Ahurried breakfast when you are late for work or

school...Thanksgiving dinner... going on a diet to
lose 5 pounds... what do these experiences all have in
common? Food. We may take food for granted, cele-
brate with it, or wish we wouldn’t eat quite so much of
it. Although food is not as immediate a need for
human beings as is oxygen, it is a very important part
of our lives. Food provides the raw materials or nutri-
ents that cells use to reproduce and to build new tis-
sue. The energy needed for cell reproduction and
tissue building is released from food in the process of
cell respiration. In fact, a supply of nutrients from reg-
ular food intake is so important that the body can even
store any excess for use later. Those “extra 5 pounds”
are often stored fat in adipose tissue.
The food we eat, however, is not in a form that our
body cells can use. A turkey sandwich, for example,
consists of complex proteins, fats, and carbohydrates.
The function of the digestive systemis to change
these complex organic nutrient molecules into simple
organic and inorganic molecules that can then be
absorbed into the blood or lymph to be transported to
cells. In this chapter we will discuss the organs of
digestion and the contribution each makes to diges-
tion and absorption.

DIVISIONS OF THE

DIGESTIVE SYSTEM

The two divisions of the digestive system are the ali-
mentary tube and the accessory organs (Fig. 16–1).
The alimentary tubeextends from the mouth to the
anus. It consists of the oral cavity, pharynx, esophagus,
stomach, small intestine, and large intestine.
Digestion takes place within the oral cavity, stomach,
and small intestine; most absorption of nutrients takes
place in the small intestine. Undigestible material, pri-
marily cellulose, is eliminated by the large intestine
(also called the colon).
The accessory organsof digestion are the teeth,
tongue, salivary glands, liver, gallbladder, and pancreas.
Digestion does not take place withinthese organs, but
each contributes something tothe digestive process.

TYPES OF DIGESTION

The food we eat is broken down in two complemen-
tary processes: mechanical digestion and chemical

digestion. Mechanical digestionis the physical break-

ing up of food into smaller pieces. Chewing is an

example of this. As food is broken up, more of its sur-

face area is exposed for the action of digestive enzymes.

Enzymes are discussed in Chapter 2. The work of the

digestive enzymes is the chemical digestionof bro-

ken-up food particles, in which complex chemical mol-

ecules are changed into much simpler chemicals that

the body can utilize. Such enzymes are specific with

respect to the fat, protein, or carbohydrate food mole-

cules each can digest. For example, protein-digesting

enzymes work only on proteins, not on carbohydrates

or fats. Each enzyme is produced by a particular diges-

tive organ and functions at a specific site. However, the

enzyme’s site of action may or may not be its site of

production. These digestive enzymes and their func-

tions are discussed in later sections.

END PRODUCTS OF DIGESTION

Before we describe the organs of digestion, let us see

where the process of digestion will take us, or rather,

will take our food. The three types of complex organic

molecules found in food are carbohydrates, proteins,

and fats. Each of these complex molecules is digested

to a much more simple substance that the body can

then use. Carbohydrates, such as starches and disac-

charides, are digested to monosaccharides such as glu-

cose, fructose, and galactose. Proteins are digested to

amino acids, and fats are digested to fatty acids and

glycerol. Also part of food, and released during diges-

tion, are vitamins, minerals, and water.

We will now return to the beginning of the alimen-

tary tube and consider the digestive organs and the

process of digestion.

ORAL CAVITY

Food enters the oral cavity(or buccal cavity) by way

of the mouth. The boundaries of the oral cavity are

the hard and soft palates superiorly; the cheeks later-

ally; and the floor of the mouth inferiorly. Within the

oral cavity are the teeth and tongue and the openings

of the ducts of the salivary glands.

TEETH

The function of the teethis, of course, chewing. This

is the process that mechanically breaks food into

smaller pieces and mixes it with saliva. An individual

370 The Digestive System