Unit 3
Digestion, Absorption, and Transport

The human gastrointestinal (GI) tract is a 26-foot long, muscular, flexible tube that begins with the mouth as one opening, and ends with the anus as the other. Its main function is to digest and absorb nutrients from food. In this unit, we review the anatomical features of the GI tract, and study how the involuntary muscles and enzymes function in the mechanical and chemical digestion of food. The processes of digestion, absorption, and transport involve specially controlled systems and mechanisms, and this unit presents only an overview of these complicated processes. We shall elaborate further as we discuss individual nutrients in later units. Finally, in this unit, we briefly discuss some common digestive problems, such as choking, vomiting, diarrhea, constipation, heartburn and “acid indigestion,” and peptic ulcer.

This unit consists of the four sections listed below.

• Section 3.1  Digestion
• Section 3.2  Absorption
• Section 3.3  Transport
• Section 3.4  Some Common Digestive Problems