Constant biological action

As soon as a drug finds its way into the blood stream, it tries to approach the site of biological action. Hence, the distribution of a drug is markedly influenced by such vital factors as tissue distribution and membrane penetration, which largely depends on the physico-chemical characteristics of the drug. For instance, the effect of the ultra-short acting barbiturate thiopental may be explained on its dissociation constant and lipid solubility. It is worthwhile to observe here that the dmation of thiopental is not influenced by its rate of excretion or metabolism, but by its rate of distribution. 

Peat was often used in earlier years as both a humus source and as a fertilizer, with the expectation that it would give results similar to those obtained with animal manures. Although increased yields were often obtained, and the physical condition of the soil was improved, it did not act like animal manures. This was due in part to deficiencies in mineral nutrients, but to a greater extent to the comparatively inert nature of the material. As already pointed out, manure is an active material and as it decomposes, nutrients are constantly released through biological action. In contrast, peat is so resistant to decomposition that much of the release of nutrients must be through exchange reactions and as chelated elements. 

Bilberry has shown vasoprotective, antiedematous, antioxidant, anti-inflammatory, and astringent actions. In particular, a standardized extract enriched with anthoeyanins and their aglycons (anthocyanidins) is endowed with constant biological activity usefiil in ophthalmology and treatment of vascular disorders including capillary weakness, venous insufficiency, and hemorrhoids [12]. 

Simple similar action (simple joint action or concentration/dose addition) is a noninteractive process in which the chemicals in the mixture do not affect the toxicity of one another. All the chemicals of concern in the mixture act on the same biological site, by the same mechanism of action, and differ only in their potencies. The correlation of tolerances is completely positive (r=+l) and each chemical contributes to the toxicity of the mixture in proportion to its dose, expressed as the percentage of the dose of that chemical alone that would be required to obtain the given effect of the mixture. Thus, the individual components of the mixture act as if they were dilutions of the same toxic compound and their relative potencies are assumed to be constant throughout all dose levels. An important implication is that, in principle, no threshold exists for dose additivity. 

The great solvent power of water, especially for ionic compounds, is due to its dielectric constant. If this were only, say 10, instead of the actual 80, it would mean that water could dissolve only a trace of sodium chloride. This solvent action of water., naturally. plays an important role in geology. In biology, water functions as a means of conveying salts and other substances which circulate in the bodies of animals and plants. It is outside the scope of this book to discuss any further the function of water on this planet, a subject which could fill many volumes. It is important in this context that we now know water molecules to possess a dipole moment and to discover whether perhaps this fact can provide an explanation of the unique properties of water. 

Reactions of the hydrated electron possibly may be somewhat relevant to the action of dose-modifying agents such as 02, NO, C02, and sulfhydryl compounds. It can safely be assumed that these exert their influence at the radiation-chemical level, and it is notable that many of them react rapidly with hydrated electrons. Table II, taken from a paper by Braams (6), compares the rate constant for reaction with the hydrated electron with the concentration at which certain compounds have been used as protective agents. It can be seen that, at the concentrations used in biological systems, those substances which are effective as protectors can compete favorably with oxygen for hydrated electrons. Penicillamine was not a good protector at the concentration used and did not compete as favorably as the other substances for hydrated electrons. Higher concentrations of penicillamine could not be... 

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