| natural language processing | Computer processing of a language with rules that reflect and describe current usage rather than prescribed usage. (12 Dec 1998) |
|---|---|
| natural mutation | A mutation which occurs by itself without first being affected by a mutagen, for example during the process of DNA replication. Spontaneous mutations arise at a remarkably constant rate. The rate that spontaneous mutations arise has been used as an evolutionary clock to estimate how closely related two (or more) separate species are to each other. (09 Oct 1997) |
| natural passive immunity | Immunity conferred by the mother on the foetus or newborn. (14 Nov 1997) |
| natural pigment | A naturally occurring coloured compound; absorbs light in the visible range of the electromagnetic spectrum. Compare: structural colour. Synonym: biochrome. (05 Mar 2000) |
| natural products | Naturally occurring compounds that are end products of secondary metabolism; often, they are unique compounds for particular organisms or classes of organisms. (05 Mar 2000) |
| natural selection | The hypothesis that genotype environment interactions occurring at the phenotypic level lead to differential reproductive success of individuals and hence to modification of the gene pool of a population. (18 Nov 1997) |
| immunity, natural | The capacity of a normal organism to remain unaffected by microorganisms and their toxins. It results from non-specific mechanisms such as genetic endowment, naturally occurring antiviral and bactericidal substances, phagocytosis, and constitutional factors such as body temperature, oxygen tension, and age. (12 Dec 1998) |
| killer cells, natural | Cells responsible for spontaneous cytotoxicity of a variety of tumour cells without prior immunization. These natural killer cells are found in non-immune humans and experimental animals and are thought by some to be the same as killer cells (killing by antibody-dependent cell cytotoxicity), but they can also kill in the absence of antibody. (12 Dec 1998) |
| alveolar gas | The gas in the pulmonary alveoli, where O2-CO2 exchange with pulmonary capillary blood occurs. Synonym: alveolar air. (05 Mar 2000) |
| alveolar gas equation | The equation defining the steady state relation of the alveolar oxygen pressure to the barometric pressure, inspired gas composition, alveolar carbon dioxide pressure, and respiratory exchange ratio; the equation is used in various forms depending upon which simplifying assumptions are acceptable for different applications. (05 Mar 2000) |
| anaesthetic gas | A gas or a liquid with sufficient vapor pressure to produce general anaesthesia when breathed. (05 Mar 2000) |
| arterial blood gas | A test which analyses arterial blood for oxygen, carbon dioxide and bicarbonate content in addition to blood pH. Used to test the effectiveness of respiration. Acronym: ABG (17 Oct 1997) |
| bivalent gas gangrene antitoxin | Antitoxin specific for the toxins of Clostridium perfringens and C. Septicum. (05 Mar 2000) |
| blood gas analysis | <investigation> A test which analyses arterial blood for oxygen, carbon dioxide and bicarbonate content in addition to blood pH. Used to test the effectiveness of respiration. (27 Sep 1997) |
| blood gas monitoring, transcutaneous | The noninvasive measurement or determination of the partial pressure (tension) of oxygen and/or carbon dioxide locally in the capillaries of a tissue by the application to the skin of a special set of electrodes. These electrodes contain photoelectric sensors capable of picking up the specific wavelengths of radiation emitted by oxygenated versus reduced haemoglobin. (12 Dec 1998) |