Webster's New World Medical Dictionary

vasculitis, allergic
See 
Churg-Strauss syn-
drome.
vasectomy
A surgical procedure designed to
make a man sterile by cutting or blocking both the
right and left vas deferens, the tubes through which
sperm pass into the ejaculate.
vasoconstriction
Narrowing of the blood vessels
that results from contraction of the muscular walls
of the vessels. The opposite of vasoconstriction is
vasodilation.
vasodepressor syncope
See 
syncope, situa-
tional.
vasodilation
Widening of blood vessels that
results from relaxation of the muscular walls of the
vessels. What widens in vasodilation is actually 
the diameter of the interior (lumen) of the vessel.
The opposite of vasodilation is vasoconstriction.
vasodilator
An agent that acts as a blood vessel
dilator, opening blood vessels by relaxing their mus-
cular walls. For example, nitroglycerin is a vasodila-
tor, as are the angiotensin converting enzyme (ACE)
inhibitors.
vasomotor
Relating to the nerves and muscles
that cause blood vessels to constrict or dilate.
vasomotor rhinitis
Inflammation of the nose
(rhinitis) due to abnormal nerve control of the
blood vessels in the nose. Vasomotor rhinitis is not
allergic rhinitis. Decongestant medications are used
to temporarily reduce swelling of sinus and nasal
tissues leading to an improvement of breathing and
a decrease in obstruction.
vasopressin
See 
antidiuretic hormone.
vasovagal attack
See 
vasovagal reaction.
vasovagal reaction
A reflex of the involuntary
nervous system that causes the heart to slow down
and, at the same time, affects the nerves to the blood
vessels in the legs, permitting those vessels to dilate
(widen). As a result, the heart puts out less blood,
the blood pressure drops, and the blood that is cir-
culating tends to go into the legs rather than to the
head. The brain is deprived of oxygen, and a faint-
ing episode (syncope) occurs. See also 
syncope.