VASOCONSTRICTION
VEGETARIANISM
VASECTOMY
man continues to achieve orgasm and
ejaculate as normal, but the
semen
no
longer contains
sperm
, which are reab-
sorbed in the testes. Male sterilization
is a safe and effective method of
contra-
ception.
However, rarely, the severed
ends of a vas deferens reunite, and
sperm appear in the ejaculate. If this
occurs, the man can safely undergo
another vasectomy. Some operations to
restore fertility after vasectomy are suc-
cessful, but the process should be
regarded as irreversible.
vasoconstriction Narrowing of blood
vessels, causing reduced blood flow to a
part of the body. Vasoconstriction under
the skin occurs in response to the cold
and reduces heat loss from the body.
vasodilation Widening of blood ves-
sels, causing increased blood flow to a
part of the body. Vasodilation under the
skin occurs in response to hot weather
and increases heat loss from the body.
vasodilator drugs A group of drugs
that widen blood vessels. Vasodilator
drugs include
ACE inhibitors
,
alpha-
blockers, calcium channel blockers, nitrate
drugs
, and sympatholytic drugs. They
are used to treat disorders in which
abnormal narrowing of blood vessels
reduces blood flow through tissues,
impairing the supply of oxygen. Such
disorders include
angina pectoris
and
peripheral vascular disease
. Vasodilators
are also used to treat
hypertension
and
heart failure
. All vasodilator drugs may
cause flushing, headaches, dizziness,
fainting, and swollen ankles.
vasopressin Another name for
ADH.
vasovagal attack Temporary loss of
consciousness due to sudden slowing
of the heartbeat, usually brought on by
severe pain, stress, shock, or fear. A
vasovagal attack, a common cause of
fainting
in healthy people, results from
overstimulation of the
vagus nerve
.
VD The abbreviation for venereal dis-
ease, an outdated term for
sexually
transmitted infections
.
vector An animal that transmits a par-
ticular
infectious disease
. A vector picks
up disease-causing organisms from a
source of infection (such as an infected
person's or animal's blood or faeces),
carries them in or on its body, and later
deposits them where they infect a new
host, directly or indirectly. Mosquitoes,
fleas, lice, ticks, and flies are the most
important vectors of disease to humans.
veganism The adoption of a diet that
excludes all meat and fish and all ani-
mal products, including milk and eggs.
A vegan diet is likely to result in
vitamin
B 12
deficiency. Supplements are essen-
tial during pregnancy.
vegetarianism Eating a diet that ex-
cludes meat and fish, and sometimes
all other animal products. Humans do
not need to eat meat or animal prod-
ucts to maintain health as long as the
nutrients supplied by plant foods pro-
vide a balanced diet (see
nutrition
).
However, people who exclude all animal
products (vegans) need to plan their
diet carefully or take supplements to
avoid
vitamin B 12
or
calcium
deficiency.
Vegetarian diets are relatively rich in
fibre
, which may help protect against
V
579
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