Human Physiology, 14th edition (2016)

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18.1 INTRODUCTION TO THE

DIGESTIVE SYSTEM

Within the lumen of the gastrointestinal tract, large food

molecules are hydrolyzed into their monomers (subunits).

These monomers pass through the inner layer, or mucosa,

of the small intestine to enter the blood or lymph in a pro-

cess called absorption. Digestion and absorption are aided

by specializations of the gastrointestinal tract.

( monomers ). Within the gastrointestinal tract, the digestion of

these large molecules into their monomers occurs by means of

hydrolysis reactions (reviewed in fig. 18.1 ). The monomers thus

formed are transported across the wall of the small intestine

into the blood and lymph in the process of absorption. Diges-

tion and absorption are the primary functions of the digestive

system.

Because the composition of food is similar to the composi-

tion of body tissues, enzymes that digest food are also capable

of digesting a person’s own tissues. This does not normally

occur, however, because a variety of protective devices inacti-

vate digestive enzymes in the body and keep them away from

the cytoplasm of the cells. The fully active digestive enzymes

are normally confined to the lumen (cavity) of the gastrointes-

tinal tract.

The lumen of the gastrointestinal tract is open at both ends

(mouth and anus) and is thus continuous with the environment.

In this sense, the harsh conditions required for digestion occur

outside the body. Indigestible materials, such as cellulose from

plant walls, pass from one end to the other without crossing

the epithelial lining of the digestive tract; because they are not

absorbed, they do not enter the body.

In Planaria (a type of flatworm), the gastrointestinal tract

has only one opening—the mouth is also the anus. Each cell that

lines the gastrointestinal tract is thus exposed to food, absorb-

able digestion products, and waste products. The two open ends

of the digestive tract of higher organisms, by contrast, permit

one-way transport, which is ensured by wavelike muscle con-

tractions and by the action of sphincter muscles. This one-way

transport allows different regions of the gastrointestinal tract to

be specialized for different functions, as a “disassembly line.”

These functions of the digestive system include:

1. Motility. This refers to the movement of food through the

digestive tract through the processes of

a. Ingestion: Taking food into the mouth.

b. Mastication: Chewing the food and mixing it with

saliva.

c. Deglutition: Swallowing food.

d. Peristalsis and segmentation: Rhythmic, wavelike con-

tractions (peristalsis), and mixing contractions in dif-

ferent segments (segmentation), move food through the

gastrointestinal tract.

2. Secretion. This includes both exocrine and endocrine

secretions.

a. Exocrine secretions: Water, hydrochloric acid, bicarbon-

ate, and many digestive enzymes are secreted into the

lumen of the gastrointestinal tract. The stomach alone, for

example, secretes 2 to 3 liters of gastric juice a day.

b. Endocrine secretions: The stomach and small intestine

secrete a number of hormones that help to regulate the

digestive system.

3. Digestion. This refers to the breakdown of food molecules

into their smaller subunits, which can be absorbed.

4. Absorption. This refers to the passage of digested end

products into the blood or lymph.

George was morbidly obese and had frequent heartburn,

for which he was prescribed omeprazole. Before under-

going a vertical sleeve gastrectomy, he had an endos-

copy that revealed Barrett’s esophagus. A year after his

successful bariatric surgery, he suffered abdominal pain

with alternating periods of diarrhea and constipation,

although a colonoscopy uncovered no structural reason

for his symptoms. The pain disappeared, but months later

a different abdominal pain began, and his eyes appeared

yellowed. An ultrasound revealed that he had gallstones,

and laboratory tests showed elevated plasma levels of

amylase and lipase. George was hospitalized and treated

for pancreatitis, and afterward underwent a cholecystec-

tomy that resolved his pain.

Some of the new terms and concepts you will

encounter include:

• Barrett’s esophagus, GERD, bariatric surgery, and
  proton pump inhibitors


• Bilirubin, jaundice, pancreatic juice enzymes,
  intestinal microbiota

Clinical Investigation

LEARNING OUTCOMES

After studying this section, you should be able to:

1. List the functions of the digestive system.


2. Describe the microscopic structure of the
   gastrointestinal tract.

Unlike plants, which can form organic molecules using inor-

ganic compounds such as carbon dioxide, water, and ammo-

nia, humans and other animals must obtain their basic organic

molecules from food. Some of the ingested food molecules are

needed for their energy (caloric) value—obtained by the reac-

tions of cell respiration and used in the production of ATP—

and the balance is used to make additional tissue.

Most of the organic molecules that are ingested are similar

to the molecules that form human tissues. These are generally

large molecules ( polymers ), which are composed of subunits

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