To understand general principles that determine fluctuations of integrative functions in multi-celled organism (MO), special theoretical analysis based on mathematical models is done. A complex model includes description of three levels of physiological processes: 1) mechanism of cell adaptation functioning against balance deficit (BD); 2) partial homeostatic systems (HS) and, 3) functional systems. Models and computer simulation allow one to make clear the main causal relations that coming up as step by step generalization of local and independent processes on cellular level. It is shown that the physiological adaptation has a goal (minimization of BD) and special moving forces activating by BD only.