Glossary
743
thyroid gland
endocrine gland in neck; secretes
thyroid hormones and calcitonin
thyroid hormone (TH)
collective term for amine
hormones released from thyroid gland—that is,
thyroxine (T
4
) and triiodothyronine (T
3
)
thyroid peroxidase
enzyme within the thyroid
gland that mediates many of the steps of
thyroid hormone synthesis
thyroid-stimulating hormone (TSH)
glycoprotein hormone secreted by anterior
pituitary; induces secretion of thyroid
hormone; also called thyrotropin
thyrotropin-releasing hormone (TRH)
hypophysiotropic hormone that stimulates
thyrotropin and prolactin secretion by anterior
pituitary
thyroxine (T
4
)
(thigh-ROCKS-in)
tetraiodothyronine; iodine-containing amine
hormone secreted by thyroid gland
tidal volume (V
t
)
air volume entering or leaving
lungs with single breath during any state of
respiratory activity
tight junction
cell junction in which extracellular
surfaces of the plasma membrane of two
adjacent cells are joined together; extends
around epithelial cell and restricts molecule
diffusion through space between cells
time-averaged mean
value obtained for a given
physiological variable over an extended period
of time; for example, average serum hormone
levels can be estimated over 24 hours by
collecting their metabolites in the total daily
urine output
tip link
small, extracellular fi ber connecting
adjacent stereocilia that activates ion channels
when the cilia are bent
tissue
aggregate of single type of specialized cell;
also denotes general cellular fabric of a given
organ
tissue factor
protein involved in initiation of
clotting via the extrinsic pathway; located on
plasma membrane of subendothelial cells
tissue factor pathway inhibitor
a plasma protein
secreted by endothelial cells; one of several
mechanisms for protecting against excessive
blood coagulation
tissue plasminogen activator (t-PA)
plasma
protein produced by endothelial cells; after
binding to fi brinogen, activates the proenzyme
plasminogen
titin
protein that extends from the Z line to the
thick fi laments and M line of skeletal muscle
sarcomere
T lymphocyte
see
T cell
tone
maintained functional activity;
see also
muscle
tone
tonic
(TAH-nik) continuous activity;
compare
phasic
tonsil
one of several small lymphoid organs in
pharynx
total blood carbon dioxide
sum total of
dissolved carbon dioxide, bicarbonate, and
carbamino-CO
2
total energy expenditure
sum of external work
done plus heat produced plus energy stored
by body
total peripheral resistance (TPR)
total
resistance to fl ow in systemic blood vessels
from beginning of aorta to ends of venae cavae
totipotent
cells of the conceptus that have the
capacity to develop into a normal, mature fetus
trace element
mineral present in body in
extremely small quantities
trachea
(TRAY-key-ah) single airway connecting
larynx with bronchi; windpipe
tract
large, myelinated nerve fi ber bundle in CNS
transamination
(trans-am-in-NAY-shun) reaction
in which an amino acid amino group (—NH
2
)
is transferred to a ketoacid, the ketoacid thus
becoming an amino acid
transcellular pathway
crossing an epithelium
by movement into an epithelial cell, diffusion
through the cytosol of that cell, and exit across
the opposite membrane
transcription
formation of RNA containing, in
linear sequence of its nucleotides, the genetic
information of a specifi c gene; fi rst stage of
protein synthesis
transcription factor
one of a class of proteins
that act as gene switches, regulating the
transcription of a particular gene by activating
or repressing the initiation process
transducin
(trans-DOO-sin) G protein in
disc membranes of photoreceptor; initiates
inactivation of cGMP
transduction
process by which stimulus energy is
transformed into a response
transepithelial transport
see
epithelial transport
transferrin
(trans-FAIR-in) iron-binding protein
that carries iron in plasma
transfer RNA (tRNA)
type of RNA; different
tRNAs combine with different amino acids and
with codon on mRNA specifi c for that amino
acid, thus arranging amino acids in sequence to
form specifi c protein
translation
during protein synthesis, assembly of
amino acids in correct order according to genetic
instructions in mRNA; occurs on ribosomes
transmembrane protein
a protein that spans
the plasma membrane and contains both
hydrophilic and hydrophobic regions; often
acts as a receptor or an ion channel
transmural pressure
pressure difference exerted
on the two sides of a wall
transporter
integral membrane protein that
mediates passage of molecule through
membrane; also called carrier
transport maximum (
T
m
)
upper limit to amount
of material that carrier-mediated transport can
move across the renal tubule
transpulmonary pressure (
P
tp
)
difference in
pressure between the inside and outside of the
lung (alveolar pressure minus the intrapleural
pressure)
transverse tubule (T-tubule)
tubule extending
from striated-muscle plasma membrane
into the fi ber, passing between opposed
sarcoplasmic reticulum segments; conducts
muscle action potential into muscle fi ber
tricarboxylic acid cycle
see
Krebs cycle
tricuspid valve
(try-CUS-pid) valve between right
atrium and right ventricle of heart
triglyceride
subclass of lipids composed of
glycerol and three fatty acids; also called
fat,
neutral fat,
or
acylglycerol
triiodothyronine (T
3
)
(try-eye-oh-doh-THIGH-
roh-neen) iodine-containing amine hormone
secreted by thyroid gland
triplet code
three-base sequence in DNA and
RNA that specifi es particular amino acid
trophic
(TROF-ik) growth promoting
trophoblast
(TROH-foh-blast) outer layer
of blastocyst; gives rise to fetal portion of
placental tissue
tropic hormone
hormone that stimulates the
secretion of another hormone
tropomyosin
(troh-poh-MY-oh-sin) regulatory
protein capable of reversibly converting
binding sites on actin; associated with muscle
thin fi laments
troponin
(troh-POH-nin) regulatory protein
bound to actin and tropomyosin of striated
muscle thin fi laments; site of calcium binding
that initiates contractile activity
trypsin
(TRIP-sin) enzyme secreted into small
intestine by exocrine pancreas as precursor
trypsinogen; breaks certain peptide bonds in
proteins and polypeptides
trypsinogen
(trip-SIN-oh-jen) inactive precursor
of trypsin; secreted by exocrine pancreas
T-tubule
see
transverse tubule
T-type calcium channel
channel that carries
inward calcium current that briefl y supports
diastolic depolarization of cardiac pacemaker
cells (T: “transient”)
tubular reabsorption
transfer of materials from
kidney tubule lumen to peritubular capillaries
tubular secretion
transfer of materials from
peritubular capillaries to kidney tubule lumen
tubulin
(TOOB-you-lin) the major protein
component of microtubules
tumor necrosis factor (TNF)
(neh-KROH-
sis) cytokine secreted by macrophages (and
other cells); has many of the same functions
as IL-1
T wave
component of electrocardiogram
corresponding to ventricular repolarization
twitch
mechanical response of muscle to single
action potential
tympanic membrane
(tim-PAN-ik) membrane
stretched across end of ear canal; also called
eardrum
type I alveolar cell
a fl at epithelial cell that with
others forms a continuous layer lining the air-
facing surface of the pulmonary alveoli
type II alveolar cell
pulmonary cell that produces
surfactant
tyrosine
(TY-roh-seen) amino acid; precursor of
catecholamines and thyroid hormones
tyrosine kinase
protein kinase that
phosphorylates tyrosine portion of proteins;
may be part of plasma membrane receptor
U
ubiquitin
(you-BIK-wit-in) small intracellular
peptide that attaches to proteins and directs
them to proteasomes
ultrafi
ltrate
(ul-tra-FIL-trate) protein-free fl uid
formed from plasma as it is forced through
capillary walls by pressure gradient
umami
(oo-MOM-ee) unique taste sensation
roughly equivalent to “fl avorfulness”
umbilical artery
artery transporting blood from
the fetus into the capillaries of the chorionic
villi
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