tachycardia (tak,t-kar'de-a) An excessively rapid heart rate, usually applied to rates in excess of 100 beats per minute (in contrast to bradycardia, in which the heart rate is very slow—below 60 beats per minute). target organ The organ that is specifically affected by the action of a hormone or other regulatory process.
T cell A type of lymphocyte that provides cellmediated immunity (in contrast to B lymphocytes, which provide humoral immunity through the secretion of antibodies). There are three subpopulations of T cells: cytotoxic (killer), helper, and suppressor. telo- (Gk.) An end, complete, final. telomere (tel o-mer) A DNA sequence at the end of a chromosome that is not copied by DNA polymerase during DNA replication. This inability to copy telomeres may contribute to cell aging and death. Germinal cells (that produce gametes) and cancer cells have an additional enzyme, telomerase, which copies the telomeres. telophase (telo-faz) The last step of mitosis and the last step of the second division of meiosis. tendon (ten dun) The dense regular connective tissue that attaches a muscle to the bones of its origin and insertion. testes (tes'tez); sing; testis. Male gonads. Testes are also known as testicles. testis-determining factor The product of a gene located on the short arm of the Y chromosome that causes the indeterminate embryonic gonads to develop into testes. testosterone (tes-tos'te-ron) The major androgenic steroid secreted by the Leydig cells of the testes after puberty. tetanus (tet n-us) In physiology, a term used to denote a smooth, sustained contraction of a muscle, as opposed to muscle twitching. tetraiodothyronine (tet'ra-i"d-do-thi'ro-nen) (T4) A hormone containing four iodine atoms; also known as thyroxine. thalassemia (thal"a-se'me-a) Any of a group of hemolytic anemias caused by the hereditary inability to produce either the alpha or beta chain of hemoglobin. It is found primarily among Mediterranean people. theophylline (the-of'i-lin) A drug found in certain tea leaves that promotes dilation of the bronchioles by increasing the intracellular concentration of cyclic AMP (cAMP) in the smooth muscle cells. This effect is due to inhibition of the enzyme phosphodiesterase, which breaks down cAMP. thermiogenesis (ther-me-o-jen'it-sis) The production of heat by the body through mechanisms such as increased metabolic rate. thorax (thor aks) The part of the body cavity above the diaphragm; the chest. threshold The minimum stimulus that just produces a response.
thrombin (throm bin) A protein formed in blood plasma during clotting that enzymatically converts the soluble protein fibrinogen into insoluble fibrin. thrombocyte (throm'bo-sit) A blood platelet; a discshaped structure in blood that participates in clot formation. thrombopoietin (throm"bo-poi-e tin) A cytokine that stimulates the production of thrombocytes (blood platelets) from megakaryocytes in the bone marrow. thrombosis (throm-bo sis) The development or presence of a thrombus. thrombus (throm bus) A blood clot produced by the formation of fibrin threads around a platelet plug.
thymus (thi mus) A lymphoid organ located in the superior portion of the anterior mediastinum. It processes T lymphocytes and secretes hormones that regulate the immune system. thyroglobulin (thi-ro-glob yut-lin) An iodine-
containing protein in the colloid of the thyroid follicles that serves as a precursor for the thyroid hormones. thyroxine (thi-rok sin) Also called tetraiodothyronine, or T4. The major hormone secreted by the thyroid gland. It regulates the basal metabolic rate and stimulates protein synthesis in many organs. A deficiency of this hormone in early childhood produces cretinism.
tinnitus (ti-ni'tus) The spontaneous sensation of a ringing sound or other noise without sound stimuli.
tolerance, immunological The ability of the immune system to distinguish self from nonself; thus, the immune system does not normally attack those antigens that are part of one's own tissues.
total minute volume The product of tidal volume (ml per breath) and ventilation rate (breaths per minute).
totipotent (toitip ot-tent) The ability of a cell to differentiate into all tissue types, and thus to form a new organism when appropriately stimulated and placed in the correct environment (a uterus). toxin (tok sin) A poison.
toxoid (tok soid) A modified bacterial endotoxin that has lost toxicity but that still has the ability to act as an antigen and stimulate antibody production.
tracts A collection of axons within the central nervous system that forms the white matter of the CNS. trans- (L.) Across, through.
transamination (trans"am-i-na'shun) The transfer of an amino group from an amino acid to an alpha-keto acid, forming a new keto acid and a new amino acid without the appearance of free ammonia.
transcription (tran-skrip shun). genetic The process by which messenger RNA is synthesized from a DNA template resulting in the transfer of genetic information from the DNA molecule to the mRNA. transducins (trans-doo sinz) The G-proteins involved in vision. When light causes the photodissociation of rhodopsin, the G-protein alpha subunit dissociates from the opsin and indirectly cause a reduction in the dark current of the photoreceptors. translation (trans-la shun), genetic The process by which messenger RNA directs the amino acid sequence of a growing polypeptide during protein synthesis. transplantation (trans"plan-ta shun) The grafting of tissue from one part of the body to another part, or from a donor to a recipient. transpulmonary (trans"pul mot-ner"-e) pressure The pressure difference across the wall of the lung; equal to the difference between intrapulmonary pressure and intrapleural pressure. triiodothyronine (tri"i-o'do-thi'ro-nen) (T3) A
hormone secreted in small amounts by the thyroid; the active hormone in target cells formed from thyroxine. tropomyosin (tro"pot-mi ot-sin) A filamentous protein that attaches to actin in the thin filaments. Together with another protein called troponin, it acts to inhibit and regulate the attachment of myosin cross bridges to actin. troponin (tro pot-nin) A protein found in the thin filaments of the sarcomeres of skeletal muscle. A subunit of troponin binds to Ca2+, and as a result causes tropomyosin to change position in the thin filament.
trypsin (trip sin) A protein-digesting enzyme in pancreatic juice that is released into the small intestine.
tryptophan (trip'' to-fan) An amino acid that also serves as the precursor for the neurotransmitter molecule serotonin. TSH Thyroid-stimulating hormone, also called thyrotropin (thi"ro-tro'pin) A hormone secreted by the anterior pituitary that stimulates the thyroid gland. tubuloglomerular (too"byu-lo-glo-mer'yu-lar)
feedback A control mechanism whereby an increased flow of fluid through the nephron tubules causes a reflex reduction in the glomerular filtration rate. tumor necrosis (nei-kro'sis) factor (TNF) A cytokine released by immune cells and mast cells that causes destruction of tumors and migration of neutrophils toward the site of a bacterial infection. TNF is also secreted by adipose cells and may be a paracrine regulator of insulin sensitivity. turgid (tur Jid) Swollen and congested. twitch A rapid contraction and relaxation of a muscle fiber or a group of muscle fibers. tympanic (tim-pan ik) membrane The eardrum; a membrane separating the external from the middle ear that transduces sound waves into movements of the middle-ear ossicles. tyrosine kinase (ti'ro-sen ki'nas) An enzyme that adds phosphate groups to tyrosine, an amino acid present in most proteins. The membrane receptor for insulin, for example, is a tyrosine kinase. When bound to insulin, the tyrosine kinase is activated, which leads to a cascade of effects that mediate insulin's action.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.