Although our body possesses organs that carry out specific functions, there are others that works as part of a group called organ system.
There are 9 systems in our body: the osteomuscular or locomotive system (some consider it two separate systems: the osseous and the muscular system), nervous, cardiovascular or circulatory, respiratory, digestive, immune, excretory, endocrine and reproductive systems.
Regarding their importance, we could say that all of them are important and also none in particular. Not one of them could be assigned a higher value than any other, as all of them are necessary to keep us alive. For example, the osteomuscular system provides support (or we would be soft as jelly); the circulatory system carries the necessary fuel and oxygen; the reproductive system preventions extinction of the human species.
Two systems order and control the functions our body must carry out: the nervous system and the endocrine system. They gather information and then send back precise instructions to each cell so that they react coordinately as a unit in a given situation. However, despite their functional similarity, these two systems work differently: the Nervous system operates through electric impulses generated by the neurons (nerve cells), while the endocrine system, controls the body through chemical messengers called hormones.