Overall decay is driven by the complex half-life that could be expressed as a combination of physical and biological half-lives: T = TB TP /  (TB + TP) or easily by decay constants: λ = λB + λP. Biological decay is individual for each organism, while the physical decay is driven just by the half-life of the nuclide. 

Excellent 

Each type of radiolabeled atom has specific physical decay to reach into half life time but biological decay i think it is the time taken by animal to excreted the radiolabeled compound from the body.