HUMAN BIOLOGY

ExplorEon Your own

SummarY

Section 11.1 The digestive system

breaks food down into molecules that

are small enough to be absorbed into the

bloodstream. It also stores and eliminates

unabsorbed materials and promotes homeo‑

stasis by its interactions with other organ

systems.

The gastrointestinal tract includes the mouth, pharynx,

esophagus, stomach, small intestine, and large intestine.

Its associated accessory organs include salivary glands, the

pancreas, the liver, and the gallbladder (Table 11.9).

The GI tract is lined with mucous membrane. From the

esophagus onward its wall consists of four layers: an innermost

mucosa, then the submucosa, then smooth muscle, then the

serosa. Sphincters at either end of the stomach and at other

locations within the GI tract control the forward movement of

ingested material.

Sections 11.2, 11.3 Starch digestion

begins in the mouth or oral cavity, where

the salivary glands secrete saliva, which

contains salivary amylase. Chewed food

mixes with saliva to form a bolus that is

swallowed. Waves of peristalsis move each

bolus down the esophagus to the stomach.

Protein digestion begins in the stomach, where gastric

fluid containing pepsins and other substances is secreted. The

stomach contents are reduced to a watery chyme that passes

through a sphincter into the small intestine.

Section 11.4 Digestion is completed

and most nutrients are absorbed in the small

intestine, which has a large surface area

for absorption due to its many villi and

microvilli.

Section 11.5 Enzymes and some other

substances secreted by the pancreas, the

liver, and the gallbladder aid digestion. Bile

(secreted by the liver and then stored and

released into the small intestine by the gallbladder) contains

bile salts that speed up the digestion of fats. Micelles aid the

This is an exercise you can eat when you’re done. All you need is a food item like a

slice of pizza or a sandwich and paper for jotting notes.

To begin, analyze your meal, noting the various kinds of biological molecules it includes.

(For this exercise, ignore nucleic acids.) Then, beginning with your mouth and teeth, write what

happens to your meal as it moves through your digestive system. Consider the following ques-

tions: What kinds of enzymes act on the different components of the meal (such as lettuce or

meat), and where do they act, as it is digested? What mechanical processes aid digestion?

Which ones can you consciously control? Using the tables in Section 11.13, list the vitamins

and minerals that your meal likely contains. Finally, analyze your meal in terms of its contribu-

tion (or lack of one) to a balanced diet.

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absorption of fatty acids and triglycerides. The hepatic portal

system diverts nutrient‑laden blood to the liver for processing.

Section 11.6 In the small intestine,

segmentation mixes material and forces it close

to the absorptive surface. Absorbed glucose

and amino acids move into blood vessels

in intestinal villi. Triglycerides enter lacteals,

then move into blood vessels.

Section 11.7 Peristalsis moves wastes

into the large intestine. Water is reabsorbed in

the colon; wastes (feces) move on to the rectum

and into the anal canal and are eliminated

via the anus. The appendix projects from the

upper part of the large intestine. It may have a

role in immunity.

Section 11.8 The nervous and endocrine

systems govern the digestive system. Many

controls operate in response to the volume

and composition of food in the gut. They cause

changes in muscle activity and in the secretion

rates of hormones or enzymes.

Sections 11.12, 11.13 Complex

carbohydrates are the body’s preferred energy

source. The diet also must provide eight

essential amino acids, some essential fatty

acids, vitamins, and minerals. Vitamins are

organic substances; minerals are inorganic.

Both have metabolic functions no other

nutrients can serve. Phytochemicals are useful

nutrients obtained from plants.

Section 11.14 Food energy is mea‑

sured in kilocalories. The basal metabolic rate

is the amount of kilocalories needed to sustain

the body when a person is awake and resting.

To maintain a healthy weight a person’s total

energy output must balance caloric intake. Obesity is a

health‑threatening condition that increases the risk of type 2

diabetes, heart trouble, and other diseases and disorders.

220 Chapter 11

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