(breaking down), absorbing, and assimilating the nutrients
in food. The absorbed substances circulate throughout the
internal environment of our bodies. We can then, by the pro-
cess of respiration, use the energy in these nutrients for such
vital functions as growth and repair of tissues. Finally, we
excrete wastes. Taken together, these physical and chemical
events that obtain, release, and use energy are a major part
o-liz-m), all of the chemical reactions
in cells.
Table 1.3
summarizes the characteristics of life.
What are the characteristics of life?
Which physical and chemical events constitute metabolism?
Glancing at the screen, Vanessa smiles. The
image reveals the fetus in her uterus, heart beat-
ing and already showing budlike structures that
will develop into arms and legs. She happily
heads home with a video of the fetus.
Vanessa’s ultrasound exam takes only a few
minutes, whereas Michael’s MR scan takes an
hour. First, Michael receives an injection of a dye
that provides contrast so that a radiologist exam-
he two patients enter the hospital medi-
cal scanning unit hoping for opposite
outcomes. Vanessa Q., who has suf-
fered several pregnancy losses, hopes that an
ultrasound exam will reveal that her current
pregnancy is progressing normally. Michael P.,
a sixteen-year-old who has excruciating head-
aches, is to undergo a magnetic resonance (MR)
scan to assure his physician (and himself!) that
the cause of the headache is not a brain tumor.
Ultrasound and magnetic resonance scans
are noninvasive procedures that provide images
of soft internal structures. Ultrasonography uses
high-frequency sound waves beyond the range
of human hearing. A technician gently presses
a device called a transducer, which emits sound
waves, against the skin and moves it slowly over
the surface of the area being examined, which in
this case is Vanessa’s abdomen (±
g. 1A).
Prior to the exam, Vanessa drank several
glasses of water. Her filled bladder will intensify
the contrast between her uterus (and its contents)
and nearby organs because as the sound waves
from the transducer travel into the body, some of
the waves re²
ect back to the transducer when they
reach a border between structures of slightly dif-
ferent densities. Other sound waves continue into
deeper tissues, and some of them are reflected
back by still other interfaces. As the re²
ected sound
waves reach the transducer, they are converted into
electrical impulses ampli±
ed and used to create a
sectional image of the body’s internal structure on
a viewing screen. This image is a sonogram (±
g. 1B).
ining the scan can distinguish certain brain struc-
tures. Then, a nurse wheels the narrow bed on
which Michael lies into a chamber surrounded by
a powerful magnet and a special radio antenna.
The chamber, which looks like a metal doughnut,
is the MR imaging instrument. As Michael settles
back, closes his eyes, and listens to the music
through earphones, a technician activates the
Ultrasonography And Magnetic Resonance Imaging: A Tale Of Two Patients
Ultrasonography uses re²
ected sound waves to visualize internal body structures.
A scene such as Judith R.’s accident and injury underscores
the delicate balance that must be maintained to sustain life.
In those seconds at the limits of life—the birth of a baby, a
trauma scene, or the precise instant of death following a long
illness—we often think about just what combination of qual-
ities constitutes this state that we call life. Indeed, although
this text addresses the human body, the most fundamental
characteristics of life are shared by all organisms.
As living organisms, we can respond to our surround-
ings. Our bodies grow, eventually becoming able to repro-
duce. We gain energy by ingesting (taking in), digesting
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