A group of two or more organs that work together to perform one or more functions. The body of any living organism may be divided into several such separate systems. The human body is divided into 11 organ systems:

1. Integumentary System
2. Skeletal System
3. Muscular System
4. Nervous System
5. Endocrine System
6. Cardiovascular System
7. Lymphatic System
8. Respiratory System
9. Digestive System
10. Urinary System
11. Reproductive System

Informally, each system contains the same organs for both males and females with one exception -- the reproductive system. For this reason, the reproductive system is sometimes subdivided into the male reproductive system and the female reproductive system.

In the past, it was thought that each system performed distinct functions, with little or no effect on other systems. This assumption is now known to be incorrect. It can be clearly demostrated that the activities of or changes in each system can have an effect on many or all other systems. For instance, some drugs taken to relax smooth muscle tissue can actually speed up heart rate. Drugs of this type can sometimes be found in some prescription asthma medications. Also see organ system level.