Journeys Absorption

For a toxic chemical to be harmful, it must first travel from the environment to a specific target site within the body. Much of this journey happens through the deceptively simple process of diffusion molecules migrating away from their source. Examples of diffusion are aU around us, from salts dissolving into water to fiimes dispersing from a smokestack at an industrial plant into the atmosphere. But diffusion becomes more complicated when the molecule must cross from one environmental compartment into another. For example, when a molecule diffuses from water into the air, properties of the molecule, air, and water all come into play to determine the rate at which diffixsion occurs. Furthermore, the distance traveled by the molecule from its source and the dimension of the surface from which the molecule is diffusing also make a difference in the overall rate of diffusion. 

For a toxic molecule to elicit a response in a target tissue, the 

Alan S. Kolok, Modern Poisons A BrirfIntroduction to Contemporary Toxicology, DOI 10.5822/ 978-1-61091-609-7 4, 2016 Alan S. Kolok. 

To reach the blood, the compound may have to repeat the effort again, moving from the watery interior of the cell, across a lipid membrane, and back into the watery realm of the bloodstream. 

Mammalian skin acts as a barrier between the outer environment and our internal ocean of blood. Our skin stretches tautiy over our body, with only a few wrinkles or invaginations that only marginally increase its overall surface area. Even though our skin has a large surface area relative to the inner organs such as the liver, it is only as large as it needs to be to stretch over the rest of our bodies. 

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The absorption is assumed to occur into elements of liquid moving around the bubble from front to rear in accordance with the penetration theory (H13). These elements maintain their identity for a distance into the fluid greater than the effective penetration of dissolving gas during the time required for this journey. The differential equation and initial and boundary conditions for the rate of absorption are then... 

The term bioavailability has various definitions. Previously, the authors of this chapter have defined bioavailability as the proportion of a nutrient (or other food component) that is digested, absorbed and utilised in normal metabolism - with the practical measurement of bioavailability usually relying upon estimates of amounts absorbed (Southon and Faulks, 2001). Biological activity, or bioactivity , has been viewed and described as a separate stage which follows on from bioavailability in the journey of a compound from food to function. However, here we present a new definition of bioavailability that recognises the functional consequences of absorption. 

Light from noctilucent clouds carries with it more than just information about how it was scattered by cloud particles, however. On its journey from the sun to the clouds, and thence to an observer, it must travel long atmospheric paths along which it suffers selective absorption and scattering by various gases and particles of uncertain kind and amount. This selective extinction... 

The "only" attitude often causes the only child in later years to retain a capacity for self-absorption, self-knowledge, self-companionship, and self-development that can all make for a rich and rewarding journey through adult life. 

A rather different source of variability may arise if different routes of administration are being compared. Consider a brand-name manufacturer who has a treatment that has been registered as a suppository but now wishes to change the formulation to an oral form. The two formulations will have to interact with different parts of the body and in particular the oral form will have a much longer journey to reach the point of absorption. It may be possible to adjust the dose of the oral form so that it delivers the same average dose into the blood, but it is plausible that the variability will be much greater from patient to patient. 

Toxicokinetics describes the journey of a toxicant within a living system. The process includes four fundamental steps absorption, distribution, metabolism, and excretion. Simplistically, toxicokinetics may be thought of as what the body does to a chemical. This is contrasted by the term toxicodynamics, which may be thought of as what the chemical does to the body. The amount of toxicant in the blood over... 

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