Webster's New World Medical Dictionary

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artificial insemination by partner
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artificial pacemaker
A device that uses electri-
cal impulses to regulate the heart rhythm or repro-
duce it. An internal pacemaker is one in which the
electrodes to the heart, the electronic circuitry, and
the power supply are all implanted internally, within
the body. Although there are different types of pace-
makers, all are designed to treat a heart rate that is
too slow (bradycardia). Pacemakers may function
continuously and stimulate the heart at a fixed rate,
or they may function at an increased rate during
exercise. A pacemaker can also be programmed to
detect an overly long pause between heartbeats and
then stimulate the heart.
artificial pancreas
A machine that constantly
measures glucose (sugar) in the blood and, in
response to an elevated level of glucose, releases an
appropriate amount of insulin. In this respect, an
artificial pancreas functions like a natural pancreas.
asbestos
A natural material made up of tiny
fibers that is used as thermal insulation. Inhalation
of asbestos fibers can lead to asbestosis and
mesothelioma.
asbestosis
Scarring of the lungs caused by
inhalation of asbestos fibers. When asbestos fibers
lodge in the lungs, they promote the development of
cancer, such as mesothelioma of the pleura (the lin-
ing of the lung) and bronchogenic carcinoma (can-
cer of the lung). See also 
mesothelioma.
ascaris
Intestinal roundworms. Infection with
ascaris is referred to as ascariasis.
ascending aorta
The first section of the aorta,
which starts from the left ventricle of the heart and
extends to the aortic arch. The right and left coro-
nary arteries that supply blood to the heart muscle
arise from the ascending aorta.
ascites
An abnormal accumulation of fluid within
the abdomen. There are many causes of ascites,
including cirrhosis of the liver, cancer within the
abdomen, congestive heart failure, and tuberculosis.
ascorbic acid
Vitamin C, an essential nutrient
found mainly in fruits and vegetables. The body
requires ascorbic acid in order to form and main-
tain bones, blood vessels, and skin. Ascorbic acid
also promotes the healing of cuts, abrasions and
wounds; helps fight infections; inhibits conversion
of irritants in smog, tobacco smoke, and certain
foods into cancer-causing substances; appears to
lessen the risk of developing high blood pressure
and heart disease; helps regulate cholesterol levels;
prevents the development of scurvy; appears to
lower the risk of developing cataracts; and aids in
iron absorption. Ascorbic acid can cause adverse
reactions when taken with some drugs.
ASCUS
An acronym for Atypical Squamous Cells
of Undetermined Significance. This term is used in
the Bethesda System for reporting Pap smear find-
ings, and indicates that some flat (squamous) cells
look unusual and may or may not be pre-malignant
or malignant.