopically, the liver is organized into hep-
atic lobules. These consist of a small central
vein with columns of hepatic cells radiating out-
wards from this central point (figures 16 and 17).
Blood flows radially inwards towards the central
vein between the columns from both hepatic arter-
ies and branches of the hepatic portal vein. The
hepatic portal vein drains the portal venous sys-
Figure 16: Diagram showing the structure of the
liver
Figure 17: Section of liver
tem and contains high concentrations of digestion
products that are processed by cells in the liver
(hepatocytes). Also present in the lobules are hep-
atic macrophages that engulf most of the bacterial
cells that have entered through the intestinal wall.
Also within the lobules are bile canals. These are
fine tubes that drain the secretory product bile out
to the periphery of the lobule where the tubes unite
to form hepatic ducts which eventually unite to
form the common hepatic duct that carries bile
out of the liver.
10 Gall Bladder
The gall bladder is a small sac-like container that
attaches by a short tube (the cystic duct) to the
common hepatic duct (figure 14). Once the com-
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Figure 18: The large intestine
mon hepatic duct has joined the cystic duct it is
renamed the common bile duct which continues
to join with the pancreatic duct just before it emp-
ties into the duodenum. The bile duct stores thebile produced by the liver, and it is able to contract
to force bile into the duodenum as required by di-
gestion.
11 Large Intestine
The large intestine is the last part of the alimentary
canal. The large in the name refers to its diam-
eter rather than its length since the small intestine
is longer in humans. The large intestine consists ofthree functionally distinct parts, all those these are
further subdivided for descriptive purposes. These
major divisions are the caecum, the colon and the
rectum (figure 18).
11.1 Caecum
The caecum is a sac-like structure at the beginning
of the large intestine, situated retroperitoneally in
the lower right quadrant of the abdomen. In hu-
mans it is rather insignificant, but in many othermammals it is the alternative site for fermentation
of food. When this is the case, it can be larger
than the stomach. In humans, a small, blind-ended
tube hangs off the caecum. This is the vermiform1
appendix which has no digestive role, although it
may have a role as part of the lymphatic system in
fighting off infection.
11.2 Colon
The colon is a long tube that functions as the main
area for water and electrolyte absorption. In many
cases, this is actually re-absorption since large
quantities of both are secreted into the gut higher
up in the alimentary canal to aid digestion. The
colon is the region where these substances are re-
covered. Descriptively, it is divided into four re-
gions.
1. The ascending colon is the first part of the
colon which ascends vertically from the cae-
cum in the lower right quadrant up towards
the liver in the upper right quadrant. It is
retroperitoneal.
2. The transverse colons hangs down from a
complex mesentery more or less horizontally
from the end of the ascending colon in the up-
per right quadrant to the descending colon in
the upper left quadrant.
3. The descending colon runs retroperitoneally,
vertically down from the upper left quadrant
to the lower left quadrant.
4. The sigmoid colon is a short length of colon
that is once again attached to a mesentery that
connects the end of the descending colon to
the rectum. It is mostly contained within the
pelvic cavity.
1Vermiform means worm-like.
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Histologically, the wall of the colon contains a
thick mucosa containing many tubular, mucus se-creting glands (figure 19). Absorption occurs pri-
marily in the proximal half of the colon. The mu-
cus is mainly lubricating and protective to allow
the easy movement of the gut contents distally.
11.3 Rectum
The final region of the large intestine is the rec-
tum. This is a short tube that lies next to the sacrum
within the pelvic cavity (figure 20).
12 Anus
The external opening of the rectum is through the
anus. This forms the last 2 to 4 cm of the alimen-
tary canal. The mucous membrane lining the anal
canal is folded into a series of 6 to 8 longitudi-
nal anal columns. Free movement of faecal mat-
ter through the anal canal is prevented by a series
of ring-like sphincters. The internal anal sphinc-
ter is a ring of smooth muscle which is under in-
voluntary control. The external anal sphincter is
skeletal muscle under voluntary control. retroperi-
toneal.
13 Portal Circulation
The blood supply of most of the digestive system
is entirely normal blood flows in from nearby
arteries and is drained into nearby veins. How-
ever, the venous drainage for the parts of the gut in-
volved in substantial amounts of absorption of di-gestion products have a different venous drainage
(figure 21). Normal venous drainage where the
blood is conveyed directly back to the right atrium
of the heart is referred to as systemic. Venous
drainage from the gut drains first to the liver, and
only after that does it pass to the heart. This
drainage from one organ system directly to another
is referred to as the portal system. The reason
for this separate drainage is that the composition
Figure 19: Cross-section of colon
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Figure 20: The rectum and anus
of blood is greatly altered by digestion products,
and these products need to be processed before the
blood can be allowed to mix back with the rest of
the blood supply. This processing occurs within
the liver, so the liver has two blood inputs: the
hepatic artery which supplies oxygenated arterial
blood that provides the liver with the oxygen thatit needs; and the hepatic portal vein which is de-
oxygenated, venous blood drained from the distal
oesophagus, the stomach, the duodenum, jejunum,
ileum, the caecum, appendix, colon and the prox-
imal part of the rectum. It also drains blood from
the spleen, the pancreas and the gall bladder. There
are some areas where the portal and systemic sys-
tem anastamose. These are generally not impor-
tant, but in certain situations (notably portal hy-
pertension) they can open up to allow an alterna-
tive route for blood draining the gut. These includethe junctions between the areas of the oesophagus
and the rectum that are drained by portal and sys-
temic veins, and umbilical vein remnants left over
from the fetal circulation.
Figure 21: Diagram of the hepatic portal system
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