2
Chapter 1
The Scope of Human Physiology
Stated most simply and broadly,
physiology
is the study of
how living organisms work. As applied to human beings,
its scope is extremely broad. At one end of the spectrum, it
includes the study of individual molecules—for example, how
a particular protein’s shape and electrical properties allow it
to function as a channel for ions to move into or out of a
cell. At the other end, it is concerned with complex processes
that depend on the integrated functions of many organs in the
body—for example, how the heart, kidneys, and several glands
all work together to cause the excretion of more sodium in the
urine when a person has eaten salty food.
Physiologists are interested in function and integra-
tion—how parts of the body work together at various levels
of organization and, most importantly, in the entire organ-
ism. Thus, even when physiologists study parts of organisms,
all the way down to individual molecules, the intention is
ultimately to apply the information they gain to the func-
tion of the whole body. As the nineteenth-century physiolo-
gist Claude Bernard put it: “After carrying out an analysis of
phenomena, we must . . . always reconstruct our physiologi-
cal synthesis, so as to see the
joint action
of all the parts we
have isolated. . . .”
In this regard, a very important point must be made
about the present and future status of physiology. It is easy
for a student to gain the impression from a textbook that
almost everything is known about the subject, but nothing
could be farther from the truth for physiology. Many areas
of function are still only poorly understood, such as how
the workings of the brain produce conscious thought and
memory.
Indeed, we can predict with certainty a continuing explo-
sion of new physiological information and understanding. One
of the major reasons is related to the recent landmark sequenc-
ing of the human genome. As the functions of all the proteins
encoded by the genome are uncovered, their application to the
functioning of the cells and organ systems discussed in this
text will provide an ever-sharper view of how our bodies work.
The integration of molecular biology with physiology has, in
fact, led to the need for a new term to describe this grow-
ing area of research—
physiological genomics.
Nowadays,
physiologists use the tools of molecular biology to ask not just
what
changes occur in the body in response to some external
or internal stimulus, but
how
the changes are produced at the
level of the gene.
Finally, in many areas of this text, we will relate physiol-
ogy to medicine. Some disease states can be viewed as physi-
ology “gone wrong,” or
pathophysiology,
which makes
an understanding of physiology essential for the study and
practice of medicine. Indeed, many physiologists are actively
engaged in research on the physiological bases of a wide
range of diseases. In this text, we will give many examples of
pathophysiology to illustrate the basic physiology that under-
lies the disease. A handy index of all the diseases and medical
conditions discussed in this text appears in Appendix C. We
begin our study of physiology by describing the organization
of the structures of the human body.
How Is the Body Organized?
Cells: The Basic Units of Living Organisms
Before exploring how the human body works, it is necessary to
understand the components of the body and their anatomical
relationships to each other.
The simplest structural units into which a complex mul-
ticellular organism can be divided and still retain the func-
tions characteristic of life are called
cells.
One of the unifying
generalizations of biology is that certain fundamental activi-
ties are common to almost all cells and represent the mini-
mal requirements for maintaining cell integrity and life. Thus,
for example, a human liver cell and an amoeba are remark-
ably similar in terms of how they exchange materials with
their immediate environments, obtain energy from organic
nutrients, synthesize complex molecules, duplicate them-
selves, and detect and respond to signals in their immediate
environments.
Each human organism begins as a single cell, a fertilized
egg, which divides to create two cells, each of which divides
in turn to result in four cells, and so on. If cell multiplica-
tion were the only event occurring, the end result would be
a spherical mass of identical cells. During development, how-
ever, each cell becomes specialized for the performance of a
particular function, such as producing force and movement
or generating electric signals. The process of transforming an
unspecialized cell into a specialized cell is known as
cell dif-
ferentiation,
the study of which is one of the most exciting
areas in biology today. Essentially all cells in a person have the
same genes. How then is one unspecialized cell instructed to
differentiate into a nerve cell, another into a muscle cell, and
so on? What are the external chemical signals that constitute
these “instructions,” and how do they affect various cells dif-
ferently? For the most part, the answers to these questions are
only beginning to be understood.
In addition to differentiating, cells migrate to new
locations during development and form selective adhesions
with other cells to produce multicellular structures. In this
manner, the cells of the body arrange themselves in various
combinations to form a hierarchy of organized structures.
Differentiated cells with similar properties aggregate to form
tissues,
such as nerve tissue or muscle tissue, which combine
with other types of tissues to form
organs,
such as the heart,
lungs, and kidneys. Organs, in turn, work together to form
organ systems,
such as the urinary system (
Figure 1–1
).
About 200 distinct kinds of cells can be identifi ed in the
body in terms of differences in structure and function. When
cells are classifi ed according to the broad types of function
they perform, however, four major categories emerge: (1) mus-
cle cells, (2) nerve cells, (3) epithelial cells, and (4) connec-
tive tissue cells. In each of these functional categories, several
cell types perform variations of the specialized function. For
example, there are three types of muscle cells—skeletal, car-
diac, and smooth. These cells differ from each other in shape,
in the mechanisms controlling their contractile activity, and
in their location in the various organs of the body, but each of
them is a muscle cell.
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