On the occasion of the first international workshop on systems radiation biology we review the role of cell renewal systems in maintaining the integrity of the mammalian organism after irradiation. First, 11 radiation emergencies characterized by chronic or protracted exposure of the human beings to ionizing irradiation were "revisited". The data provide evidence to suggest that at a daily exposure of about 10-100 mSv, humans are capable of coping with the excess cell loss for weeks or even many months without hematopoietic organ failure. Below 10 mSv/day, the organisms show some cellular or subcellular indicators of response. At dose rates above 100 mSv/day, a progressive shortening of the life span of the irradiated organism is observed. To elucidate the mechanisms relevant to tolerance or failure, the Megakaryocyte-thrombocyte cell renewal system was investigated. A biomathematical model of this system was developed to simulate the development of thrombocyte concentration as a function of time after onset of chronic radiation exposure. The hematological data were taken from experimental chronic irradiation studies with dogs at the Argonne National Laboratory, USA. The results of thrombocyte response patterns are compatible with the notion of an "excess cell loss" (compared to the steady-state) in all proliferative cell compartments, including the stem cell pool. The "excess cell loss" is a function of the daily irradiation dose rate. Once the stem cell pool is approaching an exhaustion level, a "turbulence region" is reached. Then it takes a very little additional stress for the system to fail. We conclude that in mammalian radiation biology (including radiation medicine), it is important to understand the physiology and pathophysiology of cell renewal systems in order to allow predicting the development of radiation induced lesions.