The Scope of Human Physiology
I. Physiology is the study of how living organisms work.
Physiologists are interested in the regulation of body function.
II. Disease states are physiology “gone wrong” (pathophysiology).
How Is the Body Organized?
I. Cells are the simplest structural units into which a complex
multicellular organism can be divided and still retain the
functions characteristic of life.
II. Cell differentiation results in the formation of four categories
of specialized cells:
a. Muscle cells generate the mechanical activities that produce
force and movement.
b. Nerve cells initiate and conduct electrical signals.
c. Epithelial cells selectively secrete and absorb ions and
d. Connective tissue cells connect, anchor, and support the
structures of the body.
III. Specialized cells associate with similar cells to form tissues:
muscle tissue, nerve tissue, epithelial tissue, and connective
IV. Organs are composed of the four kinds of tissues arranged
in various proportions and patterns. Many organs contain
multiple small, similar functional units.
V. An organ system is a collection of organs that together perform
an overall function.
Body Fluid Compartments
I. The body ﬂ uids are enclosed in compartments.
a. The extracellular ﬂ uid is composed of the interstitial ﬂ
(the ﬂ uid between cells) and the blood plasma. Of the
extracellular ﬂ uid, 75–80 percent is interstitial ﬂ
20–25 percent is plasma.
b. Interstitial ﬂ uid and plasma have essentially the same
composition except that plasma contains a much higher
concentration of protein.
c. Extracellular ﬂ uid differs markedly in composition from the
ﬂ uid inside cells—the intracellular ﬂ
d. Approximately one-third of body water is in the
extracellular compartment, and two-thirds is intracellular.
II. The differing compositions of the compartments reﬂ ect the
activities of the barriers separating them.
Homeostasis: A Deﬁ
ning Feature of Physiology
I. The body’s internal environment is the extracellular ﬂ
II. The function of organ systems is to maintain a stable internal
III. Numerous variables within the body must be maintained
homeostatically. When homeostasis is lost for one variable, it
may trigger a series of changes in other variables.
General Characteristics of Homeostatic
I. Homeostasis denotes the stable condition of the internal
environment that results from the operation of compensatory
homeostatic control systems.
a. In a negative feedback control system, a change in the
variable being regulated brings about responses that tend to
push the variable in the direction opposite to the original
change. Negative feedback minimizes changes from the set
point of the system, leading to stability.
b. Homeostatic control systems minimize changes in the
internal environment but cannot maintain complete
c. Feedforward regulation anticipates changes in a regulated
variable, improves the speed of the body’s homeostatic
responses, and minimizes ﬂ uctuations in the level of the
variable being regulated.
Components of Homeostatic Control Systems
I. The components of a reﬂ ex arc are receptor, afferent pathway,
integrating center, efferent pathway, and effector. The pathways
may be neural or hormonal.
II. Local homeostatic responses are also stimulus-response
sequences, but they occur only in the area of the stimulus, with
neither nerves nor hormones involved.
Intercellular Chemical Messengers
I. Intercellular communication is essential to reﬂ exes and local
responses and is achieved by neurotransmitters, hormones, and
paracrine or autocrine agents. Less common is intercellular
communication through either gap junctions or cell-bound
Processes Related to Homeostasis
I. Acclimatization is an improved ability to respond to an
a. The improvement is induced by prolonged exposure to the
stress with no change in genetic endowment.
b. If acclimatization occurs early in life, it may be irreversible
and is known as a developmental acclimatization.
II. Biological rhythms provide a feedforward component to
homeostatic control systems.
a. The rhythms are internally driven by brain pacemakers, but
are entrained by environmental cues, such as light, which
also serve to phase-shift (reset) the rhythms when necessary.
b. In the absence of cues, rhythms free run.
III. The balance of substances in the body is achieved by matching
inputs and outputs. Total-body balance of a substance may be
negative, positive, or stable.
acquired reﬂ ex
collagen ﬁ ber
connective tissue cell
elastin ﬁ ber
homeostatic control system
interstitial ﬂ uid