Effects of Radioactive Radiation Caused by Man
Jul Låg in Geomedicine, 2017
The main long-lasting problem with contamination by radioisotopes is their mobility between the different compartments of the biogeochemical cycle, which leads to a constant danger to living beings. This includes the possible enrichment related to pool transfer, as stated above. On the other hand, the persistence of some isotopes (e.g., in soil) results in a long-lasting external irradiation of animals and man. And last but not least, there is the hope that there are natural sinks, e.g., deeper layers of soil, or that man will find a safe artificial sink for man-made radioactivity, e.g., in salt mines. Related investigations on radioecology use the terms “mean residence time”, defined as the ratio of content to input resp. output in steady-state systems, and “transfer factor”, the ratio of the radionuclide activity concentration of plants to that of the soil.
Virally Induced Water and Divalent Cation Movement Across Plasma Membranes
Gheorghe Benga in Water Transport in Biological Membranes, 1989
There is an alternative explanation which does not invoke a boundary layer that is more conventional in its assumptions, though the unexpectedly slow exchange rate makes it less likely. It is as follows. In a simple two compartment system, altering the proportions of the compartments would change the apparent relaxation times by changing the residence time of the compartment. From Equation 9, the residence time by definition follows the population size provided no other parameters change. According to the relative sizes of the relaxation and residence times of a compartment, the observed relaxation time will change. If the residence time is much shorter than the relaxation time, the observed relaxation will follow the population size. Conversely, if the relaxation time is much shorter than the residence time, the population size will have little effect on the observed relaxation rate. Thus, reducing the proportion of extracellular medium by concentrating the cell suspension, from say 80 to 20%, would, if the residence time were shorter than the relaxation time, reduce the observed relaxation time of that compartment by about fourfold, as is observed (see Table 4).
Organic Matter
Michael J. Kennish in Ecology of Estuaries Physical and Chemical Aspects, 2019
Refractory organic material, chemically recalcitrant substances of low solubility, is resistant to rapid microbial degradation.30 Hence, it has a relatively long residence time compared to labile compounds, which are rapidly assimilated by heterotrophic organisms. The standing stock of refractory materials, therefore, can obscure that of labile compounds, as is evident in some salt marsh estuaries.33 Humates and fulvates are thought to be the main components of refractory DOC.22 These complex polyphenolic compounds probably are degraded little by microbes and are not important in the food web. In some estuaries — for example, the Wadden Sea — the influx of nearly all DOC in freshwater discharges — which represents the stable end product of the terrestrial carbon cycle — passes essentially unchanged through the systems to be gradually decomposed in the oceans. This DOC, therefore, does not enter the estuarine carbon cycle.
Quantifying and reducing powder shear sensitivity when manufacturing capsules with lubricants
Published in Drug Development and Industrial Pharmacy, 2018
Daniel Blackwood, William Ketterhagen, John Kresevic, Joseph Kushner, Jeffrey Moriarty, Matthew P. Mullarney
The RTD of a perfectly mixed, ideal CSTR, E(t), is given by: τ is the mean residence time [17]. The mean residence time is given by the ratio of mass holdup in the hopper to the mass throughput rate. The concept of the mean residence time is used to approximate the average length of time powder remains in either the hopper or the bowl and the average length of time the powder is exposed to shear. The rate of agitation is given by the tip speed of the shear element, K-value calculation in Equation (1) for batch blending operations, the K-value can also be estimated as follows for continuous processing systems [18] such as the encapsulator feed hopper and bowl as
Effect of diammonium glycyrrhizinate on pharmacokinetics of omeprazole by regulating cytochrome P450 enzymes and plasma protein binding rate
Published in Xenobiotica, 2019
Lu Han, Rong Wang, Bin Wu, Yanqiu Gu, Yongfang Yuan
DAS version 3.0 (BioGuider Co., Shanghai, China) was used to analyze the plasma pharmacokinetic data and calculate the parameters. The maximum plasma concentration (Cmax) and the time taken for the drug to reach Cmax (Tmax) were obtained directly from the concentration–time curves. The linear regression of plasma concentration and time was calculated by the linear regression method, and the elimination rate constant (k) was calculated. The elimination half-life (t1/2) was calculated from the formula t1/2 = 0.693/k. The linear trapezoidal rule (AUC0–t) was used to calculate the area under the concentration–time curve. The mean residence time (MRT) was calculated with AUMC0–∞/AUC0–∞, whereby AUMC0–∞ represented the area under concentration–time curve since the initiation of the experiment. All data were presented as mean ± SD.
Pharmacokinetic study of methylnaltrexone after single and multiple subcutaneous administrations in healthy Chinese subjects
Published in Xenobiotica, 2018
Dan Zhang, Jing-Yi Ma, Man Yang, Ming Deng, Huichen Liu
Pharmacokinetic parameters were calculated by non-compartmental analysis using WinNonlin 6.4 (Pharsight Corporation, St. Louis, MO, USA). The maximum plasma concentration (Cmax) and their corresponding times (Tmax) were obtained by experimental observations. The elimination rate constant (λz) was estimated as the slope of a least-square linear regression of the terminal portion of the log-transformed plasma concentration–time curve. The t1/2 was calculated as 0.693/λz. The area under the plasma concentration–time curve from zero to time t of the last measured concentration above the limit of quantification (AUC0–t) was calculated by the linear trapezoidal rule up to the last sampling point, and extrapolated to infinity to obtain AUC0–∞, by adding the quotient of the last measurable plasma concentration and λz. The plasma clearance confounded by absolute bioavailability (CL/F) was measured from the ratio dose/AUC0–∞. The apparent volume of distribution confounded by absolute bioavailability (Vd/F) was estimated as dose/(AUC0–∞ × λz). Mean residence time (MRT) was calculated as the equation:
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