Simplify the following text for a Grade 9 native English-speaking person.
Original: In addition to the specialised endocrine organs mentioned above , many other organs that are part of other body systems , such as the kidney , liver , heart and gonads , have secondary endocrine functions . For example the kidney secretes endocrine hormones such as erythropoietin and renin .
Simplified: The endocrine system includes those organs of the body which produce hormones .