Chapter 1
The Human Body: An Orientation
3
1
anatomy provides us with a static image of the body’s archi-
tecture, physiology reveals the body’s dynamic and animated
workings.
Physiology oFen focuses on events at the cellular or mo-
lecular level. Tis is because the body’s abilities depend on
those of its individual cells, and cells’ abilities ultimately
depend on the chemical reactions that go on within them.
Physiology also rests on principles of physics, which help
to explain electrical currents, blood pressure, and the way
muscles use bones to cause body movements, among other
things. We present basic chemical and physical principles
in Chapter 2 and throughout the book as needed to explain
physiological topics.
Complementarity of Structure and Function
Although it is possible to study anatomy and physiology indi-
vidually, they are really inseparable because function always re-
flects structure. Tat is, what a structure can do depends on its
specific form. Tis key concept is called the
principle of com-
plementarity of structure and function
.
±or example, bones can support and protect body organs
because they contain hard mineral deposits. Blood flows in
one direction through the heart because the heart has valves
that prevent backflow. Troughout this book, we accompany a
description of a structure’s anatomy with an explanation of its
function, and we emphasize structural characteristics contrib-
uting to that function.
Check Your Understanding
1.
In what way does physiology depend on anatomy?
2.
Would you be studying anatomy or physiology if you
investigated how muscles shorten? If you explored the
location of the lungs in the body?
For answers, see Appendix H.
Levels of Structural
Organization
Name the different levels of structural organization that
make up the human body, and explain their relationships.
List the 11 organ systems of the body, identify their
components, and briefly explain the major function(s) of
each system.
Te human body has many levels of structural organization
(Figure 1.1)
. Te simplest level of the structural hierarchy
is the
chemical level
, which we study in Chapter 2. At this
level,
atoms
, tiny building blocks of matter, combine to form
molecules
such as water and proteins. Molecules, in turn, as-
sociate in specific ways to form
organelles
, basic components
of the microscopic cells.
Cells
are the smallest units of liv-
ing things. We examine the
cellular level
in Chapter 3. All
cells have some common functions, but individual cells vary
widely in size and shape, reflecting their unique functions in
the body.
Te simplest living creatures are single cells, but in complex
organisms such as human beings, the hierarchy continues on
to the
tissue
level
.
Tissues
are groups of similar cells that have
a common function. Te four basic tissue types in the human
body are epithelium, muscle, connective tissue, and nervous
tissue.
Each tissue type has a characteristic role in the body, which
we explore in Chapter 4. Briefly, epithelium covers the body sur-
face and lines its cavities. Muscle provides movement. Connec-
tive tissue supports and protects body organs. Nervous tissue
provides a means of rapid internal communication by transmit-
ting electrical impulses.
An
organ
is a discrete structure composed of at least two
tissue types (four is more common) that performs a specific
function for the body. Te liver, the brain, and a blood vessel
are very different from the stomach, but each is an organ. You
can think of each organ of the body as a specialized functional
center responsible for a necessary activity that no other organ
can perform.
At the
organ level
, extremely complex functions become
possible. Let’s take the stomach for an example. Its lining is an
epithelium that produces digestive juices. Te bulk of its wall is
muscle, which churns and mixes stomach contents (food). Its
connective tissue reinforces the soF muscular walls. Its nerve
fibers increase digestive activity by stimulating the muscle to
contract more vigorously and the glands to secrete more diges-
tive juices.
Te next level of organization is the
organ system level
. Or-
gans that work together to accomplish a common purpose make
up an
organ system
. ±or example, the heart and blood vessels of
the cardiovascular system circulate blood continuously to carry
oxygen and nutrients to all body cells. Besides the cardiovascular
system, the other organ systems of the body are the integumen-
tary, skeletal, muscular, nervous, endocrine, lymphatic, respira-
tory, digestive, urinary, and reproductive systems. (Note that the
immune system is closely associated with the lymphatic system.)
Look ahead to ±igure 1.3 on pp. 6 and 7 for an overview of the
11 organ systems, which we discuss in the next section and study
in more detail in Units 2–5.
Te highest level of organization is the
organism
, the living
human being. Te
organismal level
represents the sum total of
all structural levels working together to keep us alive.
Check Your Understanding
3.
What level of structural organization is typical of a
cytologist’s field of study?
4.
What is the correct structural order for the following terms:
tissue, organism, organ, cell?
5.
Which organ system includes the bones and cartilages?
Which includes the nasal cavity, lungs, and trachea?
For answers, see Appendix H.
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