The structure that secretes the substance it produces directly into the bloodstream is called an endocrine gland.

Here's why:

* Endocrine glands are ductless glands. This means they don't have a system of tubes or ducts to carry their secretions.

* Instead, they release hormones directly into the surrounding interstitial fluid, which then diffuses into the bloodstream.

* These hormones travel throughout the body, acting on target cells and tissues to regulate various bodily functions.

Examples of endocrine glands include:

* Pituitary gland

* Thyroid gland

* Adrenal glands

* Pancreas

* Ovaries (in females)

* Testes (in males)

Let me know if you have any other questions!