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Transit through the Alimentary Canal (Digestive System)

The previous page identified the 4 basic stages of the digestive process.
This page describes in more detail the processes by which foodstuffs (incl. ingested liquids) are broken down and the useful parts taken into the cells of the body. This includes further consideration of the organs of the parts of the digestive system, whose locations were indicated on the first page introducing the human digestive system.

Salivary Glands Epiglottis Teeth Parts of the Respiratory System Ileocaecal Valve Small Intestine Appendix Oesophagus Duodenum Bile Duct Gall Bladder Liver Salivary Glands Oesophagus Soft Palate Tongue Parts of the Respiratory System Cardiac Sphincter Stomach Pyloric Sphincter Gall Bladder Ileocaecal Valve Small Intestine Duodenum Pyloric Sphincter Pancreas Large Intestine Rectum
 

Above: Diagram of route through Alimentary Canal

 

Notes about each stage of the alimentary canal:

The following are key points about the stages mentioned - with just sufficient extra detail to describe and explain the passage of ingested material through each part of the alimentary tract. These short points are in the form that might be required in a test or exam. For more about individual tissues and organs, use the links to further anatomical details (included in the text below).

1.

Buccal Cavity

Buccal Cavity is a medical term used to refer to the mouth.
Several digestive processes occur in the buccal cavity, including:

  • Mechanical parts of the digestive process include chewing and grinding using the teeth.
  • Saliva is produced and secreted by the salivary glands.
    The secretion of saliva by the salivary glands is called salivation.
    The production and secretion of saliva is increased in response to the chewing action of the jaws, and also in response to the thought, taste, smell, and hence the experience of ingesting foods. Saliva has several functions, including lubrication of the buccal cavity.
  • Saliva includes the enzyme salivary amylase, which begins the process of breaking-down the carbohydrates within the food. Salivary amylase has the effect of breaking down certain large molecules: polysaccharides di-saccharides.
  • Finally, after reduction to an appropriate size and consistency, food is formed into a "bolus" (i.e. a "ball" of foodstuff) to be passed down the digestive tract.

2.

Epiglottis

The epiglottis is a thin leaf-shaped flap of cartilage covered with a layer of mucous membrane. It is located immediately behind the root of the tongue and aids the digestive process by closing the trachea (which is sometimes colloquially referred to as the "windpipe") to prevent ingested materials from entering the lungs / respiratory system.

3.

Trachea

The trachea (or, colloquially, the "windpipe") is the part of the air passage between the larynx (which is sometimes colloquially referred to as the "voice box") and the main bronchi inside the lungs. Although not part of the digestive system, the trachea is labeled above to indicate the importance of the epiglottis.

4.

Oesophagus

The oesophagus (also known colloquially as the "windpipe") is the tube through which a bolus is carried from the mouth to the stomach. The bolus progresses down the oesophagus by means of peristaltic action.

5.

Cardiac Sphincter

The cardiac sphincter is the site at which material enters the stomach.
A bolus passes from the oesophagus into the stomach through the cardiac sphincter.

6.

Stomach

The stomach is a muscular sac that churns and mixes food. It also absorbs alcohol. Mucus and proteases are also present in the stomach.
Key Notes about the stomach:

  • churns / mixes food
  • mixes food with gastric acid

Enzymes in the stomach:

  • pH approx. 1-3 due to stomach acid.
  • Kill microbes.
  • Neutralise salivary amylase.
  • Provide a medium for proteases such as rennin (coagulates milk proteins) and pepsin.
    A protese is any enzyme that catalyses the splitting of a protein.
    Proteases catalyse the process: Polypeptides Di-peptides Amino acids.

7.

Pylonic Sphincter

The pylonic sphincter is the route by which material exits the stomach.
The contents of the stomach is squeezed out as chyme into the small intestine.

8.

Liver

The liver is an accessory organ (i.e. it assists the digestive process, e.g. by supplying substances useful to the digestive process - but ingested material does not pass through the liver). The liver is the largest organ in the body, the skin being the largest organ of the body.
The liver has over 500 functions, including:

  • Production and secretion of bile and bile salts.
    (Bile = blood pigments from erythrocytes + bile salts + cholesterol). Bile is alkaline and its function is to break-down ("emulsify" = "make into smaller globules") fats.
  • Phagocytosis of bacteria and dead or foreign materials.
  • Converts glucose to glycogen and vice-versa.
  • Production of cholesterol
  • Storage of glycogen
  • De-amination of excess amino acids
  • Detoxification, e.g. conversion of ammonia to urea, and processing of alcohol and/or drugs.
  • Storage of certain vitamins & minerals, e.g. iron (Fe) that can be used to produce red blood cells.

For more about these see the page about functions of the liver.

9.

Pancreas

The pancreas is an accessory organ (i.e. it assists the digestive process, e.g. by supplying substances useful to the digestive process - but ingested material does not pass through the pancreas). The pancreas is a "dual organ", i.e. it is both exocrine and endocrine.

Endocrine Functions:

Produces the hormones insulin and glucogon, which control sugar levels.

Exocrine Functions:

 

Produces enzymes:

  • Pancreatic Amylase - breaks down carbohydrates by:
    Polysaccharides Di-saccharides
  • Lipase - breaks down fats by:
    Fat Fatty Acids + Glycerol
  • Proteases e.g. typsin - break-down proteins by:
    Polypeptides Di-peptides Amino acids

10.

Small Intestine

There are three parts of the small intestine.
They are (in the order in which they are reached):

  1. The Duodenum
  2. The Jejunum
  3. The Ileum

In addition to the enzymes already contributed at previous stages in the alimentary tract, further enzymes are released from the walls of parts (1.) and (2.) of the small intestines.
Examples include: sucrase, maltase, fructase, lactase.
The '-ase' suffix indicates that the substance is an enzyme.
These facilitate reactions of the form: Di-saccharides mono-saccharides,
so, e.g.
The enzyme sucrase facilitates break-down to the mono-saccharide sucrose.
The enzyme maltase facilitates break-down to the mono-saccharide maltose.
The enzyme fructase facilitates break-down to the mono-saccharide lactose.
The enzyme lactase facilitates break-down to the mono-saccharide lactose.

Absorption takes place at (3.), then ...

  • Glucose and Amino Acids go to the Hepatic Portal Vein, and
  • Fatty Acids and Glycerol go into lacteal and are transported by the lymphatic system.

11.

Ileocaecal Valve

The iloecaecal valve is the exit through which chyme passes from the small intestine to the large intestine.

12.

Appendix

Note that the appendix is not strictly part of the alimentary tract. (It is mentioned here to complete brief notes about all of the tissues and organs labeled in the diagram above.)
The appendix is a "vestigial organ", which means that it is thought to be present in the body as a result of evolution - even though the human body has evolved in such as way as to render it (the appendix) non-essential.
Ingested matter does not pass through the appendix.
The appendix is composed of lymphatic tissue.

13.

Large Intestine

The large intestine is the final organ in the alimentary tract.
It consists of sections that have specific names, including:

The large intestine absorbs water from material passing through it - all the way along its length.
The final stages of the large intestine (the rectum and anal canal) also form and release faeces, as stated below.

14.

Rectum

The rectum is a latter part of the large intestine. Its purpose is the formation of faeces, i.e. faeces are formed in the rectum then expelled via the process of defecation.

15.

Anus

The anus is the opening at the lower-end of the alimentary tract, through which faeces are discharged. The anus opens out from the anal canal (which is the end, or "terminal", portion of the large intestine) and is kept closed by two sphincter muscles at all times except during defecation.

The above introduces the components of the alimentary tract and presents key points about the tissues and organs through which ingested matter (i.e. foodstuffs) pass during the digestive process.
The key-points listed in note form above may not include sufficient information even for introductory-level courses that include human digestion. For more about the anatomy and physiology of individual parts of the alimentary tract, see the pages about the teeth, oesophagus, stomach, liver, pancreas, small intestine and large intestine.

The next page in this series is about Digestion: The Mouth and Teeth.

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The Functions of the Liver

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