A biological system is a complex network of biologically relevant entities.
Biological organization spans several scales. Examples of biological systems at the macro scale are populations of organisms. On the organ and tissue scale in mammals and other animals, examples include the circulatory system, the respiratory system, and the nervous system.
On the micro to the nanoscopic scale, examples of biological systems are cells, organelles, macromolecular complexes and regulatory pathways.
A biological system is not to be confused with a living system, such as a living organism.