Human being digestive system

Before jumping into a discussion on the alimentary tract, we need to review some basic
terms.


Ingestion: Taking in food

Digestion: Changing the composition of food — splitting large molecules into smaller
ones — to make it usable by the cells

Deglutition: Swallowing, or moving food from the mouth to the stomach

Absorption: Occurs when digested food moves through the intestinal wall and into
the blood

Egestion: Eliminating waste materials or undigested foods at the lower end of the
digestive tract; also known as defecation


The alimentary tract develops early on in a growing embryo. The primitive gut, or archenteron,
develops from the endoderm (inner germinal layer) during the third week after conception,
a stage during which the embryo is known as a gastrula. At the anterior end (head end),
the oral cavity, nasal passages, and salivary glands develop from a small depression called
a stomodaeum in the ectoderm (outer germinal layer). The anal and urogenital structures
develop at the opposite, or posterior, end from a depression in the ectoderm called the
proctodaeum. In other words, the digestive tract develops from an endodermal tube with
ectoderm at each end.

Whereas the respiratory tract is a two-way street — oxygen flows in and carbon dioxide flows
out — the digestive tract is designed to have a one-way flow (although when you’re sick or
your body detects something bad in the food you’ve eaten, what goes down sometimes comes
back up).


Mouth Pharynx Esophagus Stomach Small intestine Large intestine

When you swallow food, it’s mixed with digestive enzymes in both saliva and stomach
acids. Circular muscles on the inside of the tract and long muscles along the outside of
the tract keep the material moving right through defecation at the end of the line.


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