G-16 Glossarysemilunar (sem 0 e-loo 9 nar) valves The valve flaps
of the aorta and pulmonary artery at their juncture
with the ventricles.
seminal vesicles (sem 9 ̆ı-nal ves 9 ̆ı-k 9 lz) The paired
organs located on the posterior border of the urinary
bladder that empty their contents into the ejaculatory
duct and thus contribute to the semen.
seminiferous tubules (sem 0 ̆ı-nif 9 er-us too 9 byoolz)
The tubules within the testes that produce
spermatozoa by meiotic division of their germinal
epithelium.
sensory neuron (noor 9 on) An afferent neuron that
conducts impulses from peripheral sensory organs
into the central nervous system.
SERCA pumps The Ca^21 ATPase carriers in
the sarcoplasmic reticulum of striated muscle
cells. These active transport pumps use the
breakdown of ATP to transport Ca^21 against its
concentration gradient out of the cytoplasm and
into the sarcoplasmic reticulum to allow for muscle
relaxation.
serosa (s ̆ı-ro 9 s ̆a) An outer epithelial membrane that
covers the surface of a visceral organ.
serotonin (ser 0 ̆o-to 9 nin) monoamine
neuro transmitter, chemically known as
5-hydroxytryptamine, derived from the amino acid
L-tryptophan. Serotonin released at synapses in
the brain have been associated with the regulation
of mood and behavior, appetite, and cerebral
circulation.
serotonin transporters Transport proteins in the
presynaptic membrane that take up serotonin from
the synaptic cleft. Serotonin transporters are inhibited
by drugs called serotonin-specific reuptake inhibitors
(SSRIs).
Sertoli (ser-to 9 e) cells Nongerminal supporting
cells in the seminiferous tubules. Sertoli cells
envelop spermatids and appear to participate
in the transformation of spermatids into spermato zoa;
also called sustentacular cells.
serum (ser 9 um) The fluid squeezed out of a clot as
it retracts; supernatant when a sample of blood clots
in a test tube and is centrifuged. Serum is plasma
from which fibrinogen and other clotting proteins
have been removed as a result of clotting.
sex chromosomes The X and Y chromosomes.
These are the unequal pairs of chromosomes
involved in sex determination (which depends
on the presence or absence of a Y chromosome).
Females lack a Y chromosome and normally have
the genotype XX; males have a Y chromosome and
normally have the genotype XY.
shock As it relates to the cardiovascular system, a
rapid, uncontrolled fall in blood pressure, which in
some cases becomes irreversible and leads to death.
sickle-cell anemia A hereditary autosomal
recessive trait that occurs primarily in people of
African ancestry, in whom it evolved apparently as
a protection (in the carrier state) against malaria. In
the homozygous state, hemoglobin S is made instead
of hemoglobin A, which leads to the characteristic
sickling of red blood cells, hemolytic anemia, and
organ damage.
sinoatrial (si 0 no-a 9 tre-al) node A mass of
specialized cardiac tissue in the wall of the right
atrium that initiates the cardiac cycle; the SA node,
also called the pacemaker.
sinus (si 9 nus) A cavity.
sinusoid (si 9 nu ̆-soid) A modified capillary with a
relatively large diameter that connects the arterioles
and venules in the liver, bone marrow, lymphoid
tissues, and some endocrine organs. In the liver,
sinusoids are partially lined by phagocytic cells of
the reticuloendothelial system.rickets (rik 9 ets) A condition caused by a deficiency
of vitamin D and associated with interference of the
normal ossification of bone.
rigor mortis (rig 9 or mor 9 tis) The stiffening of
a dead body due to the depletion of ATP and the
production of rigor complexes between actin and
myosin in muscles.
RNA ribonucleic (ri 0 bo-noo-kle 9 ik) acid A
nucleic acid consisting of the nitrogenous bases
adenine, guanine, cytosine, and uracil; the sugar
ribose; and phosphate groups. There are three
types of RNA found in cytoplasm: messenger RNA
(mRNA), transfer RNA (tRNA), and ribosomal
RNA (rRNA).
rods One of the two categories of photoreceptors
(the other being cones) in the retina of the eye. Rods
are responsible for black-and-white vision under low
illumination.S
saccadic (sa-kad 9 ik) eye movements Very rapid,
jerky eye movements that, for example, serve to
maintain a focus on the fovea centralis of the retina
while reading.
saltatory (sal 9 t ̆a-tor-e) conduction The rapid
passage of action potentials from one node of
Ranvier to another in myelinated axons.
sarcolemma (sar 0 c ̆o-lem 9 ̆a) The cell membrane of
striated muscle cells.
sarcomere (sar 9 k ̆o-m ̄er) The structural subunit of
a myofibril in a striated muscle; equal to the distance
between two successive Z lines.
sarcoplasm (sar 9 k ̆o-plaz 0 em) The cytoplasm of
striated muscle cells.
sarcoplasmic reticulum (sar 0 k ̆o-plaz 9 mik r ̆e-tik 9
y ̆u-lum) The smooth or agranular endoplasmic
reticulum of striated muscle cells. It surrounds each
myofibril and serves to store Ca^21 when the muscle
is at rest.
Schwann (shvan) cell A supporting cell of the
peripheral nervous system that forms sheaths around
peripheral nerve fibers. Schwann cells also direct
regeneration of peripheral nerve fibers to their target
cells.
second messenger A molecule or ion whose
concentration within a target cell is increased by the
action of a regulator molecule (e.g., a hormone or
neurotransmitter) so as to stimulate the metabolism
of that target cell in a way characteristic of the
actions of the regulator molecule—that is, in a way
that mediates the intracellular effects of the regulator
molecule.
secretin (s ̆e-kre 9 tin) A polypeptide hormone
secreted by the small intestine in response to acidity
of the intestinal lumen. Along with cholecystokinin,
secretin stimulates the secretion of pancreatic juice
into the small intestine.
secretion (s ̆e-kre 9 shun), renal The transport of
a substance from the blood through the wall
of the nephron tubule into the urine.
selectively permeable membrane A membrane
with pores of a size that permit the passage of solvent
and some solute molecules, while restricting the
passage of other solute molecules.
semen (se 9 men) The fluid ejaculated by a male
containing sperm and additives from the prostate and
seminal vesicles.
semicircular canals Three canals of the bony
labyrinth that contain endolymph, which is
continuous with the endolymph of the membranous
labyrinth of the cochlea. The semicircular canals
provide a sense of equilibrium.renin (re 9 nin) An enzyme secreted into the blood
by the juxtaglomerular apparatus of the kidneys.
Renin catalyzes the conversion of angiotensinogen
into angiotensin II.
repolarization (re-po 0 lar- ̆ı-za 9 shun) The
reestablishment of the resting membrane potential
after depolarization has occurred.
resorption (re-sorp 9 shun) bone The dissolution of
the calcium phosphate crystals of bone by the action
of osteoclasts.
respiratory acidosis (r ̆ı-sp ̄ır 9 ̆a-tor-e as 0 ̆ı-do 9 sis)
A lowering of the blood pH to below 7.35 as
a result of the accumulation of CO 2 caused by
hypoventilation.
respiratory alkalosis (al 0 k ̆a-lo 9 sis) A rise in
blood pH to above 7.45 as a result of the
excessive elimination of blood CO 2 caused by
hyperventilation.
respiratory distress syndrome (RDS) A lung
disease of the newborn, most frequently occurring in
premature infants, that is caused by abnormally high
alveolar surface tension as a result of a deficiency
in lung surfactant; also called hyaline membrane
disease.
respiratory zone The region of the lungs in which
gas exchange between the inspired air and pulmonary
blood occurs. It includes the respiratory bronchioles,
in which individual alveoli are found, and the
terminal alveoli.
resting membrane potential The potential
difference across a plasma membrane when the cell
is in an unstimulated state. The resting potential
is always negatively charged on the inside of the
membrane compared to the outside.
reticular (r ̆e-tik 9 y ̆u-lar) activating system (RAS)
A complex network of nuclei and fiber tracts within
the brain stem that produces nonspecific arousal of
the cerebrum to incoming sensory information. The
RAS thus maintains a state of alert consciousness
and must be depressed during sleep.
retina (ret 9 - ̆ı-n ̆a) The layer of the eye that contains
neurons and photoreceptors (rods and cones).
retinal (ret 9 i-nal) Also called retinaldehyde and
retinene, this is the visual pigment in rods and
cones. The 11-cis form of retinal is combined with
the protein opsin (in rods) or photopsin (in cones),
and dissociates from this protein when light causes
the retinal to change to its all-trans isomer in the
bleaching reaction.
retinoic (ret 0 ̆ı-no 9 ik) acid The active form of
vitamin A that binds to nuclear receptor proteins and
directly produces the effects of vitamin A.
retrograde neurotransmitter A neurotransmitter
that is released from a postsynaptic neuron and
diffuses across the synaptic cleft to the presynaptic
axon terminals, to modify the release of the axon’s
neurotransmitter.
rhodopsin (ro-dop 9 sin) Visual purple. A pigment
in rod cells that undergoes a photochemical
dissociation in response to light and, in so doing,
stimulates electrical activity in the photoreceptors.
rhythmicity center The area of the medulla
oblongata that controls the rhythmic pattern of
inspiration and expiration.
riboflavin (ri 9 bo-fla 0 vin) Vitamin B 2. Riboflavin
is a water-soluble vitamin that is used to form the
coenzyme FAD, which participates in the transfer of
hydrogen atoms.
ribosome (ri 9 bo-s ̄om) A cytoplasmic organelle
composed of protein and ribosomal RNA that is
responsible for the translation of messenger RNA
and protein synthesis.
ribozymes (ri 0 bo-zims) RNA molecules that have
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