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Some populations, such as red blood cells (RBCs), exhibit a pattern of population decline that is closer to linear rather than exponential, which has proven to be unexpectedly challenging to describe with a single simple mathematical model. Here we show that a sub-exponential model of population extinction can approximate very well the experimental curves of RBC extinction, and that one possible mechanism underlying sub-exponential decay is population heterogeneity with respect to death rates of individuals. We further show that a sub-exponential model of population decline can be derived within the frameworks of frequency-dependent model of population extinction if there exists heterogeneity with respect to mortality rates such that their initial distribution is the Gamma distribution. Notably, specific biological mechanisms that may result in linear pattern of population decay may be different depending on the specific biological system; however, in the end they must converge to individual death rates being different within the population, since uniform death rates would result in exponential population decline. As such, the proposed model is not intended to describe the complex dynamics of RBC biology but instead can provide a way to phenomenologically describe linear decay of population size. We briefly discuss the potential utility of this model for describing effects of drugs that may cause RBC depletion and conclude with a suggestion that this tool can provide a lens for discovering linear extinction patterns in other populations, which may have previously been overlooked.