Biological regulation is a fundamental characteristic of living organisms, enabling them to satisfy internal physiological needs and adapt to environmental changes. This chapter explores the relationship between biological regulation and functional integration. I argue that there is an organisational closure between the regulatory subsystem (R), the processes responsible for self-maintenance (C), and the signalling subsystem (S). Clarifying the functional integration between R, C, and S has two important explanatory outcomes: firstly, it highlights a fundamental organisational dimension of functional integration shared by both prokaryotes and eukaryotes; secondly, it provides a theoretical basis for understanding the evolution of complexity and the emergence of new forms of functional integration during eukaryogenesis.