We are all familiar with many of the organ systems that comprise the body of advanced animals such as the circulatory system, nervous system, etc. More of us are aware of the essential nature of the immune system in these days of HIV, AIDS, and emergent viral diseases such as Ebola and Hanta.

Animal organs are usually composed of more than one cell type. The stomach contains all four animal tissue types, epithelium to line the stomach and secrete gastric juices, connective tissues to give the stomach flexibility to expand after a large meal, smooth muscle tissues to churn and digest that meal without the need for conscious thought when we burp or suffer some sort of gastric distress and nervous tissues to monitor the progress of food as it is worked on by the stomach, and to direct secretion and muscle activity.

Each organ typically performs a given function set. The stomach is an organ composed of tissues that aid in the mechanical and chemical breakdown of food. Most organs have functions in only one organ system. The stomach is involved only in the digestion of food as part of the digestive system. Organ systems, such as the digestive system, are collections of organs that perform a major function for the organism.