Nutrition Science and Everyday Application - beta v 0.1

THE DIGESTIVE SYSTEM 143
Figure 3.9. An overview of the organs involved in digestion. The parts of the GI tract are
highlighted in blue, and the accessory organs are highlighted in yellow.
AN OVERVIEW OF THE DIGESTIVE PROCESS
The process of digestion includes five main activities: ingestion, mechanical digestion,
chemical digestion, absorption, and excretion.
The first of these processes, ingestion, refers to the entry of food into the GI tract through
the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that
begin breaking down the carbohydrates and lipids in food. Mastication (chewing) increases
the surface area of the food and allows for food to be broken into small enough pieces to be
swallowed safely.
Food (now called a bolus since it has been chewed and moistened) leaves the mouth
when the tongue and pharyngeal muscles propel the bolus into the esophagus. The bolus
will travel down the esophagus through an involuntary process called peristalsis. Peristalsis
consists of sequential, alternating waves of contraction and relaxation of the smooth
muscles in the GI tract, which act to propel food along (Figure 3.10). These waves also play a