natural purification (자정 작용
| natural language processing | Computer processing of a language with rules that reflect and describe current usage rather than prescribed usage. (12 Dec 1998) |
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| 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) |
| artificial pacemaker | Any device that substitutes for the normal pacemaker and controls the rhythm of the organ; especially an electronic cardiac pacemaker, which may be implanted in the chest, with electrodes attached to the external cardiac surface, or passed through the venous circulation into the right side of the heart (pervenous pacemaker). (05 Mar 2000) |
| cardiac pacemaker | An electrical device which delivers a small stimulant shock to the heart to effect cardiac contraction at a pre-determined rate. Many of today's pacemakers have two main components: the electrodes and the transducer (pulse generator). The electrodes are wires which are placed into the circulatory system and make physical contact with the heart muscle. A small electrical discharge from the pacemaker electrode stimulates the muscular wall of the heart to contract, thus pumping blood in an organised fashion. The transducer is a small device, usually implanted under the skin, that generates the electrical discharge at a pre-determined frequency. Transducers can monitor your heart's rate of contraction and deliver an electrical shock only when the heart is going too slow. (27 Sep 1997) |
| pacemaker | 1. <apparatus, physiology> An object or substance that influences the rate at which a certain phenomenon occurs, often used alone to indicate the natural cardiac pacemaker or an artificial cardiac pacemaker. 2. <biochemistry> A substance whose rate of reaction sets the pace for a series of interrelated reactions. (24 Mar 1998) |
| pacemaker, artificial | A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). (12 Dec 1998) |
| pacemaker failure | Failure of an artificial pacemaker to generate or deliver effective stimuli to the myocardium. (05 Mar 2000) |
| pacemaker output | Electrical energy delivered into a standard load (500 ohms resistance). (05 Mar 2000) |
| pacemaker potential | The voltage inscribed by impulses from an artificial electronic pacemaker. (05 Mar 2000) |