HUMAN BIOLOGY

DiGestioN aND NUtritioN 197

What does the digestive system do?

• The digestive system breaks down food by mechanical and
  chemical means, absorbs nutrients, and eliminates residues.


• The digestive tube, or gastrointestinal (GI) tract, extends from
  the mouth to the anus.


• For most of its length, the tube wall consists of four layers,
  including smooth muscle.


• Substances in the GI tract don’t enter the body until they pass
  across the tube wall into the bloodstream or lymph.

taKe-Home message

When we eat, food advances in one direction, from the
mouth (the oral cavity) through the pharynx, the esopha-
gus, stomach, small intestine, and large intestine. The large
intestine ends in the rectum, anal canal, and anus.

the digestive tube has four layers

From the esophagus onward, the digestive tube wall has
four layers (Figure 11.2). The innermost layer is a mucosa of
epithelial cells. It lines the lumen, the space through which
food passes. The mucosa is surrounded by the submucosa,
a layer of connective tissue with blood and lymph vessels
and nerve cells. The next layer is smooth muscle—usually
two sublayers, one circling the tube and the other oriented
lengthwise. An outer layer, the serosa, is a very thin serous
membrane (Section 4.7). Circular arrays of smooth muscle
called sphincters are located at the junctions between sec-
tions of the GI tract. As sphincters contract and relax, they
control the movement of material in the tube. For example,
a gastroesophageal sphincter controls the passage of food
from the esophagus into the stomach (Figure 11.3).

the different parts of the digestive system
perform five basic functions

To extract nutrients from food, the digestive system’s parts
carry out five types of tasks:

1. Mechanical processing and motility. Movements of
   various parts, such as the teeth, tongue, and muscle
   layers, break up, mix, and propel food along.

Figure 11.2 Four layers make up the wall of the digestive

tract. This diagram shows the layers in the small intestine.

(© Cengage Learning)

lumen circular

muscle

longitudinal

muscle

nerves

mucosa submucosa serosa blood vessels

Figure 11.3 Sphincters help regulate the passage of

food through parts of the Gi tract. This diagram shows

the sphincter where the esophagus opens into the

stomach. (© Cengage Learning)

Esophagus

Stomach

Sphincter muscles

relax, opening the

passageway.

Sphincter muscles

contract, closing the

passageway.

2. Secretion. Glands and accessory
   organs release enzymes or other
   chemicals used in digestion and
   absorption.


3. Digestion. Food is chemically
   broken down to nutrient
   molecules small enough to be
   absorbed.


4. Absorption. Digested nutrients
   and fluid pass across the tube
   wall and into blood or lymph.


5. Elimination. Undigested and unabsorbed residues are
   excreted from the end of the GI tract.
   Once nutrients from food have entered the bloodstream,
   cells throughout the body can take them up for use in all
   aspects of metabolism.

digestive system The tube

where food is digested and

absorbed and undigested

residues are expelled. Also

called the GI tract.

sphincter An array of cir-

cular muscles that regulates

the passage of material

between neighboring sec-

tions of the digestive tube.

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