Glossary G-13remains alive because it is nourished by means of
canaliculi within the extracellular material of bone.
osteomalacia (os 0 te-o-m ̆a-la 9 sh ̆a) Softening of
bones due to a deficiency of vitamin D and calcium.
osteoporosis (os 0 te-o-p ̆o-ro 9 sis) Demineralization
of bone, seen most commonly in postmenopausal
women and patients who are inactive or paralyzed.
It may be accompanied by pain, loss of stature, and
other deformities and fractures.
ovary (o 9 v ̆a-re) The gonad of a female that
produces ova and secretes female sex steroids.
oviduct (o 9 v ̆ı-dukt) The part of the female
reproductive tract that transports ova from the
ovaries to the uterus. Also called the uterine, or
fallopian tube.
ovulation (ov-y ̆u-la 9 shun) The extrusion of a
secondary oocyte from the ovary.
oxidation-reduction (ok 0 s ̆ı-da 9 shun-re-duk 9 shun)
The transfer of electrons or hydrogen atoms
from one atom or molecule to another. The
atom or molecule that loses the electrons
or hydrogens is oxidized; the atom or mole-
cule that gains the electrons or hydrogens is reduced.
oxidative phosphorylation (ok 0 s ̆ı-da 9 tiv fos 0 for-
̆ı-la 9 shun) The formation of ATP by using energy
derived from electron transport to oxygen. It occurs
in the mitochondria.
oxidative stress The damage to lipids, proteins,
and DNA in the body produced by the excessive
production of free radicals. Oxidative stress is
believed to contribute to aging and a variety of
diseases.
oxidizing (ok 9 s ̆ı-dizing) agent An atom that
accepts electrons in an oxidation-reduction reaction.
oxygen (ok 9 s ̆ı-jen) debt The extra amount of
oxygen required by the body after exercise to
metabolize lactic acid and to supply the higher
metabolic rate of muscles warmed during exercise.
oxyhemoglobin (ok 0 se-he 0 m ̆o-glo 9 bin) A
compound formed by the bonding of molecular
oxygen with hemoglobin.
oxyhemoglobin saturation The ratio, expressed
as a percentage, of the amount of oxyhemo globin
compared to the total amount of hemoglobin in
blood.
oxytocin (ok 0 s ̆ı-to 9 sin) One of the two hormones
produced in the hypothalamus and released from
the posterior pituitary (the other being vasopressin).
Oxytocin stimulates the contraction of uterine
smooth muscles and promotes milk letdown in
females.P
pacemaker The group of cells that has the fastest
spontaneous rate of depolarization and contraction in
a mass of electrically coupled cells; in the heart, this
is the sinoatrial, or SA, node.
pacemaker potential The spontaneous
depolarization of the pacemaker region of the heart,
the SA node, that occurs during diastole. When this
depolarization reaches threshold, an action potential
is produced and results in contraction (systole).
pacesetter potentials Changes in membrane
potential produced spontaneously by pacemaker cells
of single-unit smooth muscles.
pacinian corpuscle (p ̆a-sin 9 e-an kor 9 pus 9 l)
A cutaneous sensory receptor sensitive to pressure.
It is characterized by an onionlike layering of cells
around a central sensory dendrite.
PAH para-aminohippuric (par 9 ̆a- ̆a-me 9 no-hi-
pyoor 0 ik) acid A substance used to measure total
renal plasma flow because its clearance rate is equaloocyte (o 9 ̆o-s ̄ıt) An immature egg cell (ovum).
A primary oocyte has not yet completed the first
meiotic division; a secondary oocyte has begun
the second meiotic division. A secondary oocyte,
arrested at metaphase II, is ovulated.
oogenesis (o 0 ̆o-jen 9 ̆e-sis) The formation of ova in
the ovaries.
opsonization (op 0 s ̆o-n ̄ı-za 9 shun) The process by
which antibodies enhance the ability of phagocytic
cells to attack bacteria.
optic (op 9 tik) disc The area of the retina where
axons from ganglion cells gather to form the optic
nerve and where blood vessels enter and leave the
eye. It corresponds to the blind spot in the visual
field caused by the absence of photoreceptors.
orexigenic (o-rek-si-jen 9 ik) neurons Orexigenic
refers to the promotion of appetite and hunger. The
orexigenic neurons in the hypothalamus release
the peptide neurotransmitters neuropeptide Y and
AgRP (agouti-related protein). Anorexigenic neurons
suppress appetite and hunger, primarily by releasing
melanocyte-stimulating hormone (MSH).
organ A structure in the body composed of two
or more primary tissues that performs a specific
function.
organelle (or 0 g ̆a-nel 9 ) A structure within cells
that performs specialized tasks. Organelles include
mitochondria, the Golgi apparatus, endoplasmic
reticulum, nuclei, and lysosomes. The term is
also used for some structures not enclosed by a
membrane, such as ribosomes and centrioles.
organic anion transporters (OATs) A family of
transport proteins with a broad range of specificities
(described as multispecific or polyspecific), able to
transport many drugs and endogenous molecules
across the cell membranes of the renal tubules and
bile ductules. As a result of this action, the organic
anion transporters are responsible for the renal
secretion of many compounds (such as antibiotics)
into the renal tubules, and for the secretion of many
compounds into the bile, for elimination in the urine
and feces, respectively.
organ of Corti (kor 9 te) The structure within
the cochlea that constitutes the functional unit of
hearing. It consists of hair cells and supporting cells
on the basilar membrane that help to transduce sound
waves into nerve impulses; also called the spiral
organ.
osmolality (Osm) (oz 0 m ̆o-lal 9 ̆ı-te) A measure of
the total concentration of a solution; the number of
moles of solute per kilogram of solvent.
osmoreceptor (oz 0 m ̆o-re-cep 9 tor) A sensory
neuron that responds to changes in the osmotic
pressure of the surrounding fluid.
osmosis (oz-mo 9 sis) The passage of solvent (water)
from a more dilute to a more concentrated solution
through a membrane that is more permeable to water
than to the solute.
osmotic (oz-mot 9 ik) pressure A measure of the
tendency for a solution to gain water by osmosis
when separated by a membrane from pure water.
Directly related to the osmolality of the solution, it is
the pressure required to just prevent osmosis.
osteoblast (os 9 te- ̆o-blast) A bone-forming cell.
osteocalcin (os 9 te-o-kal 9 sin) A hormone secreted
by the osteoblasts of bone. Osteocalcin secretion is
stimulated by insulin, and the osteocalcin in blood
then stimulates the pancreatic islets to secrete more
insulin in a positive feedback manner.
osteoclast (os 9 te- ̆o-klast) A cell that resorbs bone
by promoting the dissolution of calcium phosphate
crystals.
osteocyte (os 9 te- ̆o-s ̄ıt) A mature bone cell that has
become entrapped within a matrix of bone. This cellNissl (nis 9 l) bodies Granular-appearing structures
in the cell bodies of neurons that have an affinity for
basic stain; they correspond to ribonucleoprotein;
also called chromatophilic substances.
nitric oxide (NO) A gas that functions as a neuro-
transmitter in both the central nervous system and in
peripheral autonomic neurons, and as an autocrine
and paracrine regulator in many organs. It promotes
vasodilation, intestinal relaxation, penile erection,
and aids long-term potentiation in the brain.
nociceptor (no 0 s ̆ı-sep 9 tor) A receptor for pain that
is stimulated by tissue damage.
nodes of Ranvier (ran 9 ve-a) Gaps in the myelin
sheath of myelinated axons, located approximately
1 mm apart. Action potentials are produced only at
the nodes of Ranvier in myelinated axons.
nonpolar molecule A molecule lacking positive
and negative charges and therefore not soluble in
water.
norepinephrine (nor 0 ep- ̆ı-nef 9 rin) A
catecholamine released as a neurotransmitter from
postganglionic sympathetic nerve endings and as a
hormone (together with epinephrine) by the adrenal
medulla; also called noradrenaline.
nuclear factor-kB Abbreviated NF-kB, where k is
the Greek letter “kappa.” A transcrip tion factor that
activates genes to promote inflammation; it is also
believed to contribute to cancer.
nuclear receptors Receptors that bind to both a
regulatory ligand (such as a hormone) and to DNA.
The nuclear receptors, when activated by their
ligands, regulate genetic expression (RNA synthesis).
nucleolus (noo-kle 9 ̆o-lus) A dark-staining area
within a cell nucleus; the site of production of
ribosomal RNA.
nucleoplasm (noo 9 kle- ̆o-plaz 0 em) The protoplasm
of a nucleus.
nucleosome (noo 9 kle- ̆o-s ̄om) A complex of DNA
and histone proteins that is believed to constitute an
inactive form of DNA. In the electron microscope,
the histones look like beads threaded on a string of
chromatin.
nucleotide (noo 9 kle- ̆o-t ̄ıd) The subunit of DNA
and RNA macromolecules. Each nucleotide
is composed of a nitrogenous base (adenine,
guanine, cytosine, and thymine or uracil); a sugar
(deoxyribose or ribose); and a phosphate group.
nucleus (noo 9 kl ̄e-us), brain An aggregation of
neuron cell bodies within the brain. Nuclei within the
brain are surrounded by white matter and are located
deep to the cerebral cortex.
nucleus, cell The organelle, surrounded by a
double saclike membrane called the nuclear envelope
(nuclear membrane), that contains the DNA and
genetic information of the cell.
nystagmus (n ̄ı-stag 9 mus) Involuntary oscillatory
movements of the eye.O
obese (o-b ̄es) Excessively fat.
oligodendrocyte (ol 0 ̆ı-go-den 9 dr ̆o-s ̄ıt) A type of
glial cell that forms myelin sheaths around axons in
the central nervous system.
oncogene (on 9 k ̆o-j ̄en) A gene that contributes to
cancer. Oncogenes are believed to be abnormal forms
of genes that participate in normal cellular regulation.
oncology (on-kol 9 ̆o-je) The study of tumors.
oncotic (on-kot 9 ik) pressure The colloid osmotic
pressure of solutions produced by proteins. In
plasma, it serves to counterbalance the outward
filtration of fluid from capillaries caused by
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