An endocrine gland secretes substances into:

The correct answer is that an endocrine gland secretes substances into the bloodstream. Endocrine glands are specialized organs that release hormones and other chemical messengers directly into the circulatory system, allowing these substances to be transported throughout the body to target organs and tissues. This mechanism of secretion is essential for regulating various physiological processes, including metabolism, growth, and mood.

In contrast to endocrine glands, exocrine glands secrete their products into ducts that lead to an external surface or a body cavity, like the stomach or sweat glands. Therefore, while some glands may release substances into the stomach or via ducts, endocrine glands specifically function by releasing their hormones directly into the bloodstream for systemic distribution. Additionally, while lymph vessels play a role in the immune system and fluid balance, they are not the primary pathway for secretion by endocrine glands.