glossary G -11hemostasis (hee-mow-stay-sis) [Gk. haima, blood, and stasis,
standing] Stopping of blood loss from a damaged blood ves-
sel through coagulation, blood vessel spasm, platelet plug
formation, and other mechanisms.hepatic portal system System of blood vessels that trans-
port blood from the digestive tract to and from the liver.
heterotroph (het-er-oh-trofe) [Gk. heteros, other, and trophos,
feeder] Organism that cannot synthesize its own organic
compounds and must obtain nourishment by feeding on
autotrophs, each other, or organic wastes. Animals, fungi,
many protists, and most bacteria are heterotrophs. Compare
autotroph.heterozygous (het-er-oh-zye-guss) [Gk. zygoun, join
together] For a given trait, having nonidentical alleles at a
particular locus on a pair of homologous chromosomes.hindbrain One of the three divisions of the brain; the
medulla oblongata, cerebellum, and pons; includes reflex cen-
ters for respiration, blood circulation, and other basic func-
tions; also coordinates motor responses and many complex
reflexes.
histamine Local signaling molecule that promotes inflamma-
tion; makes arterioles dilate and capillaries more permeable
(leaky).
homeostasis (hoe-me-oh-stay-sis) [Gk. homo, same, and
stasis, standing] A physiological state in which the physi-
cal and chemical conditions of the internal environment are
being maintained within ranges that enable survival of cells
and the whole body.
homeostatic feedback An interaction in which an organ (or
structure) stimulates or inhibits the output of another organ,
then shuts down or increases this activity when it detects that
the output has exceeded or fallen below a set point.
homologous chromosome (huh-moll-uh-gus) [Gk. homolo-
gia, correspondence] (also called a homologue) One of a pair of
chromosomes that resemble each other in size, shape, and the
genes they carry, and that line up with each other at meiosis I.
The X and Y chromosomes differ in these respects but still
function as homologues.
homologous structure The same body part, modified in
different ways, in different lines of descent from a common
ancestor.
homozygous (hoe-moe-zye-guss) Having two identi-
cal alleles at a given locus (on a pair of homologous
chromosomes).
homozygous dominant Having two dominant alleles at a
given gene locus (on a pair of homologous chromosomes).
homozygous recessive Having two recessive alleles at a
given gene locus (on a pair of homologous chromosomes).
hormone [Gk. hormon, to stir up, set in motion] Any of the
signaling molecules secreted from endocrine glands, endo-
crine cells, and some neurons that the bloodstream distributes
to nonadjacent target cells (any cell having receptors for that
hormone).
host An organism that can be infected by a pathogen.glycemic index A list that ranks a food according to its effect
on blood sugar (glucose).
glycolysis (gly-call-ih-sis) [Gk. glykys, sweet, and lysis, loos-
ening or breaking apart] In first stage of cellular respiration,
process by which glucose (or some other organic compound)
is partially broken down to pyruvate with a net yield of two
ATP. Glycolysis occurs in the cell cytoplasm and oxygen has
no role in it.
glycoprotein A protein having oligosaccharides covalently
bonded to it. Most human cell surface proteins and many pro-
teins circulating in blood are glycoproteins.
Golgi body (gohl-gee) Organelle in which newly synthe-
sized polypeptide chains as well as lipids are modified and
packaged in vesicles for export or for transport to specific
locations within the cytoplasm.
gonad (go-nad) Primary reproductive organ in which gam-
etes are produced. Ovaries and testes are gonads.
granulocyte Class of white blood cells that have a lobed
nucleus and various types of granules in the cytoplasm;
includes neutrophils, eosinophils, and basophils.
gray matter The dendrites, neuron cell bodies, and neuro-
glial cells of the spinal cord and cerebral cortex.
greenhouse effect Warming of the lower atmosphere due
to the presence of the following greenhouse gases: carbon
dioxide, methane, nitrous oxide, ozone, water vapor, and
chlorofluorocarbons.
growth factor A type of signaling molecule that can influence
growth by regulating the rate at which target cells divide.
guanine A nitrogen-containing base; one of those present in
nucleotide building blocks of DNA and RNA.
habitat [L. habitare, to live in] The type of place where an
organism normally lives, characterized by physical features,
chemical features, and the presence of certain other species.
hair cell Type of mechanoreceptor that may give rise to
action potentials when bent or tilted.
haploid (hap-loyd) Having only one of each pair of homolo-
gous chromosomes; gametes are haploid. Compare diploid.
HCG Human chorionic gonadotropin. A hormone that helps
maintain the lining of the uterus during the menstrual cycle
and during the first trimester of pregnancy.
HDL A high-density lipoprotein in blood; it transports
cholesterol to the liver for further processing.
heart Muscular pump that keeps blood circulating through
the body.
heat A form of kinetic energy; also called thermal energy.
helper T cell Type of T lymphocyte that produces and
secretes chemicals that promote formation of large effector
and memory cell populations.
hemoglobin (heem-oh-glow-bin) [Gk. haima, blood, and L.
globus, ball] Iron-containing, oxygen-transporting protein that
gives red blood cells their color.
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