Glossary G-11of being changed by excitable cells (neurons and
muscle cells).
membranous labyrinth (mem 9 br ̆a-nus lab 9 ̆ı-rinth)
A system of communicating sacs and ducts within
the bony labyrinth of the inner ear.
menarche (m ̆e-nar 9 ke) The first menstrual
discharge, normally occurring during puberty.
Ménière’s (m ̄an-yarz 9 ) disease Deafness, tinnitus,
and vertigo resulting from a disease of the labyrinth.
menopause (men 9 ̆o-pawz) The cessation of
menstruation, usually occurring at about age 50.
menstrual (men 9 stroo-al) cycle The cyclic changes
in the ovaries and endometrium of the uterus that
lasts about a month. It is accompanied by shedding
of the endometrium, with bleeding, and occurs only
in humans and the higher primates.
menstruation (men 0 stroo-a 9 shun) Shedding
of the outer two-thirds of the endometrium with
accompanying bleeding as a result of a lowering of
estrogen secretion by the ovaries at the end of the
monthly cycle. The first day of menstruation is taken
as day 1 of the menstrual cycle.
mesoderm (mes 9 ̆o-derm) The middle embryonic
tissue layer that gives rise to connective tissue
(including blood, bone, and cartilage); blood vessels;
muscles; the adrenal cortex; and other organs.
messenger RNA (mRNA) A type of RNA that
contains a base sequence complementary to a part of
the DNA that specifies the synthesis of a particular
protein.
metabolic acidosis (as 0 ̆ı-do 9 sis) and alkalosis
(al 0 k ̆a-lo 9 sis) Abnormal changes in arterial blood
pH due to changes in nonvolatile acid concentration
(for example, changes in lactic acid or ketone body
concentrations) or to changes in blood bicarbonate
concentration.
metabolic syndrome A combination of central
obesity, insulin resistance, type 2 diabetes, and
hypertension.
metabolism (m ̆e-tab 9 ̆o-liz-em) All of the chemical
reactions in the body. It includes those that result in
energy storage (anabolism) and those that result in
the liberation of energy (catabolism).
metastasis (m ̆e-tas 9 t ̆a-sis) A process whereby cells
of a malignant tumor separate from the tumor,
travel to a different site, and divide to produce a new
tumor.
methemoglobin (met-he 9 m ̆o-glo 0 bin) The
abnormal form of hemoglobin in which the iron
atoms in heme are oxidized to the ferrous form.
Methemoglobin is incapable of bonding with oxygen.
micelle (mi-sel 9 ) A colloidal particle formed by the
aggregation of numerous molecules.
microarray technology A technique
for identifying large numbers of genes by
complementary base pairing of single-stranded DNA
samples to spots of known single-stranded DNA on a
support medium.
microRNA (miRNA) Short polynucleotides (about
22 nucleotides in length), each coded by a different
region of DNA, that have regulatory functions. An
miRNA binds to a specific mRNA within a particle
called RISC (RNA-induced silencing complex),
which leads to suppressed translation of that mRNA.
This reduces the expression of the gene that coded
for that mRNA.
microvilli (mi 0 kro-vil 9 i) Tiny fingerlike projections
of a cell membrane. They occur on the apical
(lumenal) surface of the cells of the small intestine
and in the renal tubules.
micturition (mik 0 t ̆u-rish 9 un) Urination.
milliequivalent (mil 0 ̆ı-e-kwiv 9 ̆a-lent) The milli-
molar concentration of an ion multiplied by its
number of charges.lymphocyte (lim 9 f ̆o-s ̄ıt) A type of mononuclear
leukocyte; the cell responsible for humoral and cell-
mediated immunity.
lymphoid (lim 9 foyd) organs The organs that
produce lymphocytes. The primary lymphoid organs
are the bone marrow and thymus, and the secondary
lymphoid organs include the lymph nodes, spleen,
tonsils, and Peyer’s patches of the intestinal mucosa.
lymphokine (lim 9 f ̆o-k ̄ın) Any of a group of
chemicals released from T cells that contribute to
cell-mediated immunity.
lysosome (li 9 s ̆o-s ̆om) An organelle containing
digestive enzymes that is responsible for intracellular
digestion.M
macromolecule (mak 0 r ̆o-mol 9 ̆ı-kyool) A large
molecule; a term commonly used to refer to protein,
RNA, and DNA.
macrophage (mak 9 r ̆o-f ̄aj) A large phagocytic cell
in connective tissue that contributes to both specific
and nonspecific immunity.
macula densa (mak 9 y ̆u-l ̆a den 9 s ̆a) The region of
the distal tubule of the renal nephron in contact with
the afferent arteriole. This region functions as a
sensory receptor for the amount of sodium excreted
in the urine and acts to inhibit the secretion of renin
from the juxtaglomerular apparatus.
macula lutea (loo 9 te- ̆a) A yellowish depression in
the retina of the eye that contains the fovea centralis,
the area of keenest vision.
malignant Denoting a structure or process that is
life threatening. Of a tumor, tending to metastasize.
mast cell A type of connective tissue cell that
produces and secretes histamine and heparin.
maximal oxygen uptake The maximum rate
of oxygen consumption by the body per unit time
during heavy exercise. Also called the aerobic
capacity, the maximal oxygen uptake is commonly
indicated with the symbol
.
VO 2 max.
mean arterial pressure (MAP) An adjusted
average of the systolic and diastolic blood pressures.
It averages about 100 mmHg in the systemic
circulation and 10 mmHg in the pulmonary
circulation.
mechanoreceptor (mek 0 ̆a-no-re-sep 9 tor) A sensory
receptor that is stimulated by mechanical means.
Mechanoreceptors include stretch receptors, hair
cells in the inner ear, and pressure receptors.
medulla oblongata (m ̆e-dul 9 ̆a ob 0 long-g ̆at ̆a) A
part of the brain stem that contains neural centers
for the control of breathing and for regulation of the
cardiovascular system via autonomic nerves.
megakaryocyte (meg 0 ̆a-kar 9 e-o-s ̄ıt) A bone
marrow cell that gives rise to blood platelets.
meiosis (mi-o 9 sis) A type of cell division in which
a diploid parent cell gives rise to haploid daughter
cells. It occurs in the process of gamete production
in the gonads.
melanin (mel 9 ̆a-nin) A dark pigment found in the
skin, hair, choroid layer of the eye, and substantia
nigra of the brain. It may also be present in certain
tumors (melanomas).
melatonin (mel 0 ̆a-to 9 nin) A hormone secreted
by the pineal gland that produces darkening of the
skin in lower animals and that may contribute to
the regulation of gonadal function in mammals.
Secretion follows a circadian rhythm and peaks at
night.
membrane potential The potential difference
or voltage that exists between the two sides of a
cell membrane. It exists in all cells but is capableleukocyte (loo 9 k ̆o-s ̄ıt) A white blood cell.
Leydig (li 9 dig) cells The interstitial cells of the
testes that serve an endocrine function by secreting
testosterone and other androgenic hormones.
ligament (lig 9 ̆a-ment) A tough cord or fibrous
band of dense regular connective tissue that contains
numerous parallel arrangements of collagen fibers. It
connects bones or cartilages and serves to strengthen
joints.
ligand (li 9 gand, lig 9 and) A smaller molecule
that chemically binds to a larger molecule, which
is usually a protein. Oxygen, for example, is the
ligand for the heme in hemoglobin, and hormones
or neurotransmitters can be the ligands for specific
membrane proteins.
limbic (lim 9 bik) system A group of brain
structures, including the hippocampus, cingulate
gyrus, dentate gyrus, and amygdala. The limbic
system appears to be important in memory, the
control of autonomic function, and some aspects of
emotion and behavior.
lipid (lip 9 id) An organic molecule that is non-
polar, and thus insoluble in water. Lipids include
triglycerides, steroids, and phospholipids.
lipogenesis (lip 0 ̆o-jen 9 ̆e-sis) The formation of fat or
triglycerides.
lipolysis (li-pol 9 ̆ı-sis) The hydrolysis of
triglycerides into free fatty acids and glycerol.
lipophilic (lip 0 ̆o-fil 9 ik) Pertaining to molecules that
are nonpolar and thus soluble in lipids. The steroid
hormones, thyroxine, and the lipid-soluble vitamins
are examples of lipophilic molecules.
long-term depression (LTD) A process in which
proper stimulation of a presynaptic neuron causes
the postsynaptic neuron to release endocannabinoids,
which suppress the release of neurotransmitters
from presynaptic neurons. Depression of excitatory
input from glutamate would inhibit activation of the
postsynaptic neuron, but depression of inhibitory
input from GABA-releasing presynaptic axons
would enhance the activation of the postsynaptic
neuron.
long-term potentiation (p ̆o-ten 0 she-a 9 shun) (LTP)
The improved ability of a presynaptic neuron that has
been stimulated at high frequency to subsequently
stimulate a post synaptic neuron over a period
of weeks or even months. This may represent a
mechanism of neural learning.
low-density lipoproteins (lip 0 o-pro 9 te-inz) (LDLs)
Plasma proteins that transport triglycerides and
cholesterol to the arteries. LDLs are believed to
contribute to arteriosclerosis.
lower motor neuron The motor neuron that has
its cell body in the gray matter of the spinal cord
and that contributes axons to peripheral nerves. This
neuron innervates muscles and glands.
lumen (loo 9 men) The cavity of a tube or hollow
organ.
lung surfactant (sur-fak 9 tant) A mixture of
lipoproteins (containing phospholipids) secreted by
type II alveolar cells into the alveoli of the lungs. It
lowers surface tension and prevents collapse of the
lungs, as occurs in hyaline membrane disease when
surfactant is absent.
luteinizing (loo 9 te- ̆ı-ni 0 zing) hormone (LH)
A gonadotropic hormone secreted by the anterior
pituitary. In a female, LH stimulates ovulation and
the development of a corpus luteum; in a male, it
stimulates the Leydig cells to secrete androgens.
lymph (limf) A fluid derived from tissue fluid that
flows through lymphatic vessels, returning to the
venous bloodstream.
lymphatic (lim-fat 9 ik) system The lymphatic
vessels and lymph nodes.fox36375_glo_G1-G19.indd 11fox36375_glo_G1-G19.indd 11 2/4/15 12:29 PM2/4/15 12:29 PM