Body Structure Is Closely Linked to Function
at All Levels of Organization 2
The cardiovascular system
• Figure 1.2
• Anatomy is the science
of structure and the
is the science of how
body structures func-
• The human body consists
of six levels of organiza-
tion: chemical, cellular,
tissue, organ, system,
and organism. Cells are
the basic structural and
functional unit of any
organism. Cells that per-
form similar functions are
grouped together to form
tissues. Different tissues
join together to form or-
gans. Related organs with
common functions form
systems. Systems com-
bine to form an organism.
• There are the 11 systems
of the human body:
muscular, nervous, endocrine, cardiovascular (as shown),
lymphatic, respiratory, digestive, urinary, and reproductive.
All Living Organisms Carry Out Common
Life Processes 6
• All living organisms have certain characteristics that set
them apart from non-living things. Among the life processes
in humans are metabolism, responsiveness, movement,
growth, differentiation, and reproduction.
• Living organisms require a stable internal environment.
Homeostasis is the maintenance of that stable environ-
ment within certain limits. Homeostasis is regulated by the
nervous and endocrine systems acting together or sepa-
rately. Homeostasis is maintained by feedback systems (as
shown), which consist of receptors, a control center, and
effectors. Receptors monitor changes in a controlled con-
dition and send input to a control center that sets the val-
ue at which a controlled condition should be maintained,
evaluates the input it receives, and generates output com-
mands when they are needed. Effectors receive output
from the control center and
produce a response (effect)
that alters the controlled
• Negative feedback systems
reverse the change in the
controlled condition and are
stable, while positive feed-
back systems strengthen
the change and are unstable.
Aging and disease alter feed-
back systems and their ability
to maintain homeostasis.
Feedback systems •
Anatomical Road Maps Guide Navigation
Through the Body 10
• Descriptions of any region of the
body assume the body is in the
, Figure 1 6
anatomical position. The human
body is divided into several major
regions: the head, neck, trunk, up-
per limbs, and lower limbs. Within
body regions, specific body parts
have common names and corre-
sponding anatomical names.
• Directional terms indicate the re-
lationship of one part of the body
to another and include medial,
lateral, superior, inferior, proxi-
mal, distal, superficial, and deep.
• Planes (as shown) are imaginary flat
surfaces, as shown, that divide the
body or organs into parts or sec-
tions, which are named according to
the plane on which the cut is made:
sagittal, frontal, or transverse.
• Body cavities are spaces in the
body that contain, protect, sepa-
rate, and support internal organs. They include the cranial,
cervical, vertebral, thoracic, and abdominopelvic cavities.
• To describe the location of organs easily, the abdominopelvic
cavity may be divided into nine regions or four quadrants.
16 CHAPTER 1
Organization of the Human Body