744
Glossary
umbilical cord
(um-BIL-ih-kul) long, ropelike
structure that connects the fetus to the
placenta and contains umbilical arteries and
vein
umbilical vein
vein transporting blood from the
chorionic villi capillaries back to the fetus
unfused tetanus
stimulation of skeletal muscle at
a low-to-moderate action potential frequency
that results in oscillating, submaximal force
unsaturated fatty acid
fatty acid containing one
or more double bonds
upper airway
part of the respiratory tree
consisting of the nose, mouth, pharynx, and
larynx
upper esophageal sphincter
(ih-sof-ih-JEE-al
SFINK-ter) skeletal muscle ring surrounding
esophagus just below pharynx that, when
contracted, closes entrance to esophagus
upper motor neuron
neuron of the motor
cortex and descending pathways involved in
motor control; they are not technically “motor
neurons” because they synapse on neurons, not
muscle cells
up-regulation
increase in number of target-cell
receptors for given messenger in response to
chronic low extracellular concentration of that
messenger;
see also
supersensitivity;
compare
down-regulation
uracil (U)
(YOU-rah-sil) pyrimidine base; present
in RNA but not DNA
urea
(you-REE-ah) major nitrogenous waste
product of protein breakdown and amino acid
catabolism
ureter
(YOU-rih-ter) tube that connects kidney
to bladder
urethra
(you-REE-thrah) tube that connects
bladder to outside of body
uric acid
(YOU-rik) waste product derived from
nucleic acid catabolism
urinary bladder
see
bladder
uterus
(YOU-ter-us) hollow organ in pelvic
region of females; houses fetus during
pregnancy; also called
womb
utricle
structure in the semicircular canals that
responds to changes in linear movement of the
head by mechanical forces on otoliths located
on its surface
V
vagina
(vah-JY-nah) canal leading from uterus to
outside of body; also called
birth canal
vagus nerve
(VAY-gus) cranial nerve X; major
parasympathetic nerve
van der Waals forces
(walls) weak attractive
forces between nonpolar regions of molecules
varicosity
(vair-ih-KOS-ih-tee) swollen region of
axon; contains neurotransmitter-fi lled vesicles;
analogous to presynaptic ending
vasa recta
(VAY-zuh) blood vessels that form
loops parallel to the loops of Henle in the renal
medulla
vascular system
closed system of blood vessels
that includes all arteries, arterioles, capillaries,
venules, and veins
vas deferens
(vas DEF-er-enz) one of paired male
reproductive ducts that connect epididymis of
testis to urethra; also called
ductus deferens
vasoconstriction
(vays-oh-kon-STRIK-shun)
decrease in blood vessel diameter due to
vascular smooth muscle contraction
vasodilation
(vays-oh-dy-LAY-shun) increase in
blood vessel diameter due to vascular smooth
muscle relaxation
vasopressin
(vays-oh-PRES-sin) peptide hormone
synthesized in hypothalamus and released
from posterior pituitary; increases water
permeability of kidneys’ collecting ducts and
causes vasoconstriction; also called
antidiuretic
hormone (ADH)
vasovagal syncope
(vays-oh-VAY-gal SIN-koh-
pay) transient fainting episode occurring
when strong emotions decrease sympathetic
activity and increase parasympathetic activity
to the cardiovascular system; unconsciousness
results from reduced blood fl ow to the brain
secondary to a fall in arterial blood pressure
vault
recently discovered cytoplasmic structures
composed of protein and RNA; their function
is uncertain but may involve cytoplasmic-
nuclear transport and modulation of a cell’s
sensitivity to certain drugs
vein
any vessel that returns blood to heart;
see also
portal vein
vena cava
(VEE-nah KAY-vah) (pl. venae cavae)
one of two large veins that returns systemic
blood to heart;
see also
inferior vena cava,
superior vena cava
venous return (VR)
blood volume fl owing
to
heart per unit time
ventilation
air exchange between atmosphere and
alveoli; alveolar air fl ow
ventral
(VEN-tral) toward or at the front of body
ventral respiratory group
region of the
brainstem containing expiratory neurons
important during exercise
ventral root
one of two groups of efferent fi bers
that leave ventral side of spinal cord
ventricle
(VEN-trih-kul) cavity, as in cerebral
ventricle or heart ventricle; lower chamber of
heart
ventricular ejection
phase of the cardiac pump
cycle during ventricle contraction when blood
exits through the semilunar valves
ventricular fi
lling
phase of the cardiac pump
cycle during which the ventricles are resting
and blood enters through the atrioventricular
valves
ventricular function curve
relation of the
increase in stroke volume as end-diastolic
volume increases, all other factors being equal
venule
(VEEN-ule) small vessel that carries blood
from capillary network to vein
very-low-density lipoprotein (VLDL)
(lip-oh-
PROH-teen) lipid-protein aggregate having
high proportion of fat
vesicle
(VES-ih-kul) small, membrane-bound
organelle within cells
vestibular apparatus
see
vestibular system
vestibular receptor
hair cell in semicircular canal,
utricle, or saccule
vestibular system
sense organ in temporal bone
of skull; consists of three semicircular canals, a
utricle, and a saccule; also called
sense organ of
balance, vestibular apparatus
vestibulocochlear nerve
(ves-tibb-yoo-loh-
KOKE-lee-ar) eighth cranial nerve; transmits
sensory information about sound and motion
from the inner ear to the brain
villus
(VIL-us) fi ngerlike projection from highly
folded surface of small intestine; covered with
single-layered epithelium
virus
nucleic acid core surrounded by protein
coat; lacks enzyme machinery for energy
production and ribosomes for protein
synthesis; thus cannot survive or reproduce
except inside other cells, using their
biochemical apparatus
viscera
(VISS-er-ah) organs in thoracic and
abdominal cavities
visceral pleura
(VISS-er-al PLOO-rah) serous
membranes covering the surface of the lung
viscosity
(viss-KOS-ih-tee) measure of friction
between adjacent layers of a fl owing liquid;
property of fl uid that makes it resist fl ow
visible spectrum
wavelengths of electromagnetic
radiation capable of stimulating photoreceptors
of the eye
visual fi
eld
part of world being viewed at a given
time
vital capacity (VC)
maximal amount of air that
can be expired, regardless of time required,
following maximal inspiration
vitalism
(VY-tal-ism) view that explanation of life
processes requires a “life force” rather than
physicochemical processes alone
vitamin
organic molecule required in trace
amounts for normal health and growth;
not manufactured in the body and must be
supplied by diet; classifi ed as water-soluble
(vitamins C and the B complex) and fat-soluble
(vitamins A, D, E, and K)
vitamin B
12
an essential vitamin found in animal
products that plays an important role in the
production of red blood cells
vitamin D
secosteroid absorbed in the diet or
released from the skin under UV light. There
are two forms: D
2
is from plants whereas D
3
is
from animals
vitamin D
2
see
ergocalciferol
vitamin D
3
see
cholecalciferol
vitamin K
a lipid-soluble substance absorbed
from the diet and manufactured by bacteria of
the large intestine; required for production of
numerous factors involved in blood clotting
vitreous humor
jellylike fl uid fi lling the posterior
chamber of the eye
vocal cord
one of two elastic-tissue bands
stretched across laryngeal opening and
caused to vibrate when air moves past them,
producing sounds
volt (V)
unit of measurement of electrical
potential between two points
voltage
measure of potential of separated electrical
charges to do work; measure of electrical force
between two points
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