3.1 Digestion at a Glance

• Digestion at a Glance

Digestion is the process of breaking down food to be absorbed or excreted. There are two types of digestion in the body; mechanical and chemical. Mechanical digestion involves physically breaking food down into smaller pieces, usually through muscle contractions.

Chemical digestion uses enzymes or other chemicals to break of food into smaller nutrients. This generally involves the breaking of chemical bonds in the process.

Figure 3.11 A number of organs are involved in digestion, which collectively are referred to as the digestive system1.

Required Web LinkVideo: Enzymes and Digestion

The gastrointestinal (GI or digestive) tract, the passageway through which our food travels, is a “tube within a tube.” The trunk of our body is the outer tube and the GI tract is the interior tube, as shown below. Thus, even though the GI tract is within the body, the actual interior of the tract is technically outside of the body. This is because the contents have to be absorbed into the body. If it’s not absorbed, it will be excreted and never enter the body itself. It’s as if you never consumed it.

Figure 3.12 The digestive tract, also known as the gastrointestinal tract, is a “tube within a tube”

The organs that form the gastrointestinal tract (e.g., mouth, esophagus, stomach, small intestine, large intestine (aka colon), rectum, and anus) come into direct contact with the food or digestive contents.

Figure 3.13 The gastrointestinal or digestive tract2

The journey through the gastrointestinal tract starts in the mouth and ends in the anus as shown below:

Mouth –> Esophagus –> Stomach –> Small Intestine –> Large Intestine –> Rectum –> Anus

In addition to the GI tract, there are a number of accessory organs (e.g. salivary glands, pancreas, gallbladder, and liver) that play an integral role in digestion. The accessory organs do not come directly in contact with food or digestive contents, but still play a crucial role in the digestive process.

Figure 3.14 Digestion accessory organs1

In addition to the digestive and accessory organs, there are a number of enzymes that are involved in digestion. We will go through each one in detail later, but this table should help give an overview of which enzymes are active at each location of the GI tract.

Table 3.11 Digestive enzymes

References & Links

• http://www.wpclipart.com/medical/anatomy/digestive/Digestive_system_diagram_page.png

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• http://commons.wikimedia.org/wiki/File:Digestivetract.gif

Video

Enzymes and Digestion – http://www.youtube.com/watch?v=bNMsNHqxszc