What is Organ, organ system?

An organ is a structure made of several tissues (Chapter 4) arranged to perform a definite function: the heart pumps, the lung exchanges gases, the kidney filters. An organ system is a set of organs cooperating in one broad function: the digestive system runs from mouth to anus with the liver and pancreas attached; the circulatory system is the heart and the vessels; the excretory system, the kidneys and the urinary tract.