The Digestive System - Introduction

Diet and Nutrition Study Notes

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

Salivary Glands Esophagus Parts of the Respiratory System Epiglottis Tongue Teeth Liver Appendix Diaphragm Gall Bladder Large Intestine Appendix Diaphragm Gall Bladder Large Intestine Stomach Pancreas Small Intestine Small Intestine Rectum Anus
Salivary Glands Esophagus Parts of the Respiratory System Epiglottis Tongue Teeth Liver Appendix Diaphragm Gall Bladder Large Intestine Appendix Diaphragm Gall Bladder Large Intestine Stomach Pancreas Small Intestine Small Intestine Rectum Anus

Above: Diagram showing the location of the main
tissues and organs of the digestive system

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

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:

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:

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.

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.

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-shaped 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.

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.

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.

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.

Next Page: Before studying the digestive processes it is useful to understand some terms relating to digestion.

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