Webster's New World Medical Dictionary

headache, thunderclap
A sudden and excruci-
atingly painful headache. Some physicians feel that
in the absence of a known headache disorder, such
as migraines, a thunderclap headache may some-
times signal a ruptured aneurysm in the brain. A
person who experiences this type of headache
should immediately seek medical attention.
headache, vascular
One of a group of
headaches felt to involve abnormal sensitivity of the
blood vessels (arteries) in the brain to various trig-
gers, resulting in rapid changes in the artery size
due to spasm (constriction). Other arteries in the
brain and scalp then open (dilate), and throbbing
pain is perceived in the head. Migraine headache is
the most common type of vascular headache. See
also 
migraine headache.
health
As officially defined by the World Health
Organization, a state of complete physical, mental,
and social well-being, not merely the absence of dis-
ease or infirmity.
health, child
See 
child health.
Health and Human Services, Department of
See 
HHS.
health care proxy
An advance medical directive
in the form of a legal document that designates
another person (a proxy) to make health care deci-
sions in case a person is rendered incapable of
making his or her wishes known. The health care
proxy has, in essence, the same rights to request or
refuse treatment that the person would have if he or
she were capable of making and communicating
decisions.
health outcomes research
Research that meas-
ures the value of a particular course of therapy.
hearing aids
Small electronic devices worn
inside or behind the ear that improve the hearing
and speech comprehension of people with hearing
loss due to damage to the small sensory cells in the
inner ear. Hearing aids contain a microphone,
amplifier, and speaker.
heart
The muscle that pumps blood received from
veins into arteries throughout the body. The heart is
positioned in the chest behind the sternum (breast-
bone); in front of the trachea, esophagus, and aorta;
and above the diaphragm. A normal heart is about
the size of a closed fist and weighs about 298 grams
or 10.5 ounces. It is cone-shaped, with the point of
the cone pointing down to the left. Two-thirds of the
heart lies in the left side of the chest, with the balance
in the right side of the chest. The heart is composed
of specialized cardiac muscle, and it is four-cham-
bered, with a right atrium and ventricle, and an
anatomically separate left atrium and ventricle. The
blood flows from the systemic veins into the right
atrium, thence to the right ventricle, from which it is
pumped to the lungs and then returned into the left
atrium, thence to the left ventricle, from which it is
driven into the systemic arteries. The heart is thus
functionally composed of two hearts: the right heart
and the left heart. The right heart consists of the right
atrium, which receives deoxygenated blood from the
body, and the right ventricle, which pumps the deoxy-
genated blood to the lungs under low pressure; and
the left heart, which consists of the left atrium, which
receives oxygenated blood from the lung, and the left
ventricle, which pumps the oxygenated blood out to
the body under high pressure.