lung
The main organ of respiration would be the two things you feel move in and out every time you breath, which if you haven't caught on yet, its you lungs silly :D
Lungs.
the main organ of the muscular system is the muscles
the heart is part of the circulatory system in the body.
The Heart is the main organ of a circulatory system.
Respiratory system
The organs that are not for respiration are as follows The intestance
The lungs conduct respiration.
i believe it is the main organ of the central nervous system
The main organ in the muscular system is the muscles.
The main organ of the circulatory system is the heart.
The respiratory, circulatory, and the digestive system.