abdominal cavity Body cavity that holds the stomach, liver,
pancreas, most of the intestine, and several other organs.
ABO blood typing Method of characterizing an individual’s
blood according to whether one or both of two protein mark-
ers, A and B, are present at the surface of red blood cells. The
O signifies that neither marker is present.
abortion Spontaneous or induced expulsion of the embryo or
fetus from the uterus.
absorption The movement of nutrients, fluid, and ions
across the gastrointestinal tract lining and into the internal
environment.
accommodation In the eye, adjustments of the lens position
that move the focal point forward or back so that incoming
light rays are properly focused on the retina.
acetylcholine (ACh) A neurotransmitter that can excite or
inhibit various target cells in the brain, spinal cord, glands,
and muscles.
acid A substance that releases hydrogen ions in water.
acid–base balance State in which extracellular fluid is
neither too acidic nor too basic, an outcome of controls over
its concentrations of dissolved ions.
acid rain Wet acid deposition; falling of rain (or snow) rich
in sulfur and nitrogen oxides.
acrosome An enzyme-containing cap that covers most of
the head of a sperm and helps the sperm penetrate an egg at
fertilization.
actin (ak-tin) A globular contractile protein. In muscle cells,
actin interacts with another protein, myosin, to bring about
contraction.
action potential An abrupt, brief reversal in the steady volt-
age difference (resting membrane potential) across the plasma
membrane of a neuron.
activation energy The minimum amount of energy required
for a chemical reaction.
active immunity Immunity that develops after a person
receives a vaccine, which stimulates the immune system to
produce antibodies against a particular pathogen.
active site A crevice on the surface of an enzyme molecule
where a specific reaction is catalyzed.
active transport The pumping of one or more specific sol-
utes through a transport protein that spans the lipid bilayer of
a cell membrane. Most often, the solute is transported against
its concentration gradient. The protein is activated by an
energy boost, as from ATP.
adaptation [L. adaptare, to fit] In evolutionary biology,
the process of becoming suited (or more suited) to a given
set of environmental conditions. Of sensory neurons, a
decrease in the frequency of action potentials (or their ces-
sation) even when a stimulus is maintained at constant
strength.
adaptive immunity Immune responses that the body devel-
ops in response to antigens of specific pathogens, toxins, or
abnormal body cells.
adaptive radiation A burst of speciation events, with lin-
eages branching away from one another as they partition the
existing environment or invade new ones.
adenine (ah-de-neen) A purine; a nitrogen-containing base
in certain nucleotides; a building block of DNA.
ADH Antidiuretic hormone. Produced by the hypothalamus
and released by the posterior pituitary, it stimulates reabsorp-
tion in the kidneys and so reduces urine volume.
adhering junctions Cell junctions that cement cells together.
adipose tissue A type of connective tissue having an
abundance of fat-storing cells and blood vessels for
transporting fats.
adjuvant therapy Cancer therapy that combines surgery
with chemotherapy.
adrenal cortex (ah-dree-nul) Outer portion of the adrenal
gland; its hormones have roles in metabolism, inflammation,
maintaining extracellular fluid volume, and other functions.
adrenal medulla Inner region of the adrenal gland; its
hormones help control blood circulation and carbohydrate
metabolism.
aerobic exercise Exercise that works muscles at a rate that
does not exceed the body’s ability to keep them supplied with
oxygen (in blood).
aerobic respiration (air-oh-bik) [Gk. aer, air, and bios, life]
An oxygen-dependent pathway of ATP formation.
afferent arteriole In the urinary system, an arteriole that
delivers blood to each nephron.
age structure Of a population, the number of individuals in
each of several or many age categories.
agglutination (ah-glue-tin-ay-shun) The clumping together
of foreign cells that have invaded the body (as pathogens or
in tissue grafts or transplants). Clumping is caused by cross-
linking between antibody molecules that have already bound
an antigen at the surface of the foreign cells.Glossary
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