Chapter 1
The Human Body: An Orientation
5
1
of the entire body. Tis theme is repeated throughout the book.
Figure 1.2
identifies some of the organ systems making major
contributions to necessary life functions. Also, as you read this
section, check
Figure 1.3
for more detailed descriptions of the
body’s organ systems.
Maintaining Boundaries
Every living organism must
maintain its boundaries
so that
its internal environment (its inside) remains distinct from the
external environment surrounding it (its outside). In single-
celled organisms, the external boundary is a limiting membrane
that encloses its contents and lets in needed substances while
restricting entry of potentially damaging or unnecessary sub-
stances. Similarly, all the cells of our body are surrounded by a
selectively permeable membrane.
Additionally, the body as a whole is enclosed and pro-
tected by the integumentary system, or skin (Figure 1.3a).
Tis system protects our internal organs from drying out
(a fatal change), bacteria, and the damaging effects of heat,
sunlight, and an unbelievable number of chemicals in the
external environment.
Movement
Movement
includes the activities promoted by the muscular
system, such as propelling ourselves from one place to another
by running or swimming, and manipulating the external en-
vironment with our nimble fingers (Figure 1.3c). Te skeletal
system provides the bony framework that the muscles pull
on as they work (Figure 1.3b). Movement also occurs when
substances such as blood, foodstuffs, and urine are propelled
through internal organs of the cardiovascular, digestive, and
urinary systems, respectively. On the cellular level, the mus-
cle cell’s ability to move by shortening is more precisely called
contractility
.
Responsiveness
Responsiveness
, or
excitability
, is the ability to sense changes
(which serve as stimuli) in the environment and then respond
to them. For example, if you cut your hand on broken glass,
a withdrawal reflex occurs—you involuntarily pull your hand
away from the painful stimulus (the broken glass). You don’t
have to think about it—it just happens! Likewise, when carbon
dioxide in your blood rises to dangerously high levels, chemical
sensors respond by sending messages to brain centers control-
ling respiration, and you breathe more rapidly.
Because nerve cells are highly excitable and communicate
rapidly with each other via electrical impulses, the nervous sys-
tem is most involved with responsiveness (Figure 1.3d). How-
ever, all body cells are excitable to some extent.
Digestion
Digestion
is the breaking down of ingested foodstuffs to simple
molecules that can be absorbed into the blood. Te nutrient-rich
blood is then distributed to all body cells by the cardiovascular
system. In a simple, one-celled organism such as an amoeba,
the cell itself is the “digestion factory,” but in the multicellular
human body, the digestive system performs this function for the
entire body (Figure 1.3i).
Metabolism
Metabolism
(mĕ-tab
9
o-lizm; “a state of change”) is a broad
term that includes all chemical reactions that occur within
body cells. It includes breaking down substances into their
simpler building blocks (more specifically, the process of
ca-
tabolism
), synthesizing more complex cellular structures from
simpler substances (
anabolism
), and using nutrients and oxy-
gen to produce (via
cellular respiration
) A±P, the energy-rich
molecules that power cellular activities. Metabolism depends
on the digestive and respiratory systems to make nutrients and
oxygen available to the blood and on the cardiovascular sys-
tem to distribute them throughout the body (Figure 1.3i, h,
and f, respectively). Metabolism is regulated largely by hor-
mones secreted by endocrine system glands (Figure 1.3e).
Interstitial fluid
Heart
Nutrients
Food
Feces
Urine
O
2
O
2
CO
2
CO
2
Digestive system
Takes in nutrients, breaks them
down, and eliminates unabsorbed
matter (feces)
Integumentary system
Protects the body as a whole
from the external environment
Urinary system
Eliminates
nitrogenous
wastes and
excess ions
Nutrients and wastes pass
between blood and cells
via the interstitial fluid
Cardiovascular system
Via the blood, distributes oxygen
and nutrients to all body cells and
delivers wastes and carbon
dioxide to disposal organs
Respiratory system
Takes in oxygen and
eliminates carbon dioxide
Blood
Figure 1.2
Examples of interrelationships among body organ
systems.
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