The Human Body: An Orientation
The Language of Anatomy
Describe the anatomical position.
Use correct anatomical terms to describe body directions,
regions, and body planes or sections.
Most of us are naturally curious about our bodies, but our inter-
est sometimes dwindles when we are confronted with the termi-
nology of anatomy and physiology. Let’s face it—you can’t just
pick up an anatomy and physiology book and read it as though
it were a novel.
Unfortunately, confusion is likely without precise, special-
ized terminology. To prevent misunderstanding, anatomists use
universally accepted terms to identify body structures precisely
and with a minimum of words. We present and explain the lan-
guage of anatomy next.
Anatomical Position and Directional Terms
To describe body parts and position accurately, we need an initial
reference point, and we must indicate direction. ±e anatomical
reference point is a standard body position called the
. In the anatomical position, the body is erect with
feet slightly apart. ±is position is easy to remember because
it resembles “standing at attention,” except that the palms face
Chapter 28 describes the positive feedback mechanism in
which oxytocin, a hypothalamic hormone, intensiﬁes labor
contractions during the birth of a baby (see Figure 28.17).
Oxytocin causes the contractions to become both more fre-
quent and more powerful. ±e increased contractions cause
more oxytocin to be released, which causes more contrac-
tions, and so on until the baby is ﬁnally born. ±e birth ends
the stimulus for oxytocin release and shuts oﬀ the positive
Blood clotting is a normal response to a break in the wall of
a blood vessel and is an excellent example of an important body
function controlled by positive feedback. Basically, once a ves-
sel has been damaged, blood elements called platelets immedi-
ately begin to cling to the injured site and release chemicals that
attract more platelets. ±is rapidly growing pileup of platelets
temporarily “plugs” the tear and initiates the sequence of events
that ﬁnally forms a clot
Positive feedback mecha-
nisms are likely to race out of control, so they are rarely used
to promote the moment-to-moment well-being of the body.
However, some positive feedback mechanisms, including this
one, may have only local reach. For example, blood clotting is
accelerated in injured vessels, but does not normally spread to
the entire circulation.
Homeostasis is so important that most disease can be regarded
as a result of its disturbance, a condition called
. As we age, our body’s control systems become less
eﬃcient, and our internal environment becomes less and less
stable. ±ese events increase our risk for illness and produce the
changes we associate with aging.
Another important source of homeostatic imbalance oc-
curs when the usual negative feedback mechanisms are over-
whelmed and destructive positive feedback mechanisms take
over. Some instances of heart failure reﬂect this phenomenon.
Examples of homeostatic imbalance appear throughout this
book to enhance your understanding of normal physiological
mechanisms. ±is symbol
introduces the homeo-
static imbalance sections and alerts you to the fact that we are
describing an abnormal condition. Each Homeostatic Imbal-
ance section is numbered to correspond with critical thinking
questions available in the Study Area of MasteringA&P—visit
www.masteringaandp.com to ﬁnd Homeostatic Imbalance
questions and other helpful study tools.
Check Your Understanding
What process allows us to adjust to either extreme heat or
When we begin to get dehydrated, we usually get thirsty,
which causes us to drink ﬂuids. Is thirst part of a negative
or a positive feedback control system? Defend your
Why is the control mechanism shown in Figure 1.6 called a
positive feedback system? What event ends it?
For answers, see Appendix H.
Feedback cycle ends
when plug is formed.
cycle is initiated.
Break or tear
occurs in blood
adhere to site
Platelet plug is
A positive feedback mechanism regulates forma-
tion of a platelet plug.