11.2.3 Extrinsic Mechanisms

The extrinsic mechanism of blood coagulation begins when a blood vessel is ruptured and the surrounding tissues are damaged. The traumatized tissue releases a complex of substances referred to as tissue thromboplastin. The tissue thromboplastin further complexes with factor VII and Ca++ ions to activate factor X directly. 

Factors, such as smoking, food habits and alcohol intake influence drug responses probably by both intrinsic and extrinsic mechanisms. 

Figure 7a. Anatomy of a real fiber, b. Intrinsic and extrinsic mechanisms of fiber birefringence. From C. P. Poole and S. Nagel, [10], Polarization effects in lightwave systems, in Optical Fiber Telecommunications, illA, pp. 115-161, I. P. Kaminow and T. L. Koch, Eds., Academic Press, San Diego, CA (1997).

Fig. 8.41 Schematic illustrations of the intrinsic and extrinsic mechanisms involved in cyclic fatigue-crack growth in a metals and b ceramics, showing the relative dependencies of growth rates, da/dN, on the alternating, AK, and maximum, stress intensities [4]. With kind...

Absorption losses caused by electronic transitions and molecular vibrations are examples of intrinsic mechanisms. For sihca, the electronic transitions are in the ultraviolet (UV) wavelengths, whereas the molecnlar vibrations are in the infiared (IR). The tails of these transitions bracket the usefiil range of low loss between the visible and the near-infrared wavelengths. Impurity absorption losses are examples of extrinsic mechanisms. The main impurities in fibers made by vapor deposition techniques are the transition metal ions and OH ions. Impurities such as iron cause absorption in the UV and visible wavelengths. Their concentrations need to be lednced to the few parts per bilhon range to reduce their contributions to... 

Wear. Ceramics generally exhibit excellent wear properties. Wear is deterrnined by a ceramic s friction and adhesion behavior, and occurs by two mechanisms adhesive wear and abrasive wear (43). Adhesive wear occurs when interfacial adhesion produces a localized Kj when the body on one side of the interface is moved relative to the other. If the strength of either of the materials is lower than the interfacial shear strength, fracture occurs. Lubricants (see Lubricants and lubrication) minimize adhesion between adj acent surfaces by providing an interlayer that shears easily. Abrasive wear occurs when one material is softer than the other. Particles originating in the harder material are introduced into the interface between the two materials and plow into and remove material from the softer material (52). Hard particles from extrinsic sources can also cause abrasive wear, and wear may occur in both of the materials depending on the hardness of the particle. 

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

In type III or immunocomplex-mediated allergy, IgG antibodies form complexes with antigen. At low exposures, the body is able to remove diese complexes, but if there is a severe exposure, immunocomplexes release a variety of proinflammatory cytokines. The involvement of this mechanism is clearest in serum sickness. This mechanism is also considered to be most important in the development of extrinsic allergic alveolitis (hypersensitivity pneumonitis, especially... 

The proposed scenario is mainly based on the molecular approach, which considers conjugated polymer films as an ensemble of short (molecular) segments. The main point in the model is that the nature of the electronic state is molecular, i.e. described by localized wavefunctions and discrete energy levels. In spite of the success of this model, in which disorder plays a fundamental role, the description of the basic intrachain properties remains unsatisfactory. The nature of the lowest excited state in m-LPPP is still elusive. Extrinsic dissociation mechanisms (such as charge transfer at accepting impurities) are not clearly distinguished from intrinsic ones, and the question of intrachain versus interchain charge separation is not yet answered. 

The Extrinsic Pathway Also Leads to Activation of Factor X But by a Different Mechanism... 

It is the intrinsic defects that have the most interest for us, since they affect the chemical properties of the solid while extrinsic defects havelittle effect. Extrinsic defects are the proper study for those interested in the mechanics of solids, particularly metals. 

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