What do endocrine glands secrete directly into?

Endocrine glands are specialized organs that play a crucial role in the endocrine system by secreting hormones directly into the bloodstream. This transport mechanism allows hormones to travel throughout the body and reach various target organs and tissues where they exert their effects. By releasing hormones directly into the blood, endocrine glands can regulate numerous physiological processes, including metabolism, growth and development, immune response, and mood.

The other options are not suitable because saliva is secreted by exocrine glands, which have ducts to carry their secretions to specific locations. Nerves transmit signals through electrical impulses rather than releasing substances like hormones into the bloodstream. Similarly, while cells can receive signals or substances, they do not represent a pathway through which endocrine glands secrete their products. The direct secretion into the bloodstream allows for a faster and broader distribution of hormones compared to localized secretion methods.