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The Digestive System - Introduction
Understanding the human digestive system consists of
knowledge of the following aspects and how they interact
with each other.
- The names and locations in the body of the organs
of the digestive system.
- The digestive process
(overview).
- The passage of matter through the digestive system,
that is the digestive
process(es) by which foodstuffs are
broken down at key stages along the alimentary canal.
- The structure and
functions of the main parts (organs)
of the digestive system - considering each organ individually.
- The chemical break-down
of food,
that is how each of the important components of food
(food groups) is processed by the body, including
the basic chemistry of these processes.
- Recognising and understanding the causes and effects
of the most common disorders
of the digestive system.
Key learning objectives are highlighted in bold-green
text in the above list and are described here on pages
(or series of pages), links to which appear in the index-list
on the left.
Identification and Location of Organs of the Digestive
System
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Read down through
the centre section first. The smaller diagram below
links to a later page, which is useful for comparison:
Diagrams representing
the Digestive System:
Notice the difference
between the (small) diagram above and the larger diagram
on the right:
The diagram of
the digestive system above
is a schematic diagram, meaning that the parts of
the digestive system are not really spaced-out as
clearly as indicated in this diagram. However, they
are shown this way to
make it easier to see the route through the digestive
tract, and hence in which order each
part is reached.
The diagram of
the digestive system on
the left indicates the
locations in the body of the tissues
and organs of the digestive system. It shows that
they are packed together closely, with some organs
in front of, behind, above, and around other organs.
Remember that the reality (in 3-dimensions) is more
complicated that this 2-dimensional diagram, which
is further simplified by the use of hard lines and
shading different organs in different colours.
The labels shown
on the diagram of the
locations of digestive organs (right)
link to more information about the body-part indicated:
Visit those pages of further information then return
to reinforce your learning about the locations of
parts of the digestive system
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Above: Diagram showing the location of the
main
tissues and organs of the digestive system
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| Use the interactive diagram above to
learn and remember the main structures of the digestive
system.
The human digestive system can be described in two
parts, they are the components of the alimentary
canal (see "Transit
through the alimentary tract") and the accessory
organs.
About the
Alimentary Canal |
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About the Accessory
Organs |
The "alimentary canal"
is also known as the "alimentary tract".
It is a tube of approx. 9m long (in total, in
an adult) that passes from the mouth to the anus
and includes the following parts:
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Organs, glands, and tissues that
assist the digestive process, e.g. by supplying
fluids/chemicals, but which ingested material
does not actually pass through may
be referred to as accessories (to the digestive
process/system). These include:
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| Note that the appendix
is not mentioned in either of the two categories
above because is is a 'vestigial organ',
which means that (although it is thought to have
been useful to distant ancestors of our species),
the appendix does not play an active role in the
digestive process. It is included in descriptions
of the digestive system because it is attached
to the large intestine.
It is useful to know about the appendix to study
conditions
& disorders of the digestive system, among
which appendicitis
is often included.
Notes about the Locations of Parts of the
Digestive System:
The locations of parts of the human digestive
system are shown above but might not be completely
clear from the diagram because these tissues and
organs over-lap when drawn (in 2-dimensions).
This is unavoidable because some parts are located
in front of / behind other parts - as well as
above, below, etc.. The following notes are therefore
included for clarification:
| Teeth |
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The teeth
are located inside the mouth - which is
also known as the "buccal
cavity".
Humans have two sets of teeth, the first
during childhood and the second set (ideally)
throughout adult life. The lower row of
teeth are inset into the mandible
(lower jaw bone) and the upper row of teeth
are inset into the maxillae
(upper jaw bones). |
| Tongue |
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The tongue is located at
the floor of the mouth between the two lateral
portions of the mandible (lower-jaw). The
base of the tongue is connected to the midline
of the floor of the mouth by a fold of membrane
called the lingual frenulum and is controlled
by several muscles including the hyoglossus. |
| Salivary Glands |
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The salivary
glands are located in the lower part
of the face and secret into the buccal
cavity.
There are 3 main salivary glands:
(1) The parotid
gland is the largest salivary gland
and is located on the side of the face immediately
below and anterior to the external ear.
(2) The submaxillary
gland is located below the jaw towards
the front of the neck/lower-jaw.
(3) The sublingual
gland is the smallest salivary gland
and is located below the mucous membrane
of the floor of the mouth. It is narrow
and has a flatten shape, resembling an almond. |
| Epiglottis |
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The epiglottis
is the flap of cartilage located at the
back of the throat behind the tongue, and
in front of the larynx. |
| Trachea |
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The trachea is not part
of the digestive system but is included
on the above diagram to clarify the action
of the epiglottis,
which acts as a "switch" - directing
foodstuffs / air from the buccal cavity
into either the oesophagus (leading to the
stomach) or the trachea (leading to the
lungs). |
| Oesophagus |
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The oesophagus
is a muscular canal that extends from the
throat at the back of the mouth to the stomach.
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| Diaphragm |
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The diaphragm is a thin
musculo-fibrous septum that is not part
of the digestive system but is included
above because it separates the thorax (containing
e.g. the lungs) from the abdomen (which
contains much of the digestive system, as
shown). The diaphragm divides these two
bodily cavities, forming the floor of the
thorax and the roof of the abdomen. |
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| Stomach |
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The stomach
is located between the lower end ("termination")
of the oesophagus
and the start of the small
intestines - at which the pylonic
sphincter of the stomach releases contents
of the stomach into the duodenum
(the first and upper-most part of the small
intestines). |
| Liver |
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The liver
is located in the upper right-side of the
abdominal cavity (i.e. immediately below
the diaphragm). It is divided into two lobes,
the left-lobe being smaller than the right-lobe.
Note that the diagram
above shows the liver on the left-hand-side
because it is a diagram of the anterior
view of the digestive system - that is looking
at the front of a person, hence the left-hand-side
of the diagram represents the right-hand-side
of the body. |
| Gall Bladder |
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The gall
bladder is an approx. cone-shapes musculo-membranous
sac located in a fossa under the right-lobe
of the liver. |
| Pancreas |
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The pancreas
is located behind the stomach and
partly within the curve of the duodenum.
Note that the diagram
above appears to show the pancreas in front
of the stomach (given that the diagram above
is an anterior view of the digestive
system) and does not explicitly show the
pancreas lying partly within the curve of
the duodenum. Unfortunately this is unavoidable
when including all of the main digestive
organs in this diagram because an ordinary
anterior view would not include anything
behind something else included in
the same view. We have therefore included
the pancreas (represented in a pale colour)
apparently in front of the stomach - with
this note in clarification.
For a clearer representation of the location
of the pancreas partly within the curve
of the duodenum, see the main/largest diagram
on the page about passage
through the alimentary tract - a small
version of which also appears at the top-right
of this page. |
| Small Intestines |
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The small
intestines are located in the lower
part of the abdomen, within the large membrane
known as the peritoneum (which has 2 layers
separated by a small amount of liquid, enabling
the organs contained within it to move freely
over and around each other). The small intestines
progress from the pyloric
sphincter, through with they receive
material from the stomach, into the first
of three parts - called the duodenum.
The next two parts of the small intestines
are the jejunum
and the ileum.
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| Large Intestines |
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The large
intestines are located in the lower
part of the abdomen, within the large membrane
known as the peritoneum (which has 2 layers
separated by a small amount of liquid, enabling
the organs contained within it to move freely
over and around each other). The large intestines
begin at the bottom of the abdomen, where
material is received from the ileum,
i.e. the final part of the small intestines.
In common with the small intestines, the
large intestines are also described in three
parts:
(1) The cecum
is the first part of the large intestine
and so receives material from the ileum.
(2) The second part of the large intestines
is the colon,
which initially rises upwards within the
abdomen (ascending
colon), then moves across the body beneath
the liver and stomach (transverse
colon), then finally passes downwards
back to the lower abdomen (descending
colon).
(3) The third and final part of the large
intestines is the rectum
- also described separately - which
is located in the approx. centre of the
lower abdomen.
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| Appendix |
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The appendix
extends from the cecum,
forming a narrow tube that may pass in any
of several directions, incl. e.g. upwards
behind the cecum, to the left behind the
ileum, or downwards and inwards. The appendix
varies in length from approx 25mm to 220mm
(in adults), typical length approx. 75mm.
The appendix is held in position by an approx.
triangular fold of peritoneum.
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| Rectum |
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The rectum
is the terminal (i.e. end) part of the large
intestine and extends from the sigmoid flexure
to the anal orifice. It is approx. 12-20cm
or 5-8 inches long in total -
estimates in textbooks vary - and may
be described in three parts, according to
the curve formed by this final part of the
large intestine. The three parts have approx.
proportions 10cm, 7.5cm, and 2.5 - 4cm (the
last part being slightly longer in men than
in women), or values in similar proportions
for recta of slightly different total length.
The rectum ends at the anus,
from which indigestible matter is released
from the body during defecation. |
| Anus |
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The anus
is located at the base of the abdomen. |
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Next Page: Before studying the
digestive processes it is useful to understand some
terms relating
to digestion.
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