abdomen: The region of the trunk bounded by the diaphragm and pelvis.

abdominopelvic cavity: The portion of the ventral body cavity that contains abdominal and pelvic subdivisions; also contains the peritoneal cavity.

abducens: Cranial nerve VI, which innervates the lateral rectus muscle of the eye.

abduction: Movement away from the midline of the body, as viewed in the anatomical position.

abortion: The premature loss or expulsion of an embryo or fetus.

abruptio placentae: The premature loss of a placental connection to the uterus, leading to maternal hemorrhaging and shock.

abscess: A localized collection of pus within a damaged tissue.

absorption: The active or passive uptake of gases, fluids, or solutes.

acclimatization: The physical adaption to a long-term environmental change, such as adaptions that accompany a change in season or in latitude.

accommodation: An alteration in the curvature of the lens of the eye to focus an image on the retina.

acetabulum: The fossa on the lateral aspect of the pelvis that accommodates the head of the femur.

acetylcholine (ACh): A chemical neurotransmitter in the brain and peripheral nervous system; the dominant neurotransmitter in the peripheral nervous system, released at neuromuscular junctions and synapses of the parasympathetic division.

acetylcholinesterase (AChE): An enzyme found in the synaptic cleft, bound to the postsynaptic membrane, and in tissue fluids; breaks down and inactivates acetylcholine molecules.

acetyl-CoA: An acetyl group bound to coenzyme A, a participant in the anabolic and catabolic pathways for carbohydrates, lipids, and many amino acids.

acetyl group: —CH₃CO.

achalasia: A condition that develops when the lower esophageal sphincter fails to dilate and ingested materials cannot enter the stomach.

Achilles tendon: See calcanean tendon.

acid: A compound whose dissociation in solution releases a hydrogen ion and an anion; an acidic solution has a pH below 7.0 and contains an excess of hydrogen ions.

acidosis: An abnormal physiological state characterized by a plasma pH below 7.35.

acinus/acini: A histological term referring to a blind pocket, pouch, or sac.

acne: A condition characterized by the inflammation of sebaceous glands and follicles; commonly affects adolescents and in most cases involves the face.

acoustic: Pertaining to sound or the sense of hearing.

acquired immune deficiency syndrome (AIDS): A disease caused by the human immunodeficiency virus (HIV); characterized by the destruction of helper T cells and a resulting severe impairment of the immune response.

acromegaly: A condition caused by the overproduction of growth hormone in adults, characterized by a thickening of bones and an enlargement of cartilages and other soft tissues.

acromion: A continuation of the scapular spine that projects above the capsule of the scapulohumeral joint.

acrosomal cap: A membranous sac at the tip of a spermatozoon that contains hyaluronidase.

actin: The protein component of microfilaments that forms thin filaments in skeletal muscles and produces contractions of all muscles through interaction with thick (myosin) filaments; see also sliding filament theory.

action potential: A conducted change in the transmembrane potential of excitable cells, initiated by a change in the membrane permeability to sodium ions; see also nerve impulse.

activation energy: The energy required to initiate a specific chemical reaction.

active transport: The ATP-dependent absorption or excretion of solutes across a cell membrane.

acute: Sudden in onset, severe in intensity, and brief in duration.

adaptation: A change in pupillary size in response to changes in light intensity; a decrease in receptor sensitivity or perception after chronic stimulation; physiological responses that produce acclimatization.

Addison's disease: A condition resulting from the hyposecretion of glucocorticoids; characterized by lethargy, weakness, hypotension, and increased skin pigmentation.

adduction: Movement toward the axis or midline of the body, as viewed in the anatomical position.

adenine: A purine; one of the nitrogenous bases in the nucleic acids RNA and DNA.

adenohypophysis: The anterior portion of the pituitary gland; also called anterior pituitary.

adenoids: The pharyngeal tonsil.

adenosine: A combination of adenine and ribose.

adenosine diphosphate (ADP): A compound consisting of adenosine with two phosphate groups attached.

adenosine monophosphate (AMP): A nucleotide consisting of adenine plus a phosphate group (PO₄^3-); also called adenosine phosphate.

adenosine triphosphate (ATP): A high-energy compound consisting of adenosine with three phosphate groups attached; the third is attached by a high-energy bond.

adenylate cyclase: An enzyme bound to the inner surfaces of cell membranes that can convert ATP to cyclic-AMP; also called adenylyl cyclase.

adhesion: The fusion of two mesenterial layers after damage or irritation of their opposing surfaces; this process restricts relative movement of the organs involved; the binding of a phagocyte to its target.

adipocyte: A fat cell.

adipose tissue: Loose connective tissue dominated by adipocytes.

adrenal cortex: The superficial portion of the adrenal gland that produces steroid hormones.

adrenal gland: A small endocrine gland that secretes steroids and catecholamines and is located superior to each kidney; also called suprarenal gland.

adrenal medulla: The core of the adrenal gland; a modified sympathetic ganglion that secretes catecholamines into the blood after sympathetic activation.

adrenergic: A synaptic terminal that, when stimulated, releases norepinephrine.

adrenocortical hormone: Any steroid produced by the adrenal cortex.

adrenocorticotropic hormone (ACTH): The hormone that stimulates the production and secretion of glucocorticoids by the zona fasciculata of the adrenal cortex; released by the anterior pituitary in response to corticotropin-releasing hormone.

adventitia: The superficial layer of connective tissue surrounding an internal organ; fibers are continuous with those of surrounding tissues, providing support and stabilization.

aerobic: Requiring the presence of oxygen.

aerobic metabolism: The complete breakdown of organic substrates into carbon dioxide and water, via pyruvic acid; a process that yields large amounts of ATP but requires mitochondria and oxygen.

afferent: Toward.

afferent arteriole: An arteriole that carries blood to a glomerulus of the kidney.

afferent fiber: An axon that carries sensory information to the central nervous system.

afterbirth: The distal portions of the umbilical cord and placenta that are ejected from the uterus during the placental stage of labor.

agglutination: The aggregation of red blood cells due to interactions between surface antigens and plasma antibodies.

agglutinins: Immunoglobulins in plasma that react with antigens on the surfaces of foreign red blood cells when donor and recipient differ in blood type.

agglutinogens: Antigens on the surfaces of red blood cells whose presence and structure are genetically determined.

aggregate lymphoid nodules: Lymphoid nodules beneath the epithelium of the small intestine; also called Peyer’s patches.

agonist: A muscle responsible for a specific movement.

agranular: Without granules; agranular leukocytes are monocytes and lymphocytes.

AIDS: See acquired immune deficiency syndrome.

alba: White.

albicans: White.

albinism: The absence of pigment in hair and skin, caused by the inability of melanocytes to produce melanin.

albuginea: White.

aldosterone: A mineralocorticoid produced by the zona glomerulosa of the adrenal cortex; stimulates sodium and water conservation at the kidneys; secreted in response to the presence of angiotensin II.

aldosteronism: The condition caused by the oversecretion of aldosterone, characterized by fluid retention, edema, and hypertension.

alkalosis: The condition characterized by a plasma pH greater than 7.45; associated with a relative deficiency of hydrogen ions or an excess of bicarbonate ions.

allantois: One of the four extraembryonic membranes; provides vascularity to the chorion and is therefore essential to placenta formation; the proximal portion becomes the urinary bladder.

alleles: Alternate forms of a particular gene.

allergen: An antigenic compound that produces a hypersensitivity response.

alpha-blockers: Drugs that prevent the stimulation of alpha receptors.

alpha cells: Cells in the pancreatic islets that secrete glucagon.

alpha receptors: Membrane receptors sensitive to norepinephrine or epinephrine; stimulation normally results in the excitation of the target cell.

alveolar sac: An air-filled chamber that supplies air to several alveoli.

alveolus/alveoli: Blind pockets at the end of the respiratory tree, lined by a simple squamous epithelium and surrounded by a capillary network; sites of gas exchange with the blood; a bony socket that holds the root of a tooth.

Alzheimer's disease: A disorder resulting from degenerative changes in populations of neurons in the cerebrum, causing dementia characterized by problems with attention, short-term memory, and emotions.

amacrine cells: Modified neurons in the retina that facilitate or inhibit communication between bipolar cells and ganglion cells.

amenorrhea: A failure to commence menstruation at adolescence or the cessation of menstruation prior to menopause.

amination: The attachment of an amino group to a carbon chain; performed by a variety of cells and important in the synthesis of amino acids.

amino acids: Organic compounds whose chemical structure can be summarized as R—CHNH²—COOH.

amino group: —NH².

amnesia: A temporary or permanent memory loss.

amniocentesis: The sampling of amniotic fluid for analytical purposes; used to detect certain genetic abnormalities.

amnion: One of the four extraembryonic membranes; surrounds the developing embryo or fetus.

amniotic fluid: Fluid that fills the amniotic cavity; cushions and supports the embryo or fetus.

amphiarthrosis: An articulation that permits a small degree of independent movement; see interosseous membrane and pubic symphysis.

amphicytes: Supporting cells that surround neurons in the peripheral nervous system; also called satellite cells.

amphimixis: The fusion of male and female pronuclei after fertilization.

ampulla/ampullae: A localized dilation in the lumen of a canal or passageway.

amygdaloid body: A cerebral nucleus that is a component of the limbic system and acts as an interface between that system, the cerebrum, and sensory systems.

amylase: An enzyme that breaks down polysaccharides; produced by the salivary glands and pancreas.

anabolism: The synthesis of complex organic compounds from simpler precursors.

anaerobic: Without oxygen.

analgesic: A substance that relieves pain.

anal triangle: The posterior subdivision of the perineum.

anamnestic response: The sudden and exaggerated production of antibodies after a second exposure to a specific antigen; due to the activation of memory B cells.

anaphase: The mitotic stage in which the paired chromatids separate and move toward opposite ends of the spindle apparatus.

anaphylaxis: A hypersensitivity reaction due to the binding of antigens to immunoglobulins (IgE) on the surfaces of mast cells; the release of histamine, serotonin, and prostaglandins by mast cells then causes widespread inflammation; a sudden decline in blood pressure may occur, producing anaphylactic shock.

anastomosis: The joining of two tubes, usually referring to a connection between two peripheral vessels without an intervening capillary bed.

anatomical position: An anatomical reference position; the body viewed from the anterior surface with the palms facing forward; supine.

anatomy: The study of the structure of the body.

anaxonic neuron: A central nervous system neuron that has many processes but no apparent axon.

androgen: A steroid sex hormone primarily produced by the interstitial cells of the testis and manufactured in small quantities by the adrenal cortex in either gender.

anemia: The condition marked by a reduction in the hematocrit, the hemoglobin content of the blood, or both.

anencephaly: A developmental defect characterized by incomplete development of the cerebral hemispheres and cranium.

anesthesia: The total or partial loss of sensation from a region of the body.

aneurysm: A weakening and localized dilation in the wall of a blood vessel.

angiogram: An X-ray image of circulatory pathways.

angiography: The X-ray examination of vessel distribution after the introduction of radiopaque substances into the bloodstream.

angiotensin I: The hormone produced by the activation of angiotensinogen by renin; angiotensin-converting enzyme converts angiotensin I into angiotensin II in lung capillaries.

angiotensin II: A hormone that causes an elevation in systemic blood pressure, stimulates the secretion of aldosterone, promotes thirst, and causes the release of antidiuretic hormone; angiotensin-converting enzyme in lung capillaries converts angiotensin I into angiotensin II.

angiotensinogen: The blood protein produced by the liver that is converted to angiotensin I by the enzyme renin.

anion: An ion bearing a negative charge.

ankyloglossia: A condition characterized by an overly robust and restrictive lingual frenulum.

anulus: A cartilage or bone shaped like a ring; also spelled annulus.

anorectal canal: The distal portion of the rectum that contains the rectal columns and ends at the anus.

anorexia nervosa: An eating disorder marked by a loss of appetite and pronounced weight loss.

anoxia: Tissue oxygen deprivation.

antagonist: A muscle that opposes the movement of an agonist.

antebrachium: The forearm.

anteflexion: The normal position of the uterus, with the superior surface bent forward.

anterior: On or near the front, or ventral surface, of the body.

anterior pituitary: See pituitary gland or adenohypophysis.

anterograde amnesia: The inability to store memories of events that occur after a specific incident or time.

anthracosis: "Black lung disease," a deterioration of respiratory exchange efficiency due to the chronic inhalation of coal dust.

antibiotic: A chemical agent that selectively kills pathogens.

antibody: A globular protein produced by plasma cells that will bind to specific antigens and promote their destruction or removal from the body.

antibody-mediated immunity: The form of immunity resulting from the presence of circulating antibodies produced by plasma cells; also called humoral immunity.

anticholinesterase: A chemical compound that blocks the action of acetylcholine and causes prolonged and intensive stimulation of postsynaptic membranes.

anticoagulant: A compound that slows or prevents clot formation by interfering with the clotting system.

anticodon: Three nitrogenous bases on a tRNA molecule that interact with an appropriate codon on a strand of mRNA.

antidiuretic hormone (ADH): A hormone synthesized in the hypothalamus and secreted at the posterior pituitary; causes water retention at the kidneys and an elevation of blood pressure.

antigen: A substance capable of inducing the production of antibodies.

antigen–antibody complex: The combination of an antigen and a specific antibody.

antigenic determinant site: A portion of an antigen that can interact with an antibody molecule.

antigen-presenting cell (APC): A cell that processes antigens and displays them, bound to MHC proteins; essential to the initiation of a normal immune response.

antihistamine: A chemical agent that blocks the action of histamine on peripheral tissues.

antipyretic agents: Chemicals that reduce fever.

antrum: A chamber or pocket.

anuria: A cessation of urine production.

anus: The external opening of the anorectal canal.

aorta: The large, elastic artery that carries blood away from the left ventricle and into the systemic circuit.

aortic reflex: A baroreceptor reflex triggered by increased aortic pressures; leads to a reduction in cardiac output and a fall in systemic pressure.

Apgar test: A test used to assess the neurological status of a newborn infant.

aphasia: The inability to speak.

apnea: The cessation of breathing.

apneustic center: A respiratory center whose chronic activation would lead to apnea at full inhalation.

apocrine secretion: A mode of secretion in which the glandular cell sheds portions of its cytoplasm.

aponeurosis/aponeuroses: A broad tendinous sheet that may serve as the origin or insertion of a skeletal muscle.

apoplexy: A cerebrovascular accident (stroke).

appendicitis: An inflammation of the appendix.

appendicular: Pertaining to the upper or lower limbs.

appendix: See vermiform appendix.

appositional growth: The enlargement of a bone by the addition of cartilage or bony matrix at its surface.

aqueous humor: A fluid similar to perilymph or cerebrospinal fluid that fills the anterior chamber of the eye.

arachidonic acid: One of the essential fatty acids.

arachnoid: The middle meninx that encloses cerebrospinal fluid and protects the central nervous system.

arachnoid granulations: Processes of the arachnoid that project into the superior sagittal sinus; sites where cerebrospinal fluid enters the venous circulation.

arbor vitae: The central, branching mass of white matter inside the cerebellum.

arcuate: Curving.

areflexia: The absence of normal reflex responses to stimulation.

areola: The pigmented area that surrounds the nipple of a breast.

areolar: Containing minute spaces, as in areolar tissue.

areolar tissue: Loose connective tissue with an open framework.

arrector pili: Smooth muscles whose contractions force hairs to stand erect.

arrhythmias: Abnormal patterns of cardiac contractions.

arteriole: A small arterial branch that delivers blood to a capillary network.

artery: A blood vessel that carries blood away from the heart and toward a peripheral capillary.

arthritis: An inflammation of a joint.

arthroscope: A fiber-optic device intended for visualizing the interior of joints; also used for certain forms of joint surgery.

articular: Pertaining to a joint.

articular capsule: The dense collagen fiber sleeve that surrounds a joint and provides protection and stabilization.

articular cartilage: The cartilage pad that covers the surface of a bone inside a joint cavity.

articulation: A joint; the formation of words.

arytenoid cartilages: A pair of small cartilages in the larynx.

ascending tract: A tract carrying information from the spinal cord to the brain.

ascites: The overproduction and accumulation of peritoneal fluid.

asphyxia: Unconsciousness due to oxygen deprivation at the central nervous system.

aspirate: To remove or obtain by suction; to inhale.

association areas: Cortical areas of the cerebrum that are responsible for the integration of sensory inputs and/or motor commands.

association neuron: See interneuron.

asthma: A reversible constriction of smooth muscles around respiratory passageways, commonly caused by an allergic response.

astigmatism: A visual disturbance due to an irregularity in the shape of the cornea.

astrocyte: One of the four types of neuroglia in the central nervous system; responsible for maintaining the blood–brain barrier by the stimulation of endothelial cells.

ataxia: A failure to coordinate muscular activities normally.

atelectasis: The collapse of a lung or a portion of a lung.

atherosclerosis: The formation of fatty plaques in the walls of arteries, leading to circulatory impairment.

atom: The smallest stable unit of matter.

atomic number: The number of protons in the nucleus of an atom.

atomic weight: Roughly, the average total number of protons and neutrons in the atoms of a particular element.

atresia: The closing of a cavity, or its incomplete development; refers to the degeneration of developing ovarian follicles.

atria: Thin-walled chambers of the heart that receive venous blood from the pulmonary or systemic circuit.

atrial natriuretic peptide (ANP): The hormone released by specialized atrial cardiocytes when they are stretched by an abnormally large venous return; promotes fluid loss and reductions in blood pressure and in venous return.

atrial reflex: The reflexive increase in heart rate after an increase in venous return; due to mechanical and neural factors; also called Bainbridge reflex.

atrioventricular (AV) node: Specialized cardiocytes that relay the contractile stimulus to the bundle of His, the bundle branches, the Purkinje fibers, and the ventricular myocardium; located at the boundary between the atria and ventricles.

atrioventricular (AV) valve: One of the valves that prevent backflow into the atria during ventricular systole.

atrophy: The wasting away of tissues from a lack of use, ischemia, or nutritional abnormalities.

auditory: Pertaining to the sense of hearing.

auditory ossicles: The bones of the middle ear: malleus, incus, and stapes.

auditory tube: A passageway that connects the nasopharynx with the middle ear cavity; also called Eustachian tube or pharyngotympanic tube.

auricle: A broad, flattened process that resembles the external ear; in the ear, the expanded, projecting portion that surrounds the external auditory canal, also called pinna; in the heart, the externally visible flap formed by the collapse of the outer wall of a relaxed atrium.

autoantibodies: Antibodies that react with antigens on the surfaces of a person’s own cells and tissues.

autodigestion: The digestion of tissues by digestive acids or enzymes from the stomach or pancreas.

autoimmunity: The immune system’s sensitivity to normal cells and tissues, resulting in the production of autoantibodies.

autolysis: The destruction of a cell due to the rupture of lysosomal membranes in its cytoplasm.

automaticity: The spontaneous depolarization to threshold, characteristic of cardiac pacemaker cells.

autonomic ganglion: A collection of visceral motor neurons outside the central nervous system.

autonomic nerve: A peripheral nerve consisting of preganglionic or postganglionic autonomic fibers.

autonomic nervous system (ANS): Centers, nuclei, tracts, ganglia, and nerves involved in the unconscious regulation of visceral functions; includes components of the central nervous system and the peripheral nervous system.

autopsy: The detailed examination of a body after death, normally performed by a pathologist.

autoregulation: Changes in activity that maintain homeostasis in direct response to changes in the local environment; does not require neural or endocrine control.

autosomal: Chromosomes other than the X or Y chromosome.

avascular: Without blood vessels.

avitaminosis: A condition caused by the inadequate intake of one or more essential vitamins.

avulsion: An injury involving the violent tearing away of body tissues.

axilla: The armpit.

axolemma: The cell membrane of an axon, continuous with the cell membrane of the cell body and dendrites and distinct from any neuroglial coverings.

axon: The elongate extension of a neuron that conducts an action potential.

axon hillock: In a multipolar neuron, the portion of the cell body adjacent to the initial segment.

axoplasm: The cytoplasm within an axon.