Homeostasis: A Framework for Human Physiology
are specialized to generate the mechanical
forces that produce movement. They may be attached through
other structures to bones and produce movements of the limbs
or trunk. They may be attached to skin, such as the muscles
producing facial expressions. They may also surround hollow
cavities so that their contraction expels the contents of the cav-
ity, as in the pumping of the heart. Muscle cells also surround
many of the tubes in the body—blood vessels, for example—
and their contraction changes the diameter of these tubes.
are specialized to initiate and conduct elec-
trical signals, often over long distances. A signal may initiate
new electrical signals in other nerve cells, or it may stimulate
a gland cell to secrete substances or a muscle cell to contract.
Thus, nerve cells provide a major means of controlling the
activities of other cells. The incredible complexity of connec-
tions between nerve cells underlies such phenomena as con-
sciousness and perception.
are specialized for the selective secre-
tion and absorption of ions and organic molecules, and for
protection. They are located mainly at the surfaces that cover
the body or individual organs, and they line the walls of vari-
ous tubular and hollow structures within the body. Epithelial
cells, which rest on an extracellular protein layer called the
form the boundaries between com-
partments and function as selective barriers regulating the
exchange of molecules. For example, the epithelial cells at
the surface of the skin form a barrier that prevents most sub-
stances in the
—the environment sur-
rounding the body—from entering the body through the
skin. Epithelial cells are also found in glands that form from
the invagination of epithelial surfaces.
Connective tissue cells,
as their name implies, con-
nect, anchor, and support the structures of the body. Some
connective tissue cells are found in the loose meshwork of
cells and ﬁ
bers underlying most epithelial layers. Other types
include adipose (fat-storing) cells, bone cells, red blood cells,
and white blood cells.
Most specialized cells are associated with other cells of a simi-
lar kind to form tissues. Corresponding to the four general
categories of differentiated cells, there are four general classes
of tissues: (1)
is used in
different ways. It is formally deﬁ ned as an aggregate of a single
type of specialized cell. However, it is also commonly used to
denote the general cellular fabric of any organ or structure—
for example, kidney tissue or lung tissue, each of which in fact
usually contains all four classes of tissue.
The immediate environment that surrounds each indi-
vidual cell in the body is the extracellular ﬂ uid. Actually, this
ﬂ uid is interspersed within a complex
consisting of a mixture of protein molecules and, in some
cases, minerals, speciﬁ c for any given tissue. The matrix serves
two general functions: (1) It provides a scaffold for cellular
attachments, and (2) it transmits information, in the form of
chemical messengers, to the cells to help regulate their activ-
ity, migration, growth, and differentiation.
The proteins of the extracellular matrix consist of
—and a mixture of nonﬁ brous proteins that contain
chains of complex sugars (carbohydrates). In some ways, the
Levels of cellular organization. The nephron is not drawn to scale.