Conceptual resistance

Selecting accurate and representative estimates of 7d is a challenge because it bundles several competing processes. Figure 6.2 illustrates a conceptual resistance model for dry deposition of small and large particles. Small, low-density particles are mixed in the atmosphere like gases and do not fall under the influence of gravity. For these particles, C/d can be calculated as... 

FIGURE 6.2 Conceptual resistance model of resistances that control the rate of dry particle deposition. 

All the salient features described here can be incorporated into a quantitative framework that takes into account stratification within the tissues and the parallel pathways [39]. It is instructive to consider a simple model based on these descriptions that embodies only transepidermal and transfollicular barrier elements. We can assume that each distinct tissue acts as a homogeneous phase, a gross distortion of reality but an assumption that nevertheless leads to a useful conceptual description. The resistance by the transepidermal route would be ... 

Although mass transfer across the water-air interface is difficult in terms of its application in a sewer system, it is important to understand the concept theoretically. The resistance to the transport of mass is mainly expected to reside in the thin water and gas layers located at the interface, i.e., the two films where the gradients are indicated (Figure 4.3). The resistance to the mass transfer in the interface itself is assumed to be negligible. From a theoretical point of view, equilibrium conditions exist at the interface. Because of this conceptual understanding of the transport across the air-water boundary, the theory for the mass transport is often referred to as the two-film theory (Lewis and Whitman, 1924). 

Multidrug resistance (MDR) is the name ascribed to the phenomenon whereby cancer cells and tumors develop resistance to chemotherapeutic agents. Conceptually, this can be viewed as a survival response whereby cancer cells endeavor to ward off cytotoxic compounds. Mechanistically, MDR is typically mediated by overexpression of P-glycoprotein (P-gp aka ABCB1) or other plasma membrane ATPases that export cytotoxic drugs used in chemotherapy, thereby reducing their efficacy. 

From Eq. 3.9, the greater the thickness (5) of each phase the larger the resistance to solute transfer. Although the use of 5 is convenient for modeling and conceptualizing WBL resistance, it is largely fictitious, as complex hydrodynamics control resistance to mass transfer across the WBL in environmental exposures (see Section 3.6.5. for a more in- depth discussion on this phenomenon). 

TRD can best be conceptualized as occurring along a continuum rather than as an all or none phenomenon. Some patients have a relatively low level of therapy resistance, which can be easily handled in practice by straightforward clinical procedures other patients will follow a much more malignant course. From longitudinal studies (Keller et al. 1992), it is now clear that as many as 15% of patients with depression follow a long-term course and eventually become treatment resistant to almost all antidepressants. 

Thus Feinberg s doubts about irresistibility call attention to a conceptual point about desire rather than to an awesome volitional power of human beings. It is not that there are certain forces that, remarkably/ are no match for human determination rather, we do not stand to our desires as to slabs of stone. For this reason, desires cannot be said to be irresistible by the same criterion, and perhaps in the same sense, as forces of nature. The corollary for the concept of motivational ability is this In Feinberg s words again, that "there is no... limit to our ability to resist temptation" that is comparable to the limits of our physical capacities not, again, because of an unusual omnipotence in this region of life, but because ability means something quite different in the motivational case. 

A particularly important set of beliefs is the idea that a given substance is addictive. Once a behavioral pattern is conceptualized as an addiction, with the concomitant causal beliefs, it may change dramatically. An especially important belief is that addiction is, if not irresistible, at least very hard to resist, almost amounting to compulsive desire. Hence, to the causal beliefs about the effects of drug taking on the addict s body and socioeconomic status, we must add causa) beliefs about the effect of addiction on his will—specifically, on the ability to quit. Two opposite beliefs about this effect may have the same impact on behavior. Some addicts use their (usually self-deceptive) belief that they can quit at any time as an excuse for not quitting. Others use their (equally self-deceptive) belief that they are unable to quit as an excuse for not quitting. The belief that one is addicted may reinforce the addiction by the mechanism of dissonance reduction ... 

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