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3.1: Digestion at a Glance

Digestion is the process of breaking down food to be absorbed or excreted. The gastrointestinal (GI, digestive) tract, the passage 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.

Figure 3.11 The digestive tract, also known as the gastrointestinal tract, is a "tube within a tube"

A number of organs are involved in digestion, which collectively is referred to as the digestive system.

Figure 3.12 The digestive system1

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

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 digestion accessory organs (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 content.

Figure 3.14 Digestion accessory organs1

There are a number of enzymes that are involved in digestion. We will go through each one in detail, 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

Location Enzyme/Coenzyme


Salivary amylase

Lingual lipase



Gastric lipase




Small Intestine

Pancreatic alpha-amylase

Brush border disaccharidases

Pancreatic lipase



Cholesterol esterase


Brush border peptidases



Enzymes and Digestion -