When the activities of a cell, tissue, organ, or system change automatically due to environmental variation, the homeostatic mechanism that operates is called

Autoregulation

Explanation:

When the activities of a cell, tissue, organ, or system change automatically due to environmental variation, the homeostatic mechanism that operates is called autoregulation.

Autoregulation is a method of controlling the local blood circulation. It is characterized as an organ's inherent capacity to sustain a standard blood flow, given changes in the pressure of the perfusion.

substitution is the answer in this case

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.