16
Chapter 1
SUMMARY
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
organic molecules.
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
tissue.
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
uid
(the fl uid between cells) and the blood plasma. Of the
extracellular fl uid, 75–80 percent is interstitial fl
uid, and
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
uid.
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
uid.
II. The function of organ systems is to maintain a stable internal
environment—homeostasis.
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
Control Systems
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
constancy.
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
messengers.
Processes Related to Homeostasis
I. Acclimatization is an improved ability to respond to an
environmental stress.
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.
KEY TERMS
acclimatization
13
acquired refl ex
9
adaptation
13
afferent pathway
10
autocrine agent
12
basement membrane
3
cell
2
cell differentiation
2
circadian rhythm
13
collagen fi ber
3
connective tissue
3
connective tissue cell
3
developmental
acclimatization
13
dynamic constancy
6
effector
10
efferent pathway
10
elastin fi ber
3
endocrine gland
11
entrainment
13
epithelial cell
3
epithelial tissue
3
equilibrium
7
external environment
3
extracellular fl
uid
5
extracellular matrix
3
feedforward
9
fi ber
3
free-running rhythm
14
functional unit
4
homeostasis
6
homeostatic control system
7
hormone
11
integrating center
10
internal environment
5
interstitial fl uid
5
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