We must look also at the important concept of a system's boundary. The boundary may be concrete or conceptual, but at each level, the concept of system requires a boundary. The boundary helps maintain the internal environment. It provides appropriate entrance and exit of matter and energy, and filters improper input. However, the boundary does not define the limits of a system's influence.
Systems are open.
Systems are also characterized by internal adjustment processes. Even the equilibrium of non-living macrocosmic systems can be conceived in terms of feedback. Though a smaller body is held by the gravity of a larger one, the smaller's gravity may be seen as a feedback attracting the larger. Systems are also characterized by rhythm . The feedback signals result in repeated fluctuations or changes that at various levels are described as cycles, orbits, oscillations, periods, pulses, vibrations, waves, etc., all reflecting the dynamics of the interactions in the system.