Adaptation (ad'-ap-TA-shun) The adjustment of the pupil of the eye
to changes in light intensity. The property by which a sensory
neuron relays a decreased frequency of action potentials from a
receptor, even though the strength of the stimulus remains con-
stant; the decrease in perception of a sensation over time while
the stimulus is still present.
Adduction (ad-DUK-shun) Movement toward the midline of the body.
Adenoid (AD-e-noyd) The pharyngeal tonsil.
Adenosine triphosphate (ATP) (a-DEN-o-sen trT-FOS-fat) The main
energy currency in living cells; used to transfer the chemical
energy needed for metabolic reactions. ATP consists of the
a d e n in e
and the five-carbon sugar
r ib o s e ,
are added, in linear array, three
p h o s p h a t e
Adenylate cyclase (a-DEN-i-lat ST-klas) An enzyme that is activated
when certain neurotransmitters or hormones bind to their recep-
tors; the enzyme that converts ATP into cyclic AMP, an important
Adipocyte (AD-i-po-sTt) Fat cell, derived from a fibroblast.
Adipose tissue (AD-i-poz) Tissue composed of adipocytes special-
ized for triglyceride storage and present in the form of soft pads
between various organs for support, protection, and insulation.
Adrenal cortex (a-DRE-nal KOR-teks) The outer portion of an adrenal
gland, divided into three zones; the zona glomerulosa secretes
mineralocorticoids, the zona fasciculata secretes glucocorticoids,
and the zona reticularis secretes androgens.
Adrenal glands Two glands located superior to each kidney. Also
called the suprarenal glands (soo'-pra-RE-nal).
Adrenal medulla (me-DUL-a) The inner part of an adrenal gland,
consisting of cells that secrete epinephrine, norepinephrine, and
a small amount of dopamine in response to stimulation by sympa-
thetic preganglionic neurons.
Adrenergic neuron (ad'-ren-ER-jik) A neuron that releases epi-
nephrine (adrenaline) or norepinephrine (noradrenaline) as its
Adrenocorticotropic hormone (ACTH) (ad-re'-no-kor-ti-ko-TROP-ik)
A hormone produced by the anterior pituitary that influences the
production and secretion of certain hormones of the adrenal cortex.
Adventitia (ad-ven-TISH-a) The outermost connective tissue covering
of a structure or organ not covered by a serous coat.
Aerobic (air-O-bik) Requiring molecular oxygen.
Afferent arteriole (AF-er-ent ar-TE-re-ol) A blood vessel of a kidney
that divides into the capillary network called a glomerulus; there
is one afferent arteriole for each glomerulus.
Agglutination (a-gloo'-ti-NA-shun) Clumping of microorganisms or
blood cells, typically due to an antigen-antibody reaction.
Aggregated lymphatic follicles (AG-re-ga-ted lim-FAT-its FOL-i-kalz)
Clusters of lymph nodules that are most numerous in the ileum.
Also called Peyer's patches (PT-erz).
(AL-bin-izm) Abnormal, nonpathological, partial, or total
absence of pigment in skin, hair, and eyes.
Albumin (al-BU-min) The most abundant (60%) and smallest of
the plasma proteins; it is the main contributor to blood colloid
osmotic pressure (BCOP).
Aldosterone (al-DOS-ter-on) A mineralocorticoid produced by the
adrenal cortex that promotes sodium and water reabsorption by
the kidneys and potassium excretion in urine.
Alkaline (AL-ka-lTn) Containing more hydroxide ions (OH-) than
hydrogen ions (H+); a pH higher than 7.
Alkalosis (al-ka-LO-sis) A condition in which blood pH is higher than
7.45. Also known as alkalemia.
Allantois (a-LAN-to-is) A small, vascularized outpouching of the yolk
sac that serves as an early site for blood formation and develop-
ment of the urinary bladder.
Alleles (a-LELZ) Alternate forms of a single gene that control the
same inherited trait (such as type A blood) and are located at the
same position on homologous chromosomes.
Allergen (AL-er-jen) An antigen that evokes a hypersensitivity
Alpha cell (AL-fa) A type of cell in the pancreatic islets (islets of Lang-
erhans) in the pancreas that secretes the hormone glucagon.
Alveolar duct (al-VE-o-lar) Branch of a respiratory bronchiole around
which alveoli and alveolar sacs are arranged.
Alveolar macrophage (MAK-ro-faj) Highly phagocytic cell found in the
alveolar walls of the lungs. Also called a dust cell.
Alveolar pressure Air pressure within the lungs.
Alveolar sac A cluster of alveoli that share a common opening.
Alveolus (al-VE-o-lus) A small hollow or cavity; an air sac in the lungs;
milk-secreting portion of a mammary gland.
P lu r a l is
Alzheimer disease (AD) (ALTZ-hT-mer) Disabling neurological disorder
characterized by dysfunction and death of specific cerebral neu-
rons, resulting in widespread intellectual impairment, personality
changes, and fluctuations in alertness.
Amnesia (am-NE-ze-a) A lack or loss of memory.
Amenorrhea (a-men-o-RE-a) Absence of menstruation.
Amino acid (a-ME-no) An organic acid, containing an acidic carboxyl
group (-COOH) and a basic amino group (-NH2); the monomer
used to synthesize polypeptides and proteins.
Amnion (AM-ne-on) A thin, protective fetal membrane that develops
from the epiblast; holds the fetus suspended in amniotic fluid.
Also called the "bag of waters."
Amniotic fluid (am'-ne-OT-ik) Fluid in the amniotic cavity, the space
between the developing embryo (or fetus) and amnion; the fluid
is initially produced as a filtrate from maternal blood and later
includes fetal urine. It functions as a shock absorber, helps regu-
late fetal body temperature, and helps prevent desiccation.
Amphiarthrosis (am'-fe-ar-THRO-sis) A slightly movable joint, in
which the articulating bony surfaces are separated by fibrous
connective tissue or fibrocartilage to which both are attached;
types are syndesmosis and symphysis.
Ampulla (am-PUL-la) A saclike dilation of a canal or duct.
Anabolism (a-NAB-o-lizm) Synthetic, energy-requiring reactions
whereby small molecules are built up into larger ones.
Anaerobic (an-ar-O-bik) Not requiring oxygen.
Anal canal (A-nal) The last 2 or 3 cm (1 in.) of the rectum; opens to
the exterior through the anus.