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April/May 2007
Providing answers to science questions
Send questions to Department Editor
Marc Rosner; MARosner@aol.com
Q
What color is the
blood in a person’s
veins? I’ve heard it
described as “blue,” but it looks
to me to be dark red, as it
appears when blood is drawn
during a lab test.
A
Lori Glazik
8th Grade Science Teacher
S.E. Gross Middle School
Brookfield, Illinois
Over two thousand years
ago, Hippocrates, of the
Ancient Greeks, took an
interest in the color of blood as he
philosophized that health is derived
from the proper balance of pigmented humors. The human circulatory system drives approximately
25 trillion red blood cells through
the body. Color is somewhat subjective—a product of the nature of incident light and absorbed and reflected
wavelengths. The apparent color of
blood is determined by hemoglobin
molecules, which make up about
one-third of the red cell’s content.
The hemoglobin molecule binds
four molecules of oxygen. During
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normal conditions—that is, normal
blood, under white light—fully oxygenated hemoglobin appears bright
red. As the degree of oxygen saturation decreases, the internal molecular
arrangement of the hemoglobin molecule changes in several directions,
thereby causing it to reflect light of
different wavelengths, and consequently changing its color.
The color of deoxygenated venous
blood is fairly accurately described as
“dark maroon” or “reddish purple.”
The fact that it is not, in fact, blue can
be simply and decisively determined
by looking at it as it comes from your
vein to the collecting vacuum tube, or
by observing the color of the blood in
the collection bags at a blood donation
center. Now, if you look at the veins
of the back of your hand, they appear
more nearly blue because the blue
rays of the incident light scatter most
readily in the numerous pigment layers below the skin surface; in general,
the deeper the vein, the more blue
it will look. If there are more than 5
g of reduced (deoxygenated) hemoglobin per 100 cc. of blood (about a
third of the usual total amount of
hemoglobin), the patient will have
cyanosis, a bluish discoloration of the
skin and mucous membranes that is
also caused by the interaction of light
with skin pigments and dark-red deoxygenated blood.
Diagrams of the circulation always show oxygenated blood as red
and deoxygenated blood as blue,
because it is much easier to grasp
that difference than the one between
bright and dark red. This type of
agreed-upon depiction decision is
called a convention. Likewise, we
might use “red” to describe Mars or
copper or a positive wire, even when
an observer would say these are not
truly red. The designation of deoxygenated blood as “blue” is more
a convention based upon its relative
hue than by our choice of a label we
might apply to describe its color in
isolation. It has been a useful teaching tool for centuries, and it might
be difficult to change this convention. Perhaps “bright red” and “dark
red” would be more accurate in
discussion of blood in its oxygenated
and deoxygenated states.
William S. Rachlin,
M.D., FACS, FACG
Boston, Massachusetts
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