The fraction of an intake of a radionuclide entering the systemic circulation is referred to as the uptake. In Publication 30 ICRP reviewed information on the behaviour of radionuclides that had entered in the body. It recommended biokinetic models for radionuclides that had entered the blood for intakes by inhalation and ingestion. The ICRP 30 models applied specifically to workers and not to members of the public. More recently, Publications 56, 67, 69 and 71 revised the biokinetic models for selected radionuclides of 31 elements and these have been applied in the calculation of dose coefficients for both workers and for infants, children and adult members of the general population (ICRP [29, 30, 32, 33]. The models given in these Publications were primarily developed to provide age‑dependent dose coefficients but the models for adults were also used in Publication 68 (ICRP, [36]) which gave updated dose coefficients for workers and in Publication 78 (ICRP, [38]) on the interpretation of bioassay data.

The biokinetic models recommended for adults are presently being reviewed by ICRP and will be published probably in 2009. A key feature of the updated models is that they will be applicable for both the calculation of doses and the interpretation of bioassay data.

Radionuclides entering the blood may distribute throughout the body (e.g. ³H, ²⁴Na, ⁴²K, ¹³⁷Cs); they may selectively deposit in a particular tissue (e.g. ¹³¹I in the thyroid; ⁹⁰Sr in bone) or they may deposit in significant quantities in a number of tissues (e.g. ²³⁹Pu, ²⁴¹Am, ¹⁴⁴Ce). If a radionuclide that enters the blood is an isotope of an element that is required by the body then it will follow the normal metabolic pathways for that element (eg. ²⁴Na, ³²P, ⁴²K, ⁴⁵Ca, ⁵⁹Fe). If it has similar chemical properties to an element that is normally present then it will tend to follow the biokinetic pathways of that element, although its rate of transfer between the various compartments in the body may be different (e.g. ⁹⁰Sr and ²²⁶Ra behave similarly to Ca, ¹³⁷Cs and ⁸⁶Rb similarly to K). For other radionuclides their behaviour in the body will depend upon their affinity for biological ligands and other transport systems in the body and, as a result, the extent of uptake and retention is largely unpredictable and must be assessed from the available human or animal data (eg. ⁹⁵Nb, ¹⁰⁶Ru, ²³⁹Pu, ²⁴¹Am). Figure 15 illustrates the structure of the systemic model for plutonium and americium.