Physiological efficiency coefficient

Despite its slightly different scaling, our /x agrees with the fx in [Cu2]. Cushing calls fx the physiological efficiency coefficient of the population, since it reflects both the reproductive efficiency and the growth efficiency of the organism. 

Oxygen uptake is not very useful in studies of energy expenditure, since 30-40% of it is not used for phosphorylated oxidation or ATP resynthesis, but for other varieties of oxidation such as peroxic and enzymatic oxidation which results in the production of heat (Khaskin, 1981). In any case, the efficiency of oxygen uptake as measured from the PIO coefficient varies according to various eco-physiological factors (Verzhbinskaya, 1968 Arsan et al., 1984 Savina, 1992). The heat produced is dissipated into the environment, except in the case of certain very active fish that possess a heat-exchange mechanism (Stevens et al., 1974) permitting the body temperature to rise and thereby increase muscle power (Love, 1980, p.321). 

A physiologically based pharmacokinetics (PBPK) model based on the ventilation rate, cardiac output, tissue blood flow rates, and volumes as well as measured tissue/air and blood/air partition coefficients has been developed (Medinsky et al. 1989a Travis et al. 1990). Experimentally determined data and model simulations indicated that during and after 6 hours of inhalation exposure to benzene, mice metabolized benzene more efficiently than rats (Medinsky et al. 1989a). After oral exposure, mice and rats appeared to metabolize benzene similarly up to oral doses of 50 mg/kg, above which rats metabolized more benzene than did mice on a per kg body weight basis (Medinsky et al. 1989b). This model may be able to predict the human response based on animal data. Benzene metabolism followed Michaelis-Menton kinetics in vivo primarily in the liver, and to a lesser extent in the bone marrow. Additional information on PBPK modeling is presented in Section 2.3.5. 

All opiates produce CNS effects. However, the degree to which they are seen depends on the dose of drug that enters the CNS. Thus, factors that allow the drug to cross the blood-brain barrier more efficiently will result in increased CNS effects. These include molecular structure, degree of ionization at physiologic pH, and the partition coefficient of the drug molecule. The degree of CNS effects seen may also depend on receptor selectivity. 

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