Distribution of Uranium in the Body Biokinetic Models

In these models, the bloodstream is in contact (directly or indirectly) with each compartment and equilibrium exists between the uranium content of the compartment and the uranium couceutration in the blood. Elevated levels of uranium in the bloodstream would lead to deposition of uranium in the compartment. The exchange rate of this process is shown in the model of Eigure 4.7. If the uranium concentration in the blood is low, then uranium could be transported from the compartment (body organ) to the blood and eventually be ranoved from the body (excreted) either by the kidneys and bladder (urine), the intestine (feces), contained in keratin of the hair or nails, or even through exhaled breath or perspiration. 

FIGURE 4.7 The compartmental model of ingested uranium, based on ICRP69 but including hair excretion pathway. (From Li, W.B. et al., Health Phys., 96, 636, 2009. With permission.) The time to reach equilibrium is denoted as STO, STl and ST2 for short (2 h), intermediate (20 days) and long (100 years), respectively, the transferrates are shown as k and the absorption factors as f. 

The foundations for the model (ICRP69 1995) that describes the biokinetic behavior of ingested uranium in the human body are based on several studies, some 

The results of laboratory animal studies and of people who are chronically exposed to uranium in their drinking water show that the amount of soluble uranium accumulated internally is proportional to the intake from ingestion. The total body burden of uranium in humans is estimated to be 40 pg, with approximately 40% of this being present in the muscles, 20% in the skeleton, and 10%, 4%, 1%, and 0.3% in the blood, lungs, liver, and kidneys, respectively (Igarashi et al. 1987). 

The biokinetic models estimate the half-life of uranium in the different compartments. Thus, for (rat) kidney the half-life has been estimated to be approximately 15 days, while for the skeleton half-lives of 300 and 5000 days have been projected, based on a two-compartment model (Wrenn et al. 1985). In another study using a 10-compartment model, half-lives of 5-11 days were estimated for the (rat) kidney and 93-165 for clearance from the skeleton (Sontag 1986). The overall elimination half-life of uranium under conditions of normal daily intake has been estimated to be between 180 and 360 days in humans (Berlin 1986). 

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