Which hormone regulation system is controlled by glands that release their products directly into the blood?

The correct choice is indeed the endocrine system because it is characterized by glands that produce hormones and release them directly into the bloodstream. This allows for the hormones to be transported throughout the body and act on various target organs and tissues, regulating numerous physiological functions such as metabolism, growth, reproduction, and mood.

In contrast, the exocrine system involves glands that secrete their products through ducts to external surfaces or into body cavities, not directly into the bloodstream. The nervous system relies on electrical signals and neurotransmitters to communicate rapidly between neurons rather than using hormones. The immune system is primarily responsible for defending the body against pathogens and does not utilize hormone regulation in the same direct manner as the endocrine system. Therefore, the unique mechanism of hormone release into the blood distinguishes the endocrine system as the correct answer.