The gastrointestinal (GI) system is basically a muscular tube with one primary purpose - absorption of nutrients and water for the body's tissues.  Without these nutrients, the body could not grow or function.  Meals must be processed in the GI tract to make them easier to absorb.  This means breaking food particles into tiny parts that can be absorbed into individual cells.  Undigestible food products and waste products are removed from the body as stool. 

Digestion and absorption

Primitive one-celled organisms swam through a soup of nutrients.  These nutrients could be absorbed directly through the cell wall as waste products could be removed from the cell in a similar way.  Multi-celled organisms needed a way to spread nutrients to deeper tissues, therefore they developed primitive circulatory systems.  As animals walked out of the seas onto land, the means of circulating nutrients through the body had to adapt.  Specialized systems were developed that allowed these early animals to eat... passing nutrients into the body where they could be processed into a liquid form and delivered to the body tissues through blood. 

The process of digestion has adapted to allow humans to eat foods made mostly of large molecules in solid form.  These molecules are broken down by the digestive system and mixed with water to allow passage into the blood and ultimately into the body tissue cells.

The process by which food is broken down into a liquid suspension of small particles ready to pass into the cells lining the GI tract is called digestion. 

Absorption is the process by which nutrients pass through the cells lining the GI tract, enter the blood or lymph, and are delivered to the body tissues.

Excretion

The GI tract also serves as a major way for the body to get rid of excess or toxic substances... yep, you got it... poop.  In addtion to passage of non-digestible food residue, the GI tract assists the body in trashing toxic substances that are not easily filtered by the kidneys.  While the kidneys are good for excreting water-soluable chemicals, the GI tract is better for excreting fat-soluable substances. 

Host defense

The GI tract is a long tube whose inner surface is continuous with the outside of the body.  In a sense, the GI tract is similar to the skin in that it is in constant contact with "the outside world" of foreign substances.  Only through absorption can chemicals enter into the blood.  This gastrointestinal barrier serves as a major component of the immune system.  Like the skin, the GI tract is a potential entry point for germs.  For this reason, the gI tract has developed ways to determine which substances are good for the body and which are bad.  The bad substances are not allowed to enter the blood or they are destroyed by immune cells. 

The GI tract as seen by an engineer...

(The author has a mechanical engineering degree!)

Mouth - Chopper, shredder and moisturizer

Esophagus - Transit tube

Stomach - Blender, storage chamber, and acid sterilizer

Duodenum - Reaction chamber

Liver - Detergent supply

Pancreas - Enzyme supply and acid neutralizer

Jejunum and Ileum - Absorptive surface

Colon - Drying chamber

Rectum - Aggregation chamber

Anus - Waste emission control

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