This chapter explores the role of system-level reproduction in achieving functional integration in biological individuals by analysing and comparing binary fission in bacteria with mitosis in unicellular eukaryotes. It argues that coordinated system-level reproduction necessitates the integration of reproduction, growth, and metabolism. These processes form the physiological foundation for unifying intra- and cross-generational functions, providing insight into the continuity between constitutive processes and reproductive functions. This organisational closure is crucial for establishing physiological and evolutionary individuality, offering a cohesive perspective on what defines a biological individual.