Webster's New World Medical Dictionary

137
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requiring the use of radioisotopes or radiation-
counting apparatus.
elliptocytosis
A blood disorder characterized by
elliptically shaped red blood cells with variable
breakup of red cells (hemolysis) and varying
degrees of anemia. Inherited as a dominant trait,
elliptocytosis is due to the mutation of one of the
genes that encodes proteins of the red cell mem-
brane skeleton. There are several forms of ellipto-
cytosis caused by specific gene abnormalities.
EM
1
Electron microscope.
2
Electron
microscopy.
embolism
The obstruction of a blood vessel by a
foreign substance or a blood clot that travels
through the bloodstream, lodging in a blood vessel,
plugging the vessel. Foreign substances that can
cause embolisms include air bubbles, amniotic
fluid, globules of fat, clumps of bacteria, chemicals
(such as talc), and drugs (mainly illegal ones).
Blood clots are the most common causes of
embolisms. A pulmonary embolus is a blood clot
that has been carried through the blood into the
pulmonary artery (the main blood vessel from the
heart to the lung) or one of its branches, plugging
that vessel within the lung.
embolism, crossed
See 
embolism, paradoxical.
embolism, paradoxical
Passage of a clot
(thrombus) from a vein to an artery. When clots in
veins break off (embolize), they travel first to the
right side of the heart and, normally, then to the
lungs, where they lodge. The lungs act as a filter to
prevent the clots from entering arterial circulation.
However, when there is a hole in the wall between
the two upper chambers of the heart (atrial septal
defect), a clot can cross from the right to the left
side of the heart, and then pass into the arteries as
a paradoxical embolism. When a clot enters arterial
circulation, it can travel to the brain, block a vessel
there, and cause a stroke (cerebrovascular acci-
dent). Because of the risk of stroke from paradoxi-
cal embolism, it is usually recommended that even
small atrial septal defects be repaired. Also known
as crossed embolism
.
embolization
The clogging of small blood ves-
sels with a substance that blocks the flow of blood.
Embolization can occur as an abnormal natural
event, such as when a blood clot travels from the leg
to lodge in the blood vessels of the lungs, or it can
be used as a treatment method, such as when mate-
rial is purposely placed in blood vessels that supply
a tumor in the hopes of destroying that tumor. See
also 
embolism
.
embolus
A blockage or plug that obstructs a
blood vessel. Examples of emboli are detached
blood clots, clumps of bacteria, and clumps of other
foreign material, such as air.
embryo
An organism in the early stages of growth
and differentiation, from fertilization to the begin-
ning of the third month of pregnancy (in humans).
After that point in time, an embryo is called a fetus.