Chapter 12
Digestive System

The chapter was edited, adapted and remixed from chapter 5 “Digestive System” of the open source book Human Biology by Willy Cushwa (CC BY 4.0; original text avialable for free at https://archive.org/details/cnx-org-col11903. The source code can be found at https://github.com/cnx-user-books/cnxbook-clark-college-human-biology) and chapter 23 “The Digestive System” of the open source book Anatomy and Physiology 2e from OpenStax (original text available for free at https://openstax.org/details/books/anatomy-and-physiology-2e).

All living organisms need nutrients to survive. While plants can obtain nutrients from their roots and the energy molecules required for cellular function through the process of photosynthesis, animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. However, the food consumed consists of protein, fat, and complex carbohydrates. Animals must convert these macromolecules into the simple molecules required for maintaining cellular function. The conversion of the food consumed to the nutrients required is a multistep process involving digestion and absorption. During digestion, food particles are broken down to smaller components, which are later absorbed by the body. This happens by both physical means, such as chewing, and by chemical means, via enzyme-catalyzed reactions.

The digestive system performs the task of digestion and absorption. It is continually at work, yet people seldom appreciate the complex tasks it performs in a choreographed biologic symphony. Consider what happens when you eat an apple. Of course, you enjoy the apple’s taste as you chew it, but in the hours that follow you don’t notice that your digestive system is working. Your stomach and intestines are busy digesting it and absorbing its vitamins and other nutrients. By the time any waste material is excreted, the body has appropriated all it can use from the apple. In short, the organs of the digestive system perform their specific functions, allowing you to use the food you eat to keep you going. This chapter examines the structure and functions of these organs, and explores the mechanics and chemistry of the digestive processes.