What does the endocrine system primarily secrete?

The endocrine system primarily secretes hormones, which are chemical messengers that are released directly into the bloodstream. Hormones play a crucial role in regulating various physiological processes in the body, including metabolism, growth and development, tissue function, and mood regulation. They are produced by glands such as the pituitary, thyroid, adrenal, and pancreas, and they target specific organs or tissues to elicit a response, thus maintaining homeostasis in the body.

The other options involve functions that are not primarily associated with the endocrine system. Muscle contractions are mainly controlled by the muscular and nervous systems, nerve signals are transmitted through neurons as part of the nervous system, and enzymes are typically secreted by glands and organs in the digestive system to aid in the breakdown of food and other substances. Therefore, hormones are distinctly the primary output of the endocrine system, making this the correct choice.