Anal column A longitudinal fold in the mucous membrane of the anal
canal that contains a network of arteries and veins.
Anal triangle The subdivision of the female or male perineum that
contains the anus.
Analgesia (an-al-JE-ze-a) Pain relief; absence of the sensation of pain.
Anaphase (AN-a-faz) The third stage of mitosis in which the chroma-
tids that have separated at the centromeres move to opposite
poles of the cell.
Anaphylaxis (an'-a-fi-LAK-sis) A hypersensitivity (allergic) reaction in
which IgE antibodies attach to mast cells and basophils, causing
them to produce mediators of anaphylaxis (histamine, leukot-
rienes, kinins, and prostaglandins) that bring about increased
blood permeability, increased smooth muscle contraction, and
increased mucus production. Examples are hay fever, hives, and
Anastomosis (a-nas-to-MO-sis) An anatomical connection between
tubular structure, especially arteries.
Anatomical position (an'-a-TOM-i-kal) A position of the body univer-
sally used in anatomical descriptions in which the body is erect,
the head is level, the eyes face forward, the upper limbs are at
the sides, the palms face forward, and the feet are flat on the
Anatomic dead space Spaces of the nose, pharynx, larynx, trachea,
bronchi, and bronchioles totaling about 150 mL; air in the ana-
tomic dead space does not reach the alveoli to participate in gas
Anatomy (a-NAT-o-me) The structure or study of structure of the
body and the relation of its parts to each other.
Androgens (AN-dro-jenz) Masculinizing sex hormones produced by
the testes in males and the adrenal cortex in both genders; also
responsible for libido (sexual desire); the two main androgens are
testosterone and dihydrotestosterone.
Anemia (a-NE-me-a) Condition of the blood in which the number of
functional red blood cells or their hemoglobin content is below
Anesthesia (an'-es-THE-ze-a) A total or partial loss of feeling or sen-
sation; may be general or local.
Aneurysm (AN-u-rizm) A saclike enlargement of a blood vessel
caused by a weakening of its wall.
Angina pectoris (an-JI-na
AN-ji-na PEK-to-ris) A pain in the chest,
jaw, shoulder, or upper limb related to reduced coronary circula-
tion due to coronary artery disease (CAD) or spasms of vascular
smooth muscle in coronary arteries.
Angiotensin (an-je-o-TEN-sin) Either of two forms of a protein associ-
ated with regulation of blood pressure. Angiotensin I is produced
by the action of renin on angiotensinogen and is converted by the
action of ACE (angiotensin-converting enzyme) into angiotensin
II, which stimulates aldosterone secretion by the adrenal cortex,
stimulates the sensation of thirst, and causes vasoconstriction
with resulting increase in systemic vascular resistance.
Anion (AN-l-on) A negatively charged ion. Examples are the chloride
ion (Cl-) and bicarbonate ion (HCO3-).
Anoxia (an-OK-se-a) Deficiency of oxygen.
Antagonist (an-TAG-o-nist) A muscle that has an action opposite that
of the prime mover (agonist) and yields to the movement of the
Anterior (an-TER-e-or) Nearer to or at the front of the body. Equiva-
lent to ventral in bipeds.
Anterior pituitary (pi-TOO-i-tar-e) Anterior lobe of the pituitary gland.
Also called the adenohypophysis (ad'-e-no-hT-POF-i-sis).
Anterior root The structure composed of axons of motor (efferent)
neurons that emerges from the anterior aspect of the spinal cord
and extends laterally to join a posterior root, forming a spinal
nerve. Also called a ventral root.
Anterolateral pathway (an'-ter-o-LAT-er-al) Sensory pathway that
conveys information related to pain, temperature, tickle, and itch.
Antibody (AN-ti-bod'-e) A protein produced by plasma cells in
response to a specific antigen; the antibody combines with that
antigen to neutralize, inhibit, or destroy it. Also called an immuno-
globulin (im-u-no-GLOB-u-lin) or Ig.
Antibody-mediated immunity That component of immunity in which
B lymphocytes (B cells) develop into plasma cells that produce
antibodies that destroy antigens. Also called humoral immunity
Anticoagulant (an-tT-co-AG-u-lant) A substance that can delay, sup-
press, or prevent the clotting of blood.
Antidiuretic (an'-ti-dT-u-RET-ik) Substance that inhibits urine
Antidiuretic hormone (ADH) Hormone produced by neurosecretory
cells in the hypothalamus that stimulates water reabsorption
from kidney tubule cells into the blood and vasoconstriction of
arterioles. Also called vasopressin (vaz-o-PRES-in).
Antigen (AN-ti-jen) A substance that has the ability to provoke an
immune response and the ability to react with the antibodies
or cells that result from the immune response; contraction of
a n t ib o d y
Antigen-presenting cell (APC) Special class of migratory cell that
processes and presents antigens to T cells during an immune
response; APCs include macrophages, B cells, and dendritic cells,
which are present in the skin, mucous membranes, and lymph
Anuria (an-0-re-a) Daily urine output of less than 50 mL.
Anus (A-nus) The distal end and outlet of the rectum.
Aorta (a-OR-ta) The main systemic trunk of the arterial system of the
body that emerges from the left ventricle.
Aortic body (a-OR-tik) Cluster of chemoreceptors on or near the arch
of the aorta that respond to changes in blood levels of oxygen,
carbon dioxide, and hydrogen ions (H+).
Aortic reflex A reflex that helps maintain normal systemic blood
pressure; initiated by baroreceptors in the wall of the ascending
aorta and arch of the aorta.
Apex (A-peks) The pointed end of a conical structure, such as the
apex of the heart.
Aphasia (a-FA-ze-a) Loss of ability to express oneself properly
through speech or loss of verbal comprehension.
Apnea (AP-ne-a) Temporary cessation of breathing.