580 Introduction to Anatomy and Physiology
alimentary canal (al-i-MEHN-ter-ee) the passage
leading from the mouth to the anus through the pharynx,
esophagus, stomach, and intestine; gastrointestinal tract
allergen antigen that causes an inappropriately strong
immune system response
allergy an inappropriately strong response (hypersensi-
tivity) of the immune system to an environmental antigen
that it has previously encountered
all-or-none law the rule stating that the fi bers in a given
motor unit always develop maximum tension when
stimulated
alpha cells cells in the islets of Langerhans that secrete
glucagon
alpha interferons (IN-ter-FAIR-ahns) interferons
produced by virus-infected leukocytes
alternative pathway one of the two primary ways in
which the complement system can be activated; this
pathway is triggered when the C3b complement protein
binds to foreign material. Compare classical pathway
alveolar capillary membrane (al-VEE-oh-lar)
gas-exchange structure that contains the alveoli and the
capillaries surrounding the alveoli
alveolar ducts (al-VEE-oh-lar) ducts that connect the
bronchioles to the alveoli
alveoli (al-VEE-oh-ligh) air sacs in the lungs from which
gas is exchanged with the capillaries
Alzheimer’s disease condition involving a progressive
loss of brain function with major consequences for
memory, thinking, and behavior
amblyopia (AM-blee-OH-pee-a) the condition caused
when one eye is the extremely dominant eye and the
other becomes weak; lazy eye
amenorrhea (ay-MEN-oh-REE-a) absence of a
menstrual period in women of reproductive age
amino-acid hormones hormones composed of proteins
or protein-related substances
amino acids the building blocks of proteins
amnion (AM-nee-ahn) a membrane that forms a closed
sac around the embryo and fetus
amniotic fl uid (am-nee-AHT-ik) a clear fl uid in which the
embryo, and then the fetus, is suspended
amphiarthrosis (AM-fee-ar-THROH-sis) joint type that
permits only slight motion
amusia a condition characterized by the inability to
distinguish differences in pitch
anabolism the process through which complex molecules
are constructed from simple ones
anaphylaxis (AN-a-fi -LAK-sis) a severe and potentially
life-threatening allergic reaction that may include airway
obstruction and very low blood pressure
anatomical position erect standing position with arms at
the sides and palms facing forward
anatomy the study of the form or structure of living
things, including plants, animals, and humans
androgen the male sex hormone that controls the
development and maintenance of masculine characteristics
anemia a condition characterized by a decrease in the
number of red blood cells or an insuffi cient amount of
hemoglobin in the red blood cells
aneurysm (AN-yoo-rizm) abnormal ballooning of a blood
vessel, usually an artery, due to a weakness in the wall
of the vessel
angina pectoris (AN-ji-na) condition characterized by
severe, constricting pain or sensation of pressure in
the chest, often radiating to the left arm caused by an
insuffi cient supply of blood to the heart
angiotensin (an-jee-oh-TEHN-sin) a polypeptide
hormone in the blood that constricts blood vessels and
increases blood pressure
angiotensin-converting enzyme an enzyme in the lungs
that converts angiotensin I to angiotensin II
anorexia nervosa condition characterized by body
weight 15% or more below the minimal normal weight
range, extreme fear of gaining weight, an unrealistic body
image, and amenorrhea
antagonist (an-TAG-un-ist) role played by a skeletal
muscle acting to slow or stop a movement
anterior (ventral) body cavity continuous internal
opening that includes the thoracic and abdominopelvic
cavities
anterior pituitary one lobe of the pituitary gland that
secretes six different hormones: growth hormone,
prolactin, adrenocorticotropin hormone, thyroid
stimulating hormone, follicle-stimulating hormone, and
luteinizing hormone
antibody cells that circulate in plasma and attack red
blood cells with foreign antigens, or antigens that are
different from those of the host
antibody-mediated immunity see humoral immunity
anticodon a set of three bases on tRNA that are
complementary to a 3-base codon on mRNA
antidiuretic hormone (ADH) (AN-tee-digh-yoo-REHT-ik)
a hormone produced by the hypothalamus and stored
in the posterior pituitary that decreases urine output by
increasing water reabsorption by the kidneys, which
increases body fl uid volume; also causes blood vessels
to constrict and raises blood pressure; vasopressin
antigen (AN-ti-jehn) a molecule on the surface of cells
that identifi es cells as either “self” or “nonself” (foreign)
cells, and which triggers production of antibodies to
attack any foreign cells; a protein on the surface of RBCs
that is used to identify blood type
antigen-antibody complex antigen combined with a
specifi c antibody
antigenic determinants the distinctive parts of a
complex molecule that can be recognized by the immune
system
antigen-presenting cells (APCs) cells that process
protein antigens and present them on their surface in a
form that can be recognized by lymphocytes (white blood
cells)
antihistamines (AN-tee-HIS-ta-meenz) medications that
work to curb the activity of histamines
anus the lower opening of the gastrointestinal tract
anvil tiny bone within the middle ear that transmits sound
from the hammer to the stirrup
aorta a large arterial trunk that arises from the base of
the left ventricle and channels blood from the heart into
other arteries throughout the body
aortic arch (ay-OR-tik) the curved portion of the aorta
between the ascending and descending parts of the
aorta
aortic valve (ay-OR-tik) the semilunar valve between
the left ventricle and the aorta that prevents blood from
fl owing back into the left ventricle
apex the pointed end of the heart that lies approximately
at the fi fth rib and points down toward the left hip
apical (AY-pi-kal) toward the apex; when used to describe
an epithelium, it refers to the surface that faces the
outside world
aplastic anemia a rare but serious condition in which the
bone marrow is incapable of making new red blood cells